Women who are pregnant or trying to conceive should be advised to avoid caffeine

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Women who are pregnant or trying to conceive should be advised to avoid caffeine because the evidence suggests that maternal caffeine consumption is associated with negative pregnancy outcomes and that there is no safe level of consumption, finds an analysis of observational studies published in BMJ Evidence Based Medicine.

Caffeine is probably the most widely consumed psychoactive substance in history, and many people, including pregnant women consume it on a daily basis.

Pregnant women have been advised that consuming a small amount of caffeine daily will not harm their baby.

The UK NHS, the American College of Obstetricians and Gynecologists, the Dietary Guidelines for Americans and the European Food Safety Authority (EFSA) set this level at 200 mg caffeine, which approximates to roughly two cups of moderate-strength coffee per day.

This study undertook a review of current evidence on caffeine-related pregnancy outcomes, to determine whether the recommended safe level of consumption for pregnant women is soundly based.

Through database searches, Professor Jack James, of Reykjavik University, Iceland, identified 1,261 English language peer-reviewed articles linking caffeine and caffeinated beverages to pregnancy outcomes.

These were whittled down to 48 original observational studies and meta-analyses published in the past two decades reporting results for one or more of six major negative pregnancy outcomes: miscarriage, stillbirth, low birth weight and/or small for gestational age, preterm birth, childhood acute leukaemia, and childhood overweight and obesity.

A total of 42 separate findings were reported in 37 observational studies; of these 32 found that caffeine significantly increased risk of adverse pregnancy outcomes and 10 found no or inconclusive associations.

Caffeine-related risk was reported with moderate to high levels of consistency for all pregnancy outcomes except preterm birth.

Eleven studies reported on the findings of 17 meta-analyses, and in 14 of these maternal caffeine consumption was associated with increased risk for four adverse outcomes: miscarriage, stillbirth, low birth weight and/or small for gestational age, and childhood acute leukaemia.

The three remaining meta-analyses did not find an association between maternal caffeine consumption and preterm birth.

No meta-analyses looked at the association between maternal caffeine consumption and childhood overweight and obesity, but four of five observational studies reported significant associations.

This is an observational study, so can’t establish causation, and the author points out that the results could be impacted by other confounding factors, such as recall of caffeine consumption, maternal cigarette smoking and most importantly pregnancy symptoms.

Pregnancy symptoms such as nausea and vomiting in early pregnancy are predictive of a healthy pregnancy and women who experience them are likely to reduce their caffeine intake.

But he adds that the dose-responsive nature of the associations between caffeine and adverse pregnancy outcomes, and the fact some studies found no threshold below which negative outcomes were absent, supports likely causation rather than mere association.

Professor James concludes that there is “substantial cumulative evidence” of an association between maternal caffeine consumption and diverse negative pregnancy outcomes, specifically miscarriage, stillbirth, low birth weight and/or small for gestational age, childhood acute leukaemia and childhood overweight and obesity, but not preterm birth.

As a result, he adds, current health recommendations concerning caffeine consumption during pregnancy are in need of “radical revision.”

“Specifically, the cumulative scientific evidence supports pregnant women and women contemplating pregnancy being advised to avoid caffeine,” he says.


Caffeine (1,3,7-trimethylxanthine) is a natural alka- loid found in coffee beans, tea leaves, cocoa beans, cola nuts and other plants. It is probably the most frequently ingested pharmacologically active substance in the world, found in common beverages (coffee, tea, soft drinks), products containing cocoa  or chocolate, and medications, including headache or pain remedies and over-the-counter stimulants (Murphy and Benjamin 1981, IARC 1991b, Dlugosz and Bracken 1992, Carrillo and Benitez 1996).

The possibility that caffeine consumption can have adverse effects on human health was assessed based on the results of (primarily) published human studies obtained through a comprehensive literature search. The results of this assessment are summarized here.

Sources and prevalence of caffeine consumption

In North America, coffee (60–75%) and tea (15–30%) are the major sources of caffeine in the adult diet, whereas caffeinated soft drinks and chocolate are the major sources of caffeine in the diet of children. Coffee is also the primary source of caffeine in the diet of adults in some European countries, such as Finland, Sweden, Denmark and Switzerland. Brewed coffee contains the most caffeine (56–100 mg/100 ml), followed by instant coffee and tea (20–73 mg/100 ml) and cola (9–19 mg/100 ml).

Cocoa and chocolate products are also important sources of caffeine (e.g. 5–20 mg/100 g in chocolate candy), as are a wide variety of both prescription (30–100 mg/tablet or capsule) and non-prescription (15–200 mg/tablet or capsule) drugs (Dlugosz and Bracken 1992, Barone and Roberts 1996, Shils et al. 1999, Tanda and Goldberg 2000).

In Canada, published values for the average daily intake    of    caffeine    from    all    sources    is about 2.4 mg kg 1    body    weight    (bw)    for adults   and 1.1 mg kg –1 bw for children 5–18 years old (Chou 1992). Recently, Brown et al. (2001) reported daily caffeine intakes ranging from 288 to 426 mg (equiva- lent to 4.5–6.5 mg kg –1 bw in a 65-kg person) in the adult population (481 men and women aged 30–75 years) residing in southern Ontario, Canada.

Elsewhere, mean daily caffeine intake for adults among the general population has been given as approximately 3 mg kg –1 bw in the USA, 4 mg kg –1 bw in the UK and 7 mg kg 1 bw in Denmark. For high-level consumers, daily intakes range from 5 to 15 mg kg –1 bw. For children, daily caffeine intakes have  been  given  as  1 mg kg –1  bw  in  the  USA, <3 mg kg 1 bw in the UK and <2.5 mg kg 1 bw in Denmark (IARC 1991b, Ellison et al. 1995, Barone and Roberts 1996, Hughes and Oliveto 1997).

Note that the caffeine content of coffee and tea is dependent on their method of preparation and the product brand. In addition, variations in caffeine intake can occur due to differences in the size of the serving ‘cup’ (Stavric et al. 1988). The impact of these variations should be considered in the interpretation and comparison of clinical studies, particularly when cultural differences may be involved.

Pharmacokinetics

Following ingestion, caffeine is rapidly and essentially completely absorbed from the gastrointestinal tract into the bloodstream. Maximum caffeine concentra- tions in blood are reached within 1–1.5 h following ingestion. Absorbed caffeine is readily distributed throughout the entire body. It passes across the blood–brain barrier, through the placenta into am- niotic fluid and the foetus, and into breast milk. Caffeine has also been detected in semen (Berger 1988, Arnaud 1999).

The liver is the primary site of caffeine metabolism (Stavric and Gilbert 1990, Arnaud 1999). In adults, caffeine is virtually completely metabolized to 1- methylxanthine and 1-methyluric acid from the paraxanthine intermediate. Only 1–5% of ingested caf- feine is recovered unchanged in the urine. Infants up to the age of 8–9 months have a greatly reduced ability to metabolize caffeine, excreting about 85% of the administered caffeine in the urine unchanged (Nolen 1989, Stavric and Gilbert 1990).

The elimination half-life of caffeine ranges between 3 and 7 h and can be influenced by many factors, including sex, age, use of oral contraceptives, preg- nancy and smoking. Caffeine’s half-life has been reported to be 20–30% shorter in females than in males.

The half-life in newborns ranges from 50 to 100 h, but it gradually approaches that of an adult by 6 months of age. The half-life in females using oral contraceptive steroids is approximately twice that observed for ovulatory females. During pregnancy, the metabolic half-life increases steadily from 4 h during the first trimester to 18 h during the third trimester.

Cigarette smoking is associated with about a twofold increase in the rate at which caffeine is eliminated (Aranda et al. 1979, Dalvi 1986, Gilbert  et al. 1986, Stavric and Gilbert 1990, James 1991a, Dlugosz and Bracken 1992,  Eskenazi  1993,  Hinds et al. 1996, Arnaud 1999, Karen 2000).

General toxicity

Death due to excessive caffeine ingestion is not com- mon, and only a few cases have been reported in the literature. The acute lethal dose in adult humans has been estimated to be 10 g/person. Death has been reported after ingestion of 6.5 g caffeine, but survival of a patient who allegedly ingested 24 g caffeine was also reported (Stavric 1988, James 1991b).

Caffeine toxicity in adults can present a spectrum of clinical symptoms, ranging from nervousness, irritability and insomnia to sensory disturbances, diuresis, arrhythmia, tachycardia, elevated respiration and gastrointestinal disturbances.

Caffeine toxicity in children is manifested by severe emesis, tachycardia, central nervous system agitation and diuresis.  Chronic exposure to caffeine has been implicated  in  a range of dysfunctions involving the gastrointestinal system, liver, renal system and musculature (Stavric 1988, James 1991b).

The most important mechanism of action of caffeine is the antagonism of adenosine receptors. Adenosine is a locally released purine which acts on different receptors that can increase or decrease cellular con- centrations of cyclic adenosine monophosphate (cAMP). Caffeine selectively blocks adenosine recep- tors and competitively inhibits the action of adeno-

sine at concentrations found in people consuming caffeine from dietary sources. Caffeine results in the release of norepinephrine, dopamine and serotonin in the brain and the increase of circulating catechola- mines, consistent with reversal of the inhibitory effect of adenosine (Benowitz 1990).

It is now widely believed that habitual daily use of caffeine >500–600 mg (four to seven cups of coffee or seven to nine cups of tea) represents a significant

health risk and may therefore be regarded as ‘abuse’. Sustained abuse may in turn result in ‘caffeinism’, which refers to a syndrome characterized by a range of adverse reactions such as restlessness, anxiety, irritability, agitation, muscle tremor, insomnia, head- ache, diuresis, sensory disturbances (e.g. tinnitus), cardiovascular symptoms (e.g. tachycardia, arrhyth- mia) and gastrointestinal complaints (e.g. nausea, vomiting, diarrhoea) (James and Paull 1985).

Excessive caffeine intake (>400 mg day 1) may in- crease the risk of detrusor instability (unstable bladder) development in women. For women with pre-existing bladder symptoms, even moderate caffeine intake (200–400 mg day 1) may result in an increased risk for detrusor instability (Arya et al. 2000).

Cardiovascular effects

Clinical studies have investigated the effects of caf- feine or coffee on cardiac arrhythmia, heart rate, serum cholesterol and blood pressure. Epidemi- ological studies have largely focused on the associ- ation between coffee intake and cardiovascular risk factors, including blood pressure and serum choles- terol levels, or the incidence of cardiovascular disease itself.

Clinical studies have shown that single doses of  caffeine <450 mg do not increase the frequency or severity of  cardiac  arrhythmia  in healthy persons, patients with ischaemic heart disease or those with serious ventricular ectopia (Myers 1998). Studies conducted in healthy or hypertensive subjects suggest that when a change in heart rate is observed, it is typically a decrease at doses >150 mg/person  (James 1991c,  Green  et  al.  1996,  Myers  1998). 

The rapid development of tolerance to the heart rate effect of caffeine (Green et al. 1996) complicates data interpretation. The generally modest decrease in heart rate is likely not clinically relevant (Myers 1998).

Several clinical and epidemiological studies have suggested that coffee consumption is associated with significant increases in total and low-density lipo-protein cholesterol levels. Recent studies, however, suggest that it is not the caffeine in coffee that is responsible for its hypercholesterolaemic effect (Thelle et al. 1987, James 1991c, d, Thelle 1993, 1995, Gardner et al. 1998).

Two diterpenoid alcohols, cafestol and kahweol, found at significant levels in boiled coffee have been identified as hypercholester- olaemic components. Although these components are largely trapped by the use of a paper filter in coffee preparation, there is some evidence that consumption of filtered coffee is associated with small increases in serum cholesterol levels (Thelle 1995).

The effect of caffeine on blood pressure in habitual caffeine consumers and abstainers has been investi- gated in more than 50 acute and 19 repeated-dose clinical trials with healthy or hypertensive subjects (reviewed by Myers 1988, 1998, James 1991c, Green et al. 1996).

The results of the acute studies indicate that caffeine induces an increase in systolic (5–15 mmHg) and/or diastolic (5–10 mmHg) blood pres- sure,  most consistently at doses  >250 mg/person,  in adults of both sexes, irrespective of age, race, blood pressure status, or habitual caffeine intake. The effect is most pronounced in elderly, hypertensive or caf- feine-naive individuals.

The pressor effect of caffeine was also observed in many of the repeated-dose studies, but not as consistently as in the acute studies. It is generally agreed that tolerance to these pressor effects develops within 1–3 days, but is partially lost after abstinence for as little as 12 h.

The clinical significance of caffeine’s pressor effects and the devel- opment of tolerance continues to be discussed in the literature (James 1991c, Green et al. 1996, Myers 1998).

Epidemiological studies investigating associations be- tween caffeine and blood pressure (reviewed by Myers 1988, 1998, James 1991c, 1997, Green et al. 1996) have yielded conflicting results (i.e. positive, negative or no association). These inconsistencies may reflect methodological problems, including misclassification resulting from the use of dietary recall data, tolerance to the pressor effects of caffeine and the effect of smoking on the plasma half-life of caffeine. While James (1991c, 1997) and Green et al. (1996) indicated that further research was needed, Myers (1998) con- cluded that there was no epidemiological evidence to support any relationship between caffeine use and blood pressure.

Epidemiological studies addressing the possible asso- ciation between consumption of caffeine-containing beverages, usually coffee, and coronary heart disease include case-control, longitudinal cohort and pro- spective studies (reviewed by James 1991d,  Lynn  and Kissinger 1992, Myers and Basinski 1992, Franceschi 1993, Thelle 1995, Myers 1998); meta- analyses of case-control and/or prospective study  data were published by Greenland (1987, 1993) and Kawachi et al. (1994); and a recent case-control was published by Palmer et al. (1995) and two recent prospective studies were published by Stensvold and Tverdal (1995) and Hart and Smith (1997).

Most relied on self-administered questionnaires to determine intakes of caffeinated beverages. Cardiovascular disease was assessed by a variety of outcome vari- ables, including death from myocardial infarction or coronary heart disease, non-fatal myocardial infarction or coronary event, angina pectoris and/or hospi- talization for coronary heart disease.

The results of both case-control and prospective epidemiological studies yielded inconsistent results, although case- control studies were more likely to show a significant relationship between coffee consumption and cardiovascular disease, with an increased risk generally observed at intakes of five or more cups of coffee    per day ( 5 500 mg caffeine day 1).

Longitudinal cohort studies published from 1986 yielded more consistent positive associations than those published up to 1981 (Greenland 1993). The inconsistencies both within and between case-control and prospective studies have resulted in controversies regarding study methodologies and data interpretation (James 1991d, Myers and Basinski 1992, Franceschi 1993, Greenland 1993, Myers 1998).

While recognizing the ambiguity of the epidemiological data, Greenland (1993) and Franceschi (1993) concluded that the possibility of heavy coffee consumption (defined as  10 or more cups per day by Greenland 1993; probably four or more cups per day in Franceschi 1993) adversely affecting the incidence of coronary heart disease or mortality cannot be ruled out.

None of the epidemiological data determine whether it is caffeine per se or other components of coffee that are responsible for coffee’s association with cardiovascular disease. Although no significant association has been found between tea consumption and cardi- ovascular disease (Franceschi 1993, Thelle 1995, Myers 1998), it has been suggested that the beneficial effects of the flavonoids present in tea may offset any adverse effect of caffeine (Thelle 1995). Support for the idea that caffeine in coffee is not responsible for cardiovascular effects comes from epidemiological studies showing an increased risk of coronary events with consumption of decaffeinated coffee  (Grobbee et al. 1990, Gartside and Glueck 1993).

In summary, the data currently available indicate that moderate caffeine intake (four or fewer cups of coffee per day, or 4 400 mg caffeine day 1) does not ad- versely affect cardiovascular health. There are insuffi- cient epidemiological data to draw any conclusions about the risk for coronary heart disease or mortality associated with consumption of 10 or more cups of coffee per day ( 5 1000 mg caffeine day 1).

Effects on bone and calcium balance

The database on caffeine’s potential to adversely influence bone metabolism includes epidemiological studies investigating the relationship between caffeine and/or coffee intake and the risk of osteoporosis as characterized by low bone mineral density and in- creased susceptibility to fractures, as well as metabolic studies examining the effect of caffeine on calcium homeostasis.

Caffeine intake of 150–300 mg after a 10-h fast increased urinary calcium excretion 2–3 h after exposure in adolescent men and women (Massey and Hollingbery 1988), women 22–30 years of age (Massey and Wise 1984, Massey and Opryszek  1990), men 21–42 years of age (Massey and Berg 1985), and women 31–78 years  of  age  consuming => 200 mg caffeine day 1 (Bergman et al. 1990).

Tolerance to the renal effects of caffeine does not develop, as habitual coffee intake had no effect on the increase in calcium excretion associated with an acute caffeine dose (Massey and Opryszek 1990). Caffeine- induced hypercalciuria was not affected by oestrogen status (Bergman et al. 1990), gender or age (Massey and Wise 1992).

Barger-Lux et al. (1990) reported that caffeine intakes of 400 mg person 1 day 1 for 19 days led to evidence of altered bone remodelling in healthy premenopausal women between the ages of 35 and 44, but had no effect on fractional calcium absorption, endogenous faecal calcium or urinary calcium excretion.

An earlier study in the same population suggested that caffeine consumption of  175 mg person 1 day 1 was positively associated with increased 24-h urinary calcium excretion (Heaney and Recker 1982).

Whether it is through increased urinary calcium ex- cretion (Massey and Whiting 1993) or decreased intestinal calcium absorption (Heaney 1998), caffeine does appear to have a negative effect on calcium balance (Hasling et al. 1992, Barger-Lux  and  Heaney 1995).

Barger-Lux et al. (1990) concluded that a daily intake of 400 mg caffeine by healthy premenopausal women with a calcium intake of at least 600 mg day 1 has no appreciable effect on cal- cium excretion. Hasling et al. (1992) derived a model from  data  collected  from  postmenopausal  women that indicated coffee intakes >1000 ml day 1 (760 mg caffeine day 1) could induce excess calcium loss, while intakes of 150–300 ml coffee day 1 (112–224 mg caf- feine day 1) would have little impact on calcium balance. The biological significance of caffeine’s ne- gative effect on calcium balance has been debated (Barger-Lux et al. 1990, Massey and Whiting 1993).

Several epidemiological studies have been conducted to assess the relationship between caffeine intake and bone density.

Increasing caffeine intakes were not associated with significant decreases in bone density in adolescent women (Lloyd et al. 1998), young women 20–30 years of age (Eliel et al. 1983, McCulloch et al. 1990, Packer and Recker 1996, Conlisk and Galuska 2000), premenopausal women (Picard et al. 1988, Lacey et al. 1991, Lloyd et al. 1991, Hansen 1994), perimenopausal women (Slemenda et al. 1987, 1990), postmenopausal women (Slemenda et al. 1987, Hansen et al. 1991, Reid et al. 1994, Lloyd et al. 1997, 2000, Hannan et al. 2000) or men (Eliel et al. 1983, Glynn et al. 1995, Hannan et al. 2000).

Some negative associations between caffeine intake and bone density have been observed; these associations disappeared when confounders such as calcium intake were adjusted for in some studies (Cooper et al. 1992, Johansson et al. 1992), but not others (Herna´ ndez-Avila et al. 1993). Some research- ers have found that caffeine’s effects on bone density were dependent on calcium intakes. Harris and Dawson-Hughes (1994) concluded that two to three servings of coffee (280–420 mg caffeine day 1) may accelerate bone loss in healthy postmenopausal women with calcium intakes <800 mg day 1. Barrett- Connor et al. (1994) found that only postmenopausal women who did not report drinking at least one glass of milk per day between the ages of 20 and 50 years exhibited a coffee-associated decrease in bone mineral density.

Caffeine intake has been investigated as a potential risk factor for bone fracture, the major cause of morbidity and mortality associated with osteoporosis. In case-control studies, caffeine intakes were not associated with an increased risk of hip fracture in women >55 years of age (Nieves et al. 1992), women 18–70 years of age (Tavani et al. 1995), or men or women >65 years of age (Cumming and Klineberg 1994).

In  a cross-sectional  study, Travers-Gustafson et al. (1995) were also unable to show that caffeine intakes were related to an increased incidence of low- trauma fractures. In contrast, data from the Nurses Health Study found that women who consumed more than  four cups  of coffee  per  day (>544 mg caffeine day 1) had a higher risk of hip fracture than those who ‘almost never’ consumed coffee (Herna´ ndez- Avila et al. 1991).

Although other studies have shown an increase in the risk of hip fracture with dietary caffeine, it was not clear whether the analysis adjusted for differences in calcium intake (Holbrook et al. 1988) or whether calcium intake data were unavail- able (Kiel et al. 1990).

Interpretation of caffeine’s effects on bone metabo- lism are complicated because coffee intake is asso- ciated with other risk factors for osteoporosis: calcium intake (Heaney and Recker 1982,  Massey and  Hollingbery  1988,  Hasling  et   al.   1992, Herna´ ndez-Avila et al. 1993), age (Barger-Lux and Heaney 1995), cigarette smoking (Cooper et al. 1992, Johansson et al. 1992, Barrett-Connor et al. 1994) and alcohol consumption (Cooper et al. 1992, Barrett- Connor et al. 1994).

Collectively, the available data suggest that an increased caffeine intake is associated with a slight but biologically real deterioration in calcium balance. The majority of evidence indicates that this effect is through caffeine-induced hypercal- ciuria. The biological significance of caffeine’s nega- tive effect on calcium balance continues to be the topic of scientific debate, as studies on both bone density and fracture risk have revealed conflicting results. Bruce and Spiller (1998) suggest that a lifetime pattern of high caffeine intake (more than four cups

of coffee per day or >400 mg caffeine day 1) in women  contributes to a negative  impact  on  calcium

and bone metabolism and is correlated with bone loss or fracture risk, particularly when there is a low calcium intake. Heaney (1998) suggests that the epidemiological studies showing a negative association between caffeine intake and bone mass may be ex-  plained by an inverse relationship between consumption of milk and consumption of caffeine-containing beverages, concluding that there is no evidence that caffeine  has  any  harmful  effect  on  bone  status   or calcium economy in individuals ingesting recom- mended levels of calcium.

To date, the evidence indicates that the significance of caffeine’s potential to affect calcium balance and bone metabolism adversely is dependent on lifetime caffeine and calcium intakes and is biologically more relevant in women. Current data suggest that  caffeine intakes of <400 mg day 1 do not have significant effects on bone status or calcium balance in individuals ingesting at least 800 mg calcium day 1 (an intake that <50% of Canadian women achieve).

Effects on human behaviour

Mood and performance in adults

The results of studies on the effects of caffeine on various psychomotor tasks (reviewed by James 1991e, Smith 1998) are sometimes conflicting. For example, some studies have shown no effects of caffeine on hand steadiness, whereas others have associated caf- feine consumption with poorer performance in this parameter (Bovim et al. 1995).

Studies showing both positive effects (Jacobson and Edgley 1987, Roache and Griffiths 1987) and no effects (Zahn and  Rapoport 1987) on reaction time have also been reported.

Inconsistent results can be encountered in the literature in terms of the impact of caffeine on cognitive functioning, including alertness, vigilance, memory and mood. These inconsistencies may be due to methodological differences, personality differences (e.g. introverts versus extroverts), the time of day when tests were conducted, and uncontrolled con- founding factors (e.g. habitual caffeine, alcohol or tobacco use) (James 1991e, Smith 1998).

In general, caffeine (100 mg day 1 for 4 days, Leathwood and Pollet 1982/83; 1.5–3 mg kg 1 bw as single doses, 2 h apart, or 105–210 mg for a 70-kg adult, Smith et al. 1993; 250 mg day 1 for 2 days, Johnson et al. 1990; two doses of 200 mg, Regina et al. 1974) has been shown to increase the alertness of individuals, especially in situations where arousal is low (e.g. night-shift workers, early in the morning).

Caffeine can also increase vigilance in the daytime. In a double-blind placebo-controlled study in males, statistically significant increases were observed in two of three vigilance tests, including both visual and auditory tests, at all caffeine doses employed (as low as 32 mg caffeine up to 256 mg) (Lieberman et al. 1987).

In another investigation of the effects of caffeine on alertness, subjects given caffeine (250 mg twice per day) performed significantly better in an auditory vigilance test than did the placebo group (Zwyghuizen- Doorenbos et al. 1990).

Most studies on the effects of caffeine on psychomo- tor and cognitive parameters deal with acute admin- istration. In a study on regular consumers of coffee and tea (Jarvis 1993), higher levels of coffee consumption were associated with improved performance in reaction time, verbal memory and visuospatial reasoning.

The consumption of tea was related to an improved performance in one test of reaction  time and in visuospatial reasoning, but not in the other tests. The best performance was noted at an intake of five to six cups of coffee or tea per day.

Although the results of studies on the effects of caffeine on alertness, vigilance and memory are sometimes contradictory in terms of whether caffeine produces beneficial effects or no effects, there is little indication that intake of caffeine (up to approxi- mately 250 mg in a single dose or over a few days) affects these processes in a negative manner (Smith 1998).

However, a single caffeine dose of 100 mg was shown to affect short-term memory adversely in one study (Terry and Phifer 1986).

Some studies have noted little or no change in mood after the consumption of single doses of caffeine of 32 mg (Lieberman et al. 1987), 100 mg (Svensson et al. 1980) or 200 mg (Swift and Tiplady 1988). Larger amounts of caffeine (200, 400 or 600 mg as a single dose) have been associated not only with slight increases on an anger/hostility scale, but also with reduced ratings for drowsiness and incoordination (Roache and Griffiths 1987). Caffeine has little effect in producing depression, even at the consumption of more than eight cups of coffee per day (James 1991f).

It is unclear why some studies have found effects on mood and others have not.

The consumption of caffeine by adults has been associated with an increase in anxiety in several studies. Many studies conducted on psychiatric in- patients, for example, have shown significantly in- creased anxiety levels in heavier users of caffeine (James 1991f); however, some of these studies did not control for alcohol and tobacco use, and patients may have been primed to report more symptoms. James et al. (1987) remedied these methodological problems in a survey of 173 psychiatric in-patients, reporting no association between the consumption of caffeine and anxiety.

In patients with generalized anxiety disorder, the administration of caffeine increased their already high anxiety level in a dose- related manner (Bruce et al. 1992). Note that the results of studies using psychiatric patients or patients with anxiety disorders may not be applicable to the general population (James and Crosbie 1987).

Other studies have shown no effects of caffeine (e.g. regular consumption of up to seven or more cups of coffee or tea per day) on anxiety in psychiatric patients, non- clinical subjects or patients with anxiety disorders (Lynn 1973, Hire 1978, Eaton and McLeod 1984, Mathew and Wilson 1990, James 1991f, Smith 1998).

The literature suggests that caffeine can produce anxiety or exacerbate anxiety in adults with pre- existing anxiety disorders; however, the doses associated with these effects are large (1–2 g  caffeine  day 1) and would likely be consumed only by a small segment of caffeine consumers. In addition, it has been suggested that people experiencing the anxio- genic effects of caffeine are likely to avoid the use of this substance (James 1991f); thus, the self-limiting nature of caffeine intake would reduce any potential that caffeine had to produce anxiety in adults.

Studies have shown that caffeine can increase the time taken to fall asleep (sleep latency) and reduce sleep duration,   especially   if   large   amounts   of caffeine (>3 mg kg 1 bw, >210 mg for a 70-kg person) are ingested close to the usual bedtime of the individual (Smith 1998). High consumers of caffeine are less likely to report sleep disturbances than individuals consuming caffeine more infrequently (Snyder and Sklar 1984, Zwyghuizen-Doorenbos et al. 1990), suggesting the development of tolerance to the effects of caffeine on this parameter. It is apparent that if caffeine ingestion (especially in the late evening) affects the sleep of the individual, a self-limiting reduction in caffeine intake will likely occur to avoid any effects on sleep.

In summary, the moderate consumption of caffeine in normal adults has not been associated with any major adverse effects on mood or performance, and most effects associated with higher consumption rates would be self-limiting.

However, in light of inconsis- tent results in the literature and individual differences in sensitivity to caffeine, some people (e.g. those with anxiety disorders) need to be aware of the possible adverse effects of caffeine and to limit their intake accordingly.

Tolerance, physical dependence, and withdrawal

The literature on the development of tolerance to the effects of caffeine during prolonged ingestion is sparse and inconsistent (James 1991e). Any tolerance that may be present is likely to be dependent on the biological or behavioural effect produced by caffeine and by the level and pattern of caffeine consumption.

Cessation of caffeine ingestion has been associated with a wide variety of mainly subjective effects, in particular headache (Rubin and Smith 1999) and fatigue, characterized by such symptoms as mental depression, weakness, lethargy, apathy, sleepiness and decreased alertness (Griffiths and Woodson 1988).

The general caffeine withdrawal pattern  appears  to be an onset from 12 to 24 h after cessation, a peak at 20–48 h, and a duration of about 1 week (Griffiths and Woodson 1988). The strength of the association between caffeine cessation and withdrawal is sup- ported by the fact that symptoms can be ameliorated by administration of caffeine tablets in a dose-depen- dent manner (Griffiths and Woodson 1988).

The intensity of the symptoms has been described as mild to extreme. The presence or absence of withdrawal symptoms is not always predictable, as some heavy users have ceased ingestion of caffeine with no appar- ent withdrawal (Griffiths and Woodson 1988).

Symptoms associated with caffeine withdrawal have been noted in studies involving the cessation of regular consumption of high ( =< 1250 mg day 1, Griffiths et al. 1986; =< 2548 mg day 1, Strain et al. 1994) and much lower doses (100 mg day 1, Griffiths et al. 1990; 235 mg day 1, Silverman  et  al.  1992; 290 mg day 1, Weber et al. 1993;  428 mg day 1, Bruce et al. 1991; four to six cups of coffee per day, van Dusseldorp and Katan 1990; five cups of coffee per day, Hughes et al. 1991).

While some studies have shown a dose-dependent increase in the effects of withdrawal  (increased  headaches  after  the stoppage of regular consumption of >700 mg caffeine day 1 compared with =< 700 mg day 1; Weber et al. 1993), others  have  shown  little  correlation  between  daily intake and withdrawal symptoms (in a range of regular intake of 231–2548 mg day 1; Strain et al. 1994).

In Strain et al. (1994), the most severe effects upon cessation were noted with the lowest consump- tion, while the individual with the highest regular consumption reported only moderate effects.

Dews et al. (1998) hypothesized that bias and priming of the subjects in caffeine withdrawal studies led to the exaggeration of the incidence and severity of symptoms of caffeine withdrawal.

They suggested that the prevalence and severity of withdrawal symp- toms have been exaggerated in the literature, as illustrated by the variability among published reports of both the symptoms associated with caffeine withdrawal and the incidence rates, and concluded that the true level of caffeine withdrawal is low and near background levels.

Also, there are reports of caffeine withdrawal continuing for long periods, which may  be the result of a return of performance and alertness to pre-caffeine conditions. Since caffeine has been shown to improve these parameters, the return to normalcy may be associated with reduced perfor- mance and alertness compared with  caffeine  use, and these effects may be attributed to a caffeine withdrawal syndrome or as a sign that physical dependence has been produced during caffeine consumption.

In a blinded study by Dews et al. (1999), subjects were given coffee and then subjected to continued caffeine intake, abrupt caffeine cessation or gradual caffeine cessation (from 100 to 0% over 7 days). Subjects in the gradual cessation group reported no adverse effects of caffeine cessation, while females (but not males) in the abrupt cessation group had adverse effects, as evidenced by reduced mood/attitude scores on no-caffeine days (reductions in scores were small).

This study showed that the blinding of subjects to caffeine cessation reduced the incidence of reported symptoms of caffeine withdrawal, as about half of the subjects reporting severe withdrawal symptoms in a prior telephone interview experienced no symptoms of withdrawal in the blinded study.

The literature thus supports the existence of caffeine withdrawal in some individuals, with variability in the severity of symptoms. When withdrawal occurs, it is short-lived and relatively mild in the majority of people affected.

Effects on children

Scientific studies have shown a variety of effects of caffeine consumption in children, although it is sur- prising that so few studies have specifically addressed effects in this population.

At low doses, an increased performance in attention tests has been noted in children. A double-blind and

placebo-controlled study was conducted in which 21 children (mean body weight 38.1 +- 12.5 kg; average age 10.6 +- 1.3 years) were administered a placebo, a low dose of caffeine (single dose of 2.5 mg kg 1 bw) or a high dose of caffeine (single dose of 5.0 mg kg 1 bw) (Bernstein et al. 1994).

The authors noted a statisti- cally significant, dose-dependent improvement in a performance test of attention after caffeine adminis- tration compared with the placebo group. A significant but non-dose-related improvement in a manual dexterity test was also noted.

In a double-blind placebo-controlled cross-over study (Elkins et al. 1981, Rapoport et al. 1981b), a group of 19 pre-adolescent boys were tested for a number of parameters after the ingestion of a placebo or a single caffeine dose of 3 or 10 mg kg 1 bw on three separate occasions (each separated by 48 h).

The children in the high-dose group showed a significant increase in motor activity compared with the control and low- dose groups, an increase in speech rate compared with the low-dose group, a significant reduction in reaction time in a vigilance test, and a reduced number of errors in a sustained attention measure test compared with the placebo group. Stratification of usual, pre- study caffeine use was not conducted for the subjects in this study.

Anxiety, measured both subjectively and objectively, has also been associated with the administration of low doses of caffeine in children in a number of studies. In the Bernstein et al. (1994) study described above, there was a trend (although it was statistically non-significant) towards a higher level of anxiety in one of the subsets of the Visual Analogue Scale for state anxiety (‘how I feel right now’) just after caffeine administration.

There was a statistically significant correlation between salivary caffeine concentration and the severity of the state anxiety as measured by the Visual Analogue Scale. It was noted in this study that the levels of salivary caffeine were significantly correlated with the dose of caffeine administered.

Other anxiety measurements conducted in this study (all self-reported, including other measurements of state and trait anxiety) showed no difference after caffeine administration. While this study randomized the order of testing, there was a lack of participant stratification based on regular, pre-study caffeine consumption.

Even so, the level of caffeine adminis- tered to children in the Bernstein et al. (1994) study is the lowest in the available literature, and this study should be considered along with the wider body of evidence.

Other reviewed studies showing manifestations of anxiety in children associated with caffeine were those by Rapoport et al. (1981a) (10 mg kg 1 bw day 1), Rapoport et al. (1981b) (3 and 10 mg kg 1 bw day 1) and Rapoport et al. (1984) (10 mg kg 1 bw day 1).

In all of these studies, effects on anxiety were noted at all doses tested. Other effects in these studies included being nervous, fidgety, jittery, and restless and experiencing hyperactivity and difficulty sleeping. Positive dose–responses were noted for skin conductance (a measure of anxiety) as well as for nervous/jittery behaviour in the children in the Rapoport et al. (1981b) study.

When subjects were stratified by pre- study caffeine intake (Rapoport et al. 1981a), differ- ences between low and high dose consumers (pre- study  intake  of  <50  and  => 300 mg  caffeine  day 1, respectively)  were  apparent. 

High  dose   consumers were more easily frustrated, with a greater feeling of nervousness on baseline tests, than the low consumer group, possibly pointing to caffeine withdrawal dur- ing this period of testing. In terms of reported side- effects, the low users could distinguish between the placebo and the caffeine treatment (according to a variety of self-reported side-effects), while the high users could not.

The high users given placebo and then caffeine experienced more side-effects during the initial placebo administration than they did when administered caffeine. The study by Rapoport et al. (1981a) appears to provide evidence of tolerance in the high regular consumers, and this group also appeared to show withdrawal in the baseline and placebo conditions.

In Rapoport et al. (1984), a number of differences were noted between high and low consumers in terms of behaviour. During the screening, baseline and initial pre-study caffeine-free periods, the high consumers reported significantly more symptoms of anxiety and were reported to be more ‘disobedient’ than the low consumers.

There appeared to be many differences between the groups when caffeine was administered for 2 weeks. Low consumers exhibited a significant increase in restlessness and fidgety behaviour, while the high-dose group showed a decrease in this behaviour.

Statistically significant differences between the groups were mood changes, excitability, inattentiveness, restlessness and crying (the direction of these changes between the two groups was not mentioned in the paper). In terms of side-effects during this period, the low consumers reported headache, stomach-ache and nausea. These effects were not noted in the high consumers. A feeling of faintness and of being flushed was signifi- cantly  increased  in  the  low  consumers  and significantly decreased in the high consumers.

Also, the low consumers had difficulty sleeping and a decreased appetite compared with the high consumer group. It was suggested by the authors that child consumers of high-caffeine diets differ inherently from those con- suming low-caffeine diets in certain ways, namely having lower autonomic arousal and being more impulsive, leading to the self-administration of caf- feine.

In this study the initial pre-study stratification of subjects into high and low consumers (18+- 18 and 641 +- 350 mg day 1, respectively) was based on a 24-h recall; however, based on a 7-day food diary for the pre-study baseline period, it was observed that there was a large overlap between the low and high con- sumer groups (95+- 84 and 290 +- 275 mg week 1  or  about 41.4 and 13.6 mg day 1, respectively).

The over- lap in pre-study caffeine intake may reduce the ability to evaluate the differential effects of caffeine on high and low consumers that were noted.

Other studies dealing with the effects of caffeine on children were those by Baer (1987), Hale et al. (1995) and Davis and Osorio (1998). The study by Baer (1987) used six 5-year-old children who were admi- nistered either a caffeine-free or a caffeinated soft drink each day for 2 weeks, resulting in a dose of 1.6–2.5 mg kg 1 bw day 1 when caffeine was adminis- tered.

Drink conditions were reversed at the end of the 2 weeks. Effects noted on behaviour (e.g. off-task behaviour, motor activity, continuous performance) were inconsistent and small. No testing for anxiety was conducted. Hale et al. (1995) examined the self- administration of caffeine in 18 adolescent children of both sexes (age 11–15) in a double-blind, placebo- controlled study. Soft drinks containing either caf- feine (33.3 mg/8 ounce serving) or a placebo were supplied to the participants.

The children  consumed  a particular drink one day (either caffeinated or placebo) followed by another drink (either the same as the previous day or different) the next day. Consumption of all drinks was ad libitum. Four children met the criteria for repeatable self-adminis- tration, preferring the caffeinated drink to the place- bo; however, only one child had a statistically significant self-administration.

In these four children, the average intake of caffeine was 169 mg day 1 com- pared with 62 mg day 1 in those where self-adminis- tration was not evident. No behavioural symptoms were consistently reported in any participant. When the results were analysed across all participants, it was noted that on caffeine-free beverage days, there was significantly more depression, drowsiness and fatigue.

No differences between the consumption of caffeinated or non-caffeinated drinks were observed in the children when a parent rating scale for anxiety, hyperactivity or impulsivity was employed. No infor- mation was provided in this study about the pre-study intake of caffeine. Davis and Osorio (1998) reported that caffeine intake can worsen and trigger the ap- pearance of tics in children, based on two children aged 11 and 13.

The authors concluded that con- sumption of caffeine can trigger the appearance of tics in susceptible children, although they made no indi- cation of how the determination of a ‘susceptible’ child could be made. It is possible that genetic factors play a role, since the two children in this study were related. It should be recognized that with only two children, this study is only suggestive of a problem; however, it is an area that deserves further research.

In a meta-analysis of nine studies (Stein et al. 1996), caffeine showed no significant deleterious acute effects on behaviour or cognition in children. The results of the meta-analysis with respect to anxiogenic effects are difficult to interpret, for several reasons. For example, tests of anxiety were grouped with a number of other tests to form an ‘internalizing’ category.

This may have diluted any effects of anxiety. In addition, the tests used to assess anxiety were not the same in each study, making comparisons between these stu- dies more difficult. Of the nine studies used for the meta-analysis, four dealt with normal children, while the remainder used children who had attention deficit hyperactivity disorder. Again, this makes the inter- comparison of studies difficult.

The cessation of caffeine intake in normally high consuming children ( => 300 mg day 1) or those admi- nistered larger amounts of caffeine (10 mg kg 1 bw day 1) over a period of weeks has resulted in the production of symptoms associated with caffeine withdrawal (Rapoport et al. 1981a).

Bernstein et al. (1998) studied the single-blinded withdrawal of caf- feine in 30 normal pre-pubertal children (mean age 10 years) having an average pre-study consumption of at least 20 mg caffeine day 1.

Children were adminis- tered 150 mg caffeine day 1 for 13 days followed by a non-caffeinated drink for 1 day, then resumed their normal diet. While on caffeine, the subjects responded significantly faster in the test of attention than in the withdrawal period and resumption to normal diet period.

During the withdrawal period, the response time was significantly increased compared with the pre-caffeine (baseline) period. This increased response time was still significantly elevated 1 week post- caffeine cessation. The authors suggested that the children had developed a physical  dependence  on  the caffeine and exhibited withdrawal effects upon removal of the caffeine.

Anxiety was observed to be higher during the baseline period in this study, with scores decreasing over time, possibly related to an increasing familiarity of the children with the testing procedure.

Caffeine has been tested for use in the treatment of hyperactivity/attention deficit disorder in children (James 1991e, Leviton 1992). A few early studies showed beneficial effects of caffeine intake at doses ranging from 175 to 600 mg day 1; in these studies, few adverse effects were noted, although some effect on sleep (dose-dependent insomnia) was noted in one study (100–400 mg caffeine day 1), and minor group increases in blood pressure and heart rate were noted in another (300 mg day 1).

Many other studies, how- ever, have shown no benefit of caffeine use in children with attention deficit disorder. Some studies, in fact, suggest that caffeine ingestion can lead to symptoms of hyperactivity in natural low consumers.

In a study in which the 7-day food diaries from 30 low- and 30 high-caffeine-consuming school children were ana- lysed, 30% of the high consumers met criteria for attention deficit disorder with hyperactivity, and the high consumers were perceived as being more restless than the low consumers (Rapoport et al. 1984). Problems with this study in terms of overlap between the low and high consumers’ pre-study intake of caffeine have been noted above.

The studies reviewed here and their sometimes conflicting results can be difficult to compare, since they employed either different endpoints or different ways to assess similar endpoints. In addition, most studies used a small number of subjects.

The problems associated with differing groups of caffeine consumers within the population of children and the potential differential susceptibility to caffeine of certain sub- populations need to be clarified. Another difficulty with some studies is the non-stratification of children based on their usual (pre-study) caffeine intake, since high consumers and low consumers may not always respond in the same manner to additional adminis- tered caffeine. In addition, no studies have been designed to test for potential chronic effects of caf- feine consumption by children.

In conclusion, it is unknown if long-term daily con- sumption of caffeine would produce effects similar   to those observed in the studies reviewed above. However, it is known that the human nervous system (including the brain) continues to develop and mature throughout childhood. It is possible that the pro- tracted development of the nervous system may ren- der children more sensitive to any adverse effects of caffeine.

Mutagenicity/genotoxicity

Caffeine not only induces mutations in bacteria in the absence of mammalian metabolic activation, but also can exhibit weak antimutagenic activity in some microorganisms (Legator and Zimmering 1979, Brusick et al. 1986, Rosenkranz and Ennever 1987, Pons and Muller 1990).

In eukaryotic organisms, including fungi and yeasts (Legator and Zimmering 1979, Osman and McCready 1998), higher plants (Gonzalez-Fernandez et al. 1985, Manandhar et al. 1996), rodent cell lines (Jenssen and Ramel 1980, Aeschbacher et al. 1986, Brusick et al. 1986, Haynes et al. 1996, Kiefer and Wiebel 1998), and human cell lines (Lachance 1982, Bernhard et al. 1996, Roldan- Reyes et al. 1997), caffeine inhibits cell cycle-depen- dent DNA repair induced by a variety of physical and chemical mutagens, leading to the potentiation of clastogenic effects (D’Ambrosio 1994, Puck et al. 1998, Harish et al. 2000, Jiang et al. 2000).

In chick embryo cells, DNA damage was induced (Mu¨ ller et al. 1996) at dose levels in the => 1 mm range, not considered toxicologically relevant (Tempel and von Zallinger 1997). Genotoxic activity in Drosophila was weakly positive or inconclusive for chromosomal effects, dominant lethals, the somatic mutation and recombination test, and chromatid aberrations (Legator and Zimmering 1979, Graf and Wurgler 1986, 1996), while X-ray damage was enhanced (De Marco and Cozzi 1980).

Only at high levels of caffeine were clastogenic effects reported in somatic cells of rodents (Jenssen and Ramel 1980, Aeschbacher et al. 1986, Haynes et al. 1996), while no specific locus mutations or chromosomal effects were induced in germ cells or embryonic cells (Legator and Zimmering 1979, Mailhes et al. 1996, Muller et al. 1996).

Antigenotoxic activity on somatic or germ cells ex- posed to a variety of physical and chemical mutagens, following ingestion of caffeinated or decaffeinated coffees was weak or negative (Legator and Zimmering 1979, Everson et al. 1988, Reidy et al. 1988, Chen et al. 1989, MacGregor 1990, Smith et al. 1990, Robbins et al. 1997, Vine et al. 1997, Abraham and Singh 1999).

Although evidence for the mutagenic potential of caffeine is conflicting (Lachance 1982, Grice 1987, Rosenkranz and Ennever 1987,  D’Ambrosio  1994), it appears to be unlikely that at normal, physiologi- cally relevant levels of consumption (i.e. at less than systemic toxicity ranges), caffeine would result in mutagenic effects in humans.

Carcinogenicity

The evidence from several oral oncogenicity/chronic toxicity studies in mice (Bauer et al. 1977, Macklin and Szot 1980, Stalder et al. 1990) and rats (Wurzner et al. 1977, Johansson 1981, Takayama and Kuwabara 1982, Mohr et al. 1984) indicate that caffeine is not a carcinogen, up to dose levels of 391 and 230 mg kg 1 bw day 1, respectively.

The most common clinical sign observed in these studies was a decrease in body weight, with no concomitant de- crease in food consumption.

Epidemiological studies on the carcinogenicity of caffeine as present in coffee have consistently shown that caffeine is not associated with cancer develop- ment at several tissue and organ sites. For example, caffeine consumption, from three or more cups of coffee  per  day  ( => 300 mg  caffeine   day  1)  was not associated with cancer development in the follow- ing sites: large bowel in 13 case-control studies (cited in IARC 1991a, Lee et al. 1993, Olsen and Kronborg 1993); stomach in six case-control studies (cited in IARC 1991a, Agudo et al. 1992); prostate in one case- control study (cited in IARC 1991a); liver in one case- control study (cited in IARC 1991a); lung in two cohort studies and one case-control study (cited in IARC 1991a); and vulva in one case-control study (Sturgeon et al. 1991).

Higher caffeine consumption, specifically drinking seven or more cups of coffee per day ( => 700 mg caffeine day 1) was not associated with breast cancer in 11  case-control studies  (cited  in Rohan et al. 1989, IARC 1991a, McLaughlin et al. 1992, Folsom et al. 1993, Smith et al. 1994, Tavani  et al. 1998).

On the other hand, caffeine intake, as measured by coffee consumption, was occasionally associated with cancer development at some sites. In the urinary bladder, four cohort studies showed no effect with doses of five or more cups of coffee per day ( => 500 mg caffeine day 1) (cited in IARC 1991a, Chyou et al. 1993, Stensvold and Jacobsen 1994). In 26 case-control studies, 17 studies showed no effect with doses of five or more cups of coffee per day. Nine studies were positive, and three of these studies showed a dose– response (cited in IARC 1991a, Vena et al. 1993, Donato et al. 1997).

Of these three studies, two showed a positive increase with any coffee consump- tion, and the third study was significant only when consumption was five or more cups of coffee per day. In the pancreas, out of nine cohort studies, eight showed no significant effect with doses of five or more cups of coffee per day ( =>500 mg caffeine day 1), while one study was positive for any coffee consumption (cited in IARC 1991a, Stensvold and Jacobsen 1994, Harnack et al. 1997). Of 24 case-control studies, 21 showed no effect on pancreas with doses of five or more cups per day.

In one of the three positive case-control studies, a significant effect was observed only when four cups of coffee per day were drunk (400 mg caffeine day 1).

In a second study, doses exceeding two cups of coffee per day (200 mg caffeine day 1) were associated with an increase. In the third positive study, any level of coffee drinking resulted in an increased risk. Of the three positive studies, two studies showed a dose-related response. When smok- ing was taken into consideration, the positive responses in these studies were weakened (cited in IARC 1991a, Bueno de Mesquita et al. 1992, Lyon  et al. 1992, Partanen et al. 1995, Nishi et al. 1996).

In the ovary, two case-control studies showed a sig- nificant increase in cancer incidence with doses of more than one cup of coffee per day, while five case-control studies showed no effect with doses of five    or more cups per day (cited in IARC 1991b, Polychronopoulou et al. 1993).

In the skin, a case- control study showed that the risk of basal cell carcinoma was increased with doses of more than two-and-a-half cups of coffee per day (>250 mg caffeine day 1) (Sahl et al. 1995).

Overall, the evidence indicates that caffeine, as present in coffee, is not a chemical that causes breast or bowel cancer. Results on the association between caffeine and the development  of  urinary  bladder  and  pancreatic  cancer  are  inconsistent  and  the  data are not conclusive.

At other sites (e.g. ovary, stomach, liver) the data are insufficient to conclude that caffeine consumption is related to carcinogenesis. Based on the studies reviewed in this report, caffeine is not likely to be a human carcinogen at a dose less than five cups of coffee per day (<500 mg caffeine day 1).

Reproductive and developmental effects

There is evidence that many women spontaneously reduce their caffeine intake during pregnancy, some apparently developing a temporary ‘loss of taste’ for the substance. Nevertheless, caffeine consumption in this group can remain relatively high.

About 98% of women of reproductive age regularly consume caf- feine in the form of caffeinated beverages or in caffeine-containing medications, while 72% of them continue to do so during pregnancy (James 1991g). Epidemiological investigations reviewed for this paper showed that a majority of women consumed caffeine  during  pregnancy  in  a  range  of   100–300 mg day 1 (Fenster et al. 1991a, Fortier et al.  1993, Mills et al. 1993, Dominguez-Rojas et  al.  1994, Rondo et al. 1996). A small proportion of pregnant women in the population may ingest a much greater amount, => 400 mg  caffeine  day  1  (Kurppa et al. 1983, Toubas et al. 1986, Olsen et al. 1991, Armstrong et al. 1992, McDonald et al. 1992a).

During the past 20 years, a great deal of evidence has accumulated concerning the effects of caffeine consumption on reproduction and pre- and postnatal development. Although the results from studies re- viewed for this publication have not been entirely consistent, the bulk of evidence suggests that caffeine intake at dose levels of => 300 mg day 1 may have adverse effects on some reproductive/developmental parameters when exposure takes place during certain periods (Dlugosz and Bracken 1992).

Christian and Brent (2001) reviewed published animal and human epidemiological studies investigating the association between caffeine ingestion and adverse reproductive/developmental effects and concluded that pre-pregnant or pregnant women who do not smoke or drink alcohol and who consume moderate amounts of caffeine ( =< 5–6 mg kg 1 bw day 1 spread throughout the day) will be unlikely to develop reproductive problems.

The effects of caffeine on the outcome of pregnancy appear biologically plausible. Published data suggest that the human foetus and neonate may be exposed to substantial amounts of caffeine or its metabolites, as caffeine ingested by the mother is rapidly absorbed from the gastrointestinal tract, readily crosses the placenta and is distributed to all foetal tissues, including the central nervous system. Caffeine is also excreted in mother’s milk. In addition, exposure of the foetus and newborn to caffeine is enhanced due to the half-life of caffeine being markedly increased in the foetus (the enzymes involved in the oxidation of methylated xanthines are absent in the foetus), new- born infant and pregnant woman in comparison with non-pregnant adults and older children (James and Paull 1985, James 1991g, Dlugosz and Bracken 1992).

Effects on conception and female fertility

Caffeine consumption is one of many factors impli- cated in the reduction of fecundity, or the capacity    to reproduce. There are several plausible biological mechanisms by which caffeine could delay concep- tion. Caffeine consumption has been associated with alteration of hormone levels (e.g. oestradiol), with tubal disease or endometriosis, with altered tubal transport time, and with reduced viability of the fertilized ovum (Alderete et al. 1995).

Caffeine metabolism varies during the menstrual cycle, with re- duced clearance during the luteal phase, resulting in greater accumulation during the period of implanta- tion and early embryonic development. Caffeine consumption may lead to pregnancy loss, which might result in prolongation of the waiting time required to achieve a clinically recognized pregnancy (Stanton and Gray 1995).

Thirteen epidemiological studies (retrospective and prospective data collection) investigating the relationship between coffee/caffeine consumption and time to conception (fecundability) present conflicting results. Five studies reported no delay in conception in women who consumed up to => 700 mg caffeine day 1 before pregnancy.

In a multicentre study conducted in the USA and Canada, caffeine consumption was not associated with decreased fertility in a group of 2817 women whose caffeine consumption from all sources ranged from 100 to => 240 mg day 1 (Joesoef et al. 1990).

Results of a study by Olsen (1991) showed no association between subfecundity and  consumption of coffee or tea at any dose level (none to eight cups per day) among non-smoking women. Florack et al. (1994) showed that participants (male and female partners) with caffeine intake of 400–700 mg day 1 had a higher fecundability than those with a lower intake      level;      only      heavy      caffeine     intake (>700 mg day 1) among partners was negatively related to fecundability when compared with the lowest intake level (<300 mg day 1).

Caan et al. (1998) found no association between caffeine intake at a mean dose level of about 90 mg day  1 and a reduction in fertility of women trying to conceive for at least 3 months. Alderete et al. (1995) examined the independent and combined effects of smoking and coffee consumption on time to conception in 1341 primigravid women and found that women who consumed more than  three cups of coffee per day (>300 mg caffeine day 1) but did not smoke showed no decrease in fertility when compared with non-coffee-drinking women (adjusted odds ratio [OR]=1.0–1.2) who did not smoke.

Results from two studies showed a significant decrease in monthly probability of pregnancy among women who consumed the equivalent of three or more cups of coffee per day (=>300mg caffeine day -1).

In a retrospective study of 2465 women, Stanton and Gray (1995) found that the adjusted OR of delayed conception for >1 year was not increased among women who consumed =<300mg caffeine day -1, but the OR was 2.65 (95% confidence interval [CI]=1.38–5.07) among non-smokers who consumed => 301mg caffeine day -1.

(In this study, no effect of high caffeine consumption was observed among women who smoked.) In a study of 430 Danish couples planning their first pregnancy, Jensen et al. (1998) found that compared with nonsmoking couples with caffeine intake <300mg day -1, non-smoking females and males who consumed 300– 700mg caffeine day -1 had fecundability ORs of 0.88 and 0.87 (95% CI=0.60–1.31 and 0.62–1.22), respectively, whereas females and males with a higher caffeine intake (>700mg day – 1) had ORs of 0.63 and 0.56 (95% CI = 0.25–1.60 and 0.31–0.89), respectively.

No dose–response relationship was found among smokers. Smoking women whose only source of caffeine was coffee (>300mg day -1) had a reduced fecundability OR=0.34 (95% CI = 0.12–0.98), and non-smoking women with a caffeine intake of 300mg day – 1 from other sources had a low, but non-significant, OR =0.43 (95% CI=0.16–1.13) compared with non-smoking women consuming <300mg caffeine day -1.

The authors concluded that the results indicated a possible association between
male and female caffeine intake and decreased fecundability only among non-smokers.

Another four studies reported delayed conception in women who consumed => 400, => 500, or => 800 mg caffeine day 1. Data collected by Christianson et al. (1989) showed a dose-related effect of coffee consumption on reported difficulties in becoming pregnant.

Women who were heavy coffee drinkers before pregnancy (four to seven or more cups of coffee per day) experienced almost double the time in becoming pregnant compared with women who consumed none or one cup of coffee per day.

Williams et al. (1990) examined data from a large cross-sectional study on 3010 postpartum women, finding that times to con- ception for women who consumed three, two, one or no cups of coffee per day were similar (ranging from 4.8 to 5.0 months), whereas time to conception was longer (6.6 months) for the 129 women who con- sumed four or more cups of coffee per day (approxi- mately 400 mg caffeine day 1).

In a retrospective study by Bolumar et al. (1997), a significantly increased OR (1.45, 95% CI=1.03–2.04) for subfecundity in the first pregnancy was observed among women consuming >500 mg caffeine day -1. Women in this highest level of consumption had an increase of 11% in the time leading to the first pregnancy. (The effect of drinking >500mg caffeine day -1 was relatively
stronger in smokers [OR = 1.56, 95% CI=0.92–2.63] than in non-smokers [OR=1.38,95% CI=0.85–2.23].)

In Olsen (1991), a statistically significant association was observed (OR=1.35, 95% CI=1.02–1.48) for a delay of =>1 year in women who smoked and also consumed at least eight cups of coffee per day (or an equivalent amount of caffeine from 16 cups of tea).

Three studies found modest positive associations with delayed conception from maternal consumption of more than one caffeinated beverage per day. A prospective study by Wilcox et al. (1988) showed that women who consumed more than one cup of coffee per day (126 mg caffeine day -1) were half as likely to conceive during a given menstrual cycle.

In a crosssectional study, Hatch and Bracken (1993) found that intake of caffeine from coffee, tea and caffeinated soft drinks was associated with an increased risk of a delay of conception of =>1 year. Compared with no caffeine use, consumption of 1–150mg caffeine day -1
resulted in an OR for delayed conception of 1.39 (95% CI=0.90–2.13), consumption of 151–300mg day -1 was associated with an OR=1.88 (95% CI=1.13–3.11), and consumption of 300mg day -1 resulted in an OR=2.24 (95% CI=1.06–4.73).

Women who reported drinking 300mg caffeine day -1 had a 27% lower chance of conceiving for each cycle, and those who reported drinking <300mg day -1 had a 10% reduction in conception rates per cycle compared with women who consumed no caffeine. Hakim et al. (1998) examined the effects of caffeine consumption on conception in a prospective study of 124 women, finding that the consumption of the equivalent of more than one cup of coffee per day among the sample of women who neither smoked nor drank alcohol was associated with a decreased risk of conception (18.0%, adjusted OR =0.56, 95% CI =0.23–1.33),
which did not reach statistical significance.

In one of the above-described studies, delayed con- ception  was  observed  among  non-smoking  women who consumed >300 mg caffeine day 1, but not among   women   who   smoked   (Stanton   and  Gray 1995). Also, Jensen et al. (1998) found no dose response relationship among smokers at caffeine doses of up to => 700 mg day 1, whereas non-smoking males and females who consumed 300–700 mg day 1 exhibited decreased fecundability compared with non-smoking    couples    with    caffeine    intake    of <300 mg day 1.

However, in Olsen (1991), no asso- ciation  was  found  among  non-smokers  at  any dose level of caffeine, just for women who smoked and also consumed at least eight cups of coffee per day. Bolumar et al. (1997) also found that the effect of drinking >500 mg caffeine day 1 was relatively stron- ger in smokers than in non-smokers.

An interaction between caffeine and smoking is biologically plaus- ible. Reports in the literature have shown that cigar- ette smoking significantly increases the rate of caffeine metabolism (see ‘Pharmacokinetics’). The enhanced caffeine metabolism in smokers also accelerates caf- feine clearance and, as a result, reduces the duration and magnitude of the exposure.

Most epidemiological studies reviewed here were affected by methodological issues, including inade- quate measurement of caffeine intake, failure to dis- tinguish among different types of preparation and different strengths of coffee, inadequate control for possible confounding effects, recall bias in retrospective studies, lack of data on frequency of unprotected intercourse, and, in some studies, inadequate sample size.

Despite these limitations, epidemiological studies are an important source of information on potential adverse effects of caffeine on fertility (delayed con- ception) in humans.

The evaluated epidemiological studies generally indi- cate that consumption of caffeine at dose levels of >300 mg day 1 may reduce fecundability in fertile women.

Effects on sperm and male fertility

Although ingested caffeine is capable of crossing the blood–testis barrier, caffeine consumption as a factor that could alter male reproductive function has not  been investigated extensively. Data from in vitro studies suggest that caffeine has variable, dose-related effects on human sperm motility, number and structure (Dlugosz and Bracken 1992).

It has been reported that women undergoing artificial insemination were twice as likely to become pregnant if their husbands’ semen had been treated with caffeine than if it had not.

Scanning electron microscopic examin- ation of fresh semen showed no morphological changes caused by in vitro treatment with caffeine (IARC 1991b, Dlugosz and Bracken 1992).

In an investigation of semen quality and its associa- tion with coffee drinking, cigarette smoking and alcohol consumption in 445 men attending an inferti- lity clinic, coffee drinking was correlated with in- creases in sperm density  and  percentage  of abnormal forms, but not in a dose-dependent man- ner. Men who drank one to two cups of coffee per day had increased sperm motility and density compared with subjects who drank no coffee. However, men who drank more than two cups per day had decreased sperm motility and density.

The combination of drinking more than four cups of coffee per  day  (>400 mg caffeine day 1) and smoking >20 cigarettes

per  day  diminished  spermatozoan  motility  and  increased the percentage of dead spermatozoa. No alteration in the fertility of individuals who consumed these substances was observed (Marshburn et al. 1989, IARC 1991b, Dlugosz and Bracken 1992).

Jensen et al. (1998) found no association between caffeine intake and semen quality in men exposed to caffeine for an extended period at dose levels as high as => 700 mg day 1.

Based on the limited data, it is concluded that caffeine consumption at dose levels of >400 mg day 1 may decrease sperm motility and/or increase the percentage of dead spermatozoa (only in heavy smokers), but will be unlikely to adversely affect male fertility in general.

Spontaneous abortion (miscarriage)

The influence of caffeine on the risk of spontaneous abortion in humans is difficult to assess. A number of studies have been conducted that show either a positive effect or a lack of effect of caffeine on this pregnancy outcome. Shortcomings in the literature include small sample size and inadequate adjustment for potential confounders.

A major potential confounder is the presence of nausea in the first trimester of pregnancy, as a lack of nausea early in pregnancy has been associated with a significantly increased risk of miscarriage (Stein and Susser 1991).

Nausea in pregnancy may cause a reduction in the consumption of coffee/caffeine, while a lack of nausea may lead to continued ingestion. This may result in an erroneous association of caffeine intake with increased risk of spontaneous abortion.

Another drawback is the general lack of accurate measurement of actual caffeine consumption by the participants in the epidemiological studies. Stavric et al. (1988), for example, found a marked variation in caffeine content of coffee and tea depending on the method of preparation and brand, and errors also arise from differences in the size of the serving ‘cup’ used by different participants.

Another serious limitation is the potential for poor identification of foetal loss due to enrolment of women later in the pregnancy or only those who presented to hospitals, as many early foetal losses go unnoticed by women.

Studies measuring human chorionic gonadotrophin levels, such as those of Wilcox et al. (1990), Mills et al. (1993) and Hakim et al. (1993), should reduce any bias in this factor. In addition, the majority of the studies showing positive associations between caffeine and spontaneous abortion are retrospective in nature, and at least one study depended on information recalled after several pregnancies (Armstrong et al. 1992).

Most of the studies have shown no association be-  tween a caffeine intake of <300 mg day 1 and an increased  risk  of  spontaneous  abortion  (Watkinson

and Fried 1985, Wilcox et al. 1990, Armstrong et al. 1992, Mills et al. 1993, Dlugosz et al. 1996, Wen et al. 2001). In the one study that accurately assessed caffeine intake (the prospective study by Watkinson and Fried 1985), 284 mothers were interviewed about their caffeine intake from coffee, tea, caffeinated soft drinks, chocolate bars, chocolate drinks and caffeine- containing medicines 3 years before pregnancy, during each trimester of pregnancy and the year after pregnancy. Caffeine consumption was measured and categorized into <100, 100–300 and >300 mg day  1.

There was no association between caffeine consumption and risk of miscarriage. In this study, there was a long period for which the women had to recall their caffeine consumption, so all recalled intakes may not have been accurate. Another study that found no association between caffeine consumption  at  levels of  =>  300 mg day  1  and  an  increase  in spontaneous abortion was the prospective study by Mills et al. (1993).

The meta-analysis conducted by Fernandes et al. (1998), using data from six original epidemiological studies (including 42988 pregnancies), showed a positive association (small but statistically significant) of spontaneous abortion with the consumption of 150mg caffeine day -1 (OR=1.36, 95% CI= 1.29– 1.45).

No other more definitive consumption categories were used in this study, and adjusting for confounders was not possible. The authors described the increased risk as small and noted that ‘a possible contribution to these results of maternal age, smoking, ethanol use or other confounders could not be excluded’.

Srisuphan and Bracken (1986) conducted a prospective cohort study with 3135 pregnant women whose caffeine consumption was estimated from their reported consumption of coffee, tea, caffeinated soft drinks and caffeine-containing drugs.

In terms of a crude association, the rate of spontaneous abortion was 1.8% for those who did not use caffeine (<1mg day -1), 1.8% for the light users (1– 150mg day -1) and 3.1% for the moderate/heavy users (=>151mg day -1).

When exposure was divided into 50-mg increments, there was a ‘marked increase’ in the relative risk for spontaneous abortion at use levels of >150mg day -1, but no dose–response was noted, as no further risk was associated with exposures 200mg caffeine day -1.

This study also pointed out that coffee consumption rather than caffeine consumption per se may have contributed to the risk of spontaneous abortion, as those who had a caffeine consumption from coffee alone had an increased
crude relative risk compared with those consuming tea or caffeinated soft drinks alone, although the differences were not statistically significant.

In this study, there was no more definitive categorization of intake >150mg day – 1. Al-Ansary and Babay (1994) conducted a retrospective case-control study with 226 women in Saudi Arabia and found an increased risk of miscarriage with the consumption of >150mg caffeine day – 1 (OR=1.0 [referent] and OR=1.9 [95% CI=1.2– 3.0] for consumption of 1–150 and >150mg day -1,respectively).

No subclassification of intake 150mg day -1 was conducted in this study, and it appears that no confounders were taken into consideration in the analysis. Only cases that had presented to a hospital were included, which may not give a complete picture of all possible miscarriages.

The retrospective case-control study by Infante-Rivard et al. (1993) is one of the better papers of those showing an association between lower levels of caffeine consumption and the risk of spontaneous abortion. In total, there were 331 cases and 993
controls.

The investigators found significant increases in OR for the risk of foetal loss in high consumers of caffeine when it was ingested before and during pregnancy (>321mg caffeine day-1 before pregnancy, OR=1.85, 95% CI=1.18–2.89; 163–321 and 321mg caffeine day -1 during pregnancy, OR=1.95, 95% CI=1.29–2.93, and OR =2.62, 95% CI=1.38–5.01, respectively).

For caffeine consumption before pregnancy, the OR increased by a factor of 1.10 for each 100mg caffeine ingested per day. For consumption during pregnancy, the OR increased by a factor of 1.22 for each 100mg ingested per day. The conclusion was that the incidence of spontaneous abortions was strongly associated with caffeine intake during pregnancy and moderately associated with caffeine use before pregnancy.

The majority of papers that showed an increased risk of spontaneous abortion with caffeine consumption showed associations at levels of =>300mg caffeine day -1. In a prospective cohort study by Dlugosz et al.(1996), for example, only the highest use of coffee and tea (three or more cups per day, about =>300mg caffeine day -1) was associated with an increased risk of spontaneous abortions (OR=2.63, 95% CI=1.29–5.34, for coffee; OR=2.33, 95% CI=0.92–5.85, for tea).

Armstrong et al. (1992), in a retrospective study of 35 848 pregnancies in Quebec, Canada, found the percentage of subjects with spontaneous abortions to be 20.4, 21.3, 24.1, 28.1 and 30.9% for persons consuming none, one to two, three to four, five to nine and 10 or more cups per day, respectively.

The ORs in these consumption categories were 1.00 (referent), 0.98 (95% CI=0.93–1.04), 1.02 (0.94–1.12), 1.17 (1.03–1.32) and 1.19 (0.97–1.45), respectively.

In this paper, the time lag between the actual abortion and the interview may have introduced errors in recall about the amount of coffee consumed in previous pregnancies. Subjects in this paper were questioned about the incidence of spontaneous abortion and caffeine intake in all previous pregnancies.

Wen et al. (2001) studied the association between caffeine consumption and nausea and the risk of spontaneous abortion. The categories of caffeine consumption (based on periodic food frequency questionnaires) were: <20, 20–99, 100–299 and =>300mg day -1.

Caffeine consumption was calculated for the periods before pregnancy, in the first trimester of pregnancy, and up to the date of any spontaneous abortion if it occurred before the end of the first trimester. The presence and duration of nausea were monitored. Potential confounders were analysed, including demographic factors, smoking and the con- sumption of alcohol. Parity and body mass index  were also considered. None of these parameters caused any important confounding and,  therefore,  the data were left unadjusted for these factors.

Overall, 7.2 versus 29.6% of the women who experienced any nausea or no nausea, respectively, had spontaneous abortions. In this study, no  increased risk of spontaneous abortion was noted with any level of pre-pregnancy intake of caffeine. The data showed that the consumption of caffeine did not increase the risk of spontaneous abortion in women who were already at risk due to a lack of nausea or a reduced frequency/duration of nausea.

However, in those women who had nausea in their first trimester and who were consequently at a reduced risk of spontaneous abortion, increased caffeine consumption during the first trimester was associated with abortion.

The risk ratios and 95% CIs were: <20mg caffeine day -1, 1.0 (reference category); 20–99mg day -1, 1.8(0.8–3.9); 100–299mg day -1, 2.4 (0.9–6.2); and =>300mg day -1, 5.4 (2.0–14.6). The risk of spontaneous abortion was elevated significantly with a consumption of caffeine =>300mg day-1.

Klebanoff et al. (1999), using actual serum measure-ments of paraxanthine, a major caffeine metabolite, showed an increased risk of spontaneous abortion at an estimated 600–1100 mg caffeine day 1.

In this retrospective study of 591 women who had sponta- neous abortions and 2558 matched controls, women with spontaneous abortions had significantly higher serum paraxanthine levels than the controls (752 and 583 ng ml 1 in women having spontaneous abortions and controls, respectively).

The increased risk of spontaneous abortions (OR=1.9, 95% CI=1.2–2.8) was noted only in those women with serum paraxanthine concentrations >1845 ng ml -1.

The authors concluded that the daily intake of caffeine needed to

reach 1845 ng paraxanthine/ml serum in a 60-kg woman would be about 600 mg for  those  who  do not smoke and 1100 mg in those who smoke. This would correlate with about six and 11 cups of coffee per day, respectively.

Some studies have revealed the possibility that con- stituents in coffee or tea other than caffeine may be

related to an increased risk of spontaneous abortion  in women (Watkinson and Fried 1985, Srisuphan and Bracken 1986, Dlugosz et al. 1996). The one study that accurately measured caffeine consumption (Watkinson and Fried 1985) found no association between caffeine intake and spontaneous  abortion, but did find a statistically significant larger proportion of coffee and tea drinkers in the group of women who had spontaneous abortions. Dlugosz et al. (1996) found that caffeinated soft drink use (up to three or more cans per day) did not increase the risk of spontaneous abortions.

Tea and coffee (at consump- tion of up to three or more cups of either drink per day) produced similar risks, despite these products having differing caffeine contents.

Although much epidemiological work has been conducted, additional prospective studies that measure actual caffeine intake in the participants and that adjust for potential confounders such as nausea and vomiting during pregnancy would be beneficial.

In the absence of these data, however, there appear to be reasonable grounds for limiting the consumption of caffeine to <300 mg day 1 in women who are, or who are planning to become, pregnant.

Foetal growth

The potential adverse impact of caffeine consumption during pregnancy on foetal growth has been a con- cern for many years. Caffeine increases the levels of cAMP through inhibition of phosphodiesterases, and the rise in cAMP might interfere with foetal cell growth and development (Karen 2000). 

Caffeine  may also block specific adenosine receptors. As adenosine is involved in maintaining the balance between the availability and the use of tissue oxygen, blockage of its receptors could increase the susceptibility of the cell to hypoxia.

Consumption of two cups of coffee has been reported to increase maternal epinephrine concentration and decrease intervillous placental blood flow (Fortier et al. 1993). As smoking is closely associated with caffeine consumption, it is important to stress that caffeine and smoking impose similar adverse physiological effects on foetal development (Fortier et al. 1993).

Results from epidemiological studies investigating the association between caffeine consumption and foetal growth have been conflicting. Of 18 original epidemiological studies, three indicate an association be tween either low birth weight (body weight <2500 g at birth) or intrauterine growth retardation (defined as birth weight <10th percentile of the sex-specific and gestation age-specific distribution of birth weight) and caffeine consumption <300 mg day 1.

In a population-based  study  by  Fortier  et  al.  (1993),  caffeine intake by 7025 women living in the Quebec City, Canada, area was not related to  low  birth  weight  but was associated with an increased risk of intrauterine growth retardation.

For women whose average daily caffeine consumption was 0–10, 11–150, 151–300 or >300 mg, the adjusted ORs for deliver- ing a newborn with growth retardation were 1.00,1.28 (95% CI 1.04–1.59), 1.42 (1.07–1.87) and 1.57 (1.05–2.33), respectively.

In a Brazilian unmatched case-control study by Rondo et al. (1996), results showed that the proportion of mothers who delivered babies with intrauterine growth retardation increased as the average consumption of coffee increased during pregnancy.
Compared with mothers whose babies’ growth was appropriate for gestation age, the ORs of mothers with babies with intrauterine growth retardation were 1.55 (95% CI =0.99–2.44), 2.25 (1.34–3.78) and 2.07 (1.14–3.78) for caffeine consumption levels of approximately < 140, 141–280 and => 281 mg caffeine day⁻¹, respectively, following adjustment for confounders such as cigarette smoking, alcohol intake and per capita income. Vlajinac et al. (1997), in an investigation of the effect of caffeine consumption during the third trimester on birth weight, found that birth weight decreased as caffeine consumption increased at levels ranging from 71 to 5 140 mg day⁻¹ in non-smokers.

Five studies reported an increased risk for foetal growth retardation in infants whose mothers were exposed to caffeine at dose levels of => 300 mg day⁻¹ during pregnancy after adjustment for potential confounders, including cigarette smoking and alcohol consumption (especially binge drinking). In the pro- spective study by Watkinson and Fried (1985) in which data were collected on maternal use of tea, coffee, caffeinated soft drinks, chocolate bars, choco- late drinks and caffeinated medication, the most marked effects associated with heavy caffeine use (>300 mg day⁻¹) were reduced birth weight and small head circumference; the associations were still significant after adjustment for maternal nicotine use.

The mean weight of babies born to 12 heavy users was 3158 compared with 3537 g for the remaining sample. The results suggest that daily caffeine intake of => 300 mg can interfere with normal foetal growth.

In a prospective study investigating the effects of caffeine consumption on intrauterine growth retardation, Martin and Bracken (1987) found that low birth weight was most common among offspring of women consuming >300 mg caffeine day⁻¹, the rate being 7.3% compared with the unexposed group rate of 4.1%.

Heavy caffeine intake (>300 mg day⁻¹) was associated with a 120-g reduction in birth weight compared with the untreated group.

Moderate use of caffeine (151–300 mg day⁻¹) was also associated with a decrease in birth weight, but to a lesser extent. When a comparison was made with women who had no caffeine exposure, the relative risks (RR) of low birth weight after adjustment for confounding factors (maternal age, ethnicity, education, previous spontaneous abortions, previous stillbirth, weight gain, body mass index, smoking and alcohol intake) were 1.4 (95% CI = 0.70–3.00) for 1–150 mg caffeine day⁻¹, 2.3 (1.1–5.2) for 151–300 mg, and 4.6 (2.0–10.5) for 300 mg. Beaulac-Baillargeon and Desrosiers (1987) found that birth weight was significantly less for women who consumed > 300 mg caffeine day⁻¹ and who smoked 15 or more cigarettes per day.

In a case-control study by Caan and Goldhaber (1989), the data showed no increased risk of low birth weight with light to moderate consumption of caffeine (<300 mg day⁻¹) (adjusted OR 0.90, 95% CI 0.4–1.92) but a small but measurable increased risk with heavy consumption of caffeine (>300 mg day⁻¹) (adjusted OR 2.94, 95% CI 0.89–9.65). One limitation of this study was its small sample size (131 cases, 136 controls). Fenster et al. (1991b) found that heavy caffeine consumption of 300 mg day⁻¹ significantly increased the risk for foetal growth retardation.

The mean birth weights for no, light (1–150 mg day⁻¹), moderate (151– 300 mg day⁻¹) and heavy (>300 mg day⁻¹) caffeine use were 3327, 3311, 3288 and 3170 g (reduction of 0, 0.5, 1.2 and 4.7%), respectively. Adjusted ORs for low birth weight for women consuming 1–150, 150–300 and 300 mg caffeine day⁻¹ were 0.78 (95% CI 0.45–1.35), 1.07 (0.51–2.21) and 2.05 (0.86– 4.88), respectively.
Three studies reported a reduction in birth weight for infants born to mothers who consumed caffeine dur- ing gestation at 400, 500 or => 800 mg caffeine day⁻¹. Olsen et al. (1991), in a study of 11 858 pregnant women in Denmark, found that maternal coffee consumption of four or more cups per day (400 mg caffeine day⁻¹) was associated with a moderate de- crease in birth
weight.

The adjusted OR for women consuming 400–700 mg caffeine day⁻¹ was 1.4 (95% CI = 1.10–1.70); for those consuming => 800 mg day⁻¹,the OR was 1.2 (0.90–1.80).

No dose–response rela- tionship was observed. One explanation for the re- sults might be that individuals who drink many cups of coffee may tend to drink weaker coffee, and therefore the caffeine intake may have been overestimated in the group drinking more coffee. In this study, the women assigned to the control group consumed 0– 300 mg caffeine day 1. McDonald et al. (1992a), in a study of 40 455 pregnancies in Montreal, Canada, found that coffee consumption at levels of 10 or more cups per day was associated with low birth weights and that consumption at levels of five to nine cups per day was associated with lower birth weight for gestational age, after adjusting for such confounders as maternal age, smoking and alcohol consumption.

Adjusted ORs for low birth weight at one to two, three to four, five to nine and 10 or more cups per day were 1.05 (95% CI 0.95–1.16), 1.08 (0.93–1.25), 1.13 (0.92–1.39) and 1.43 (1.02–2.02), respectively. For low birth weight for gestational age, the ORs at one to two, three to four, five to nine and 10 or more cups per day were 1.05 (95% CI  0.94–1.16), 1.15 (0.99– 1.34),  1.34  (1.10–1.65)  and  1.39  (0.97–1.98),  respectively, when compared with the controls (no coffee consumption).

Although Larroque et al. (1993) found no clear relation between caffeine consumption and birth weight in different groups of maternal tobacco use,  there  was  a  decreasing  trend  in  non-smokers;women who drank >800 mg caffeine day 1 had in- fants weighing 187 g less than the infants of those who drank =< 400 mg day 1, and this difference was at the limit of significance. In this study, non-users and users of <400 mg caffeine day 1 were combined and used as the control group.

Seven studies reported no association of caffeine consumption with birth weight or foetal growth retardation at levels of 300 to =>400mg day -1 during pregnancy. In a study of 12 205 women in the Boston area in the USA, Linn et al. (1982) found no relation between low birth weight and coffee consumption of up to four cups per day after controlling for confounders, including smoking and alcohol intake.

The adjusted OR among heavy coffee drinkers (four or more cups per day) was 1.19 (95% CI=0.86–1.65).
These negative results suggest that coffee consumption had a minimal effect, if any, on birth weight under the conditions of this study. Brooke et al. (1989) found no significant effects of caffeine consumption on birth weight in 1513 women in England
after controlling for smoking with caffeine intakes of 0, 1–200, 201–400 and =>401mg day -1. Barr and
Streissguth (1991) reported no undesirable changes in birth weight, length or head circumference for infants born to mothers exposed to caffeine at doses up to 750mg day -1 during the entire pregnancy.
Godel et al. (1992) found no association between caffeine ingestion (>300mg day -1) and birth weight, length or head circumference in the babies of 162 women in northern Canada when the data were adjusted for smoking and alcohol intake. Mills et al. (1993), in a prospective study of 423 women in the USA, found that moderate caffeine consumption (=<300 mg day-1) was not associated with a reduction in early foetal growth.

Although heavy caffeine consumption (>300mg day -1) appeared to have a negative effect on intrauterine growth and head circumference, the negative effect was no longer significant after adjusting for other risk factors, notably smoking and maternal age. In a prospective study by Shu et al. (1995), caffeine consumption at dose levels up to 300 mg day -1 (three cups of coffee per day) showed no relation to foetal growth.

Although heavy caffeine consumption (=>300mg day -1) in the first or second trimester was related to a reduction of crude
mean birth weight (93 g for the first trimester, 141 g for the second trimester), the study reported no decrease in foetal growth in any trimester when the data were adjusted for parity, pre-pregnancy weight, income, smoking and nausea. A matched case-control study by Santos et al. (1998) found no association between caffeine consumption at an average dose level of approximately 150mg day -1 and increased risk of low birth weight or intrauterine growth retardation.
The interaction of caffeine consumption and smoking and their association with low birth weight were also reported. Several studies have found a marked positive correlation between smoking and caffeine intake, including Godel et al. (1992), Fortier et al. (1993), and Vlajinac et al. (1997).

Beaulac-Baillargeon and Desrosiers (1987) found that birth weight was not statistically different with a caffeine consumption of 300mg day -1 for non-smokers and women who smoked one to 14 cigarettes per day, but the birth weight of babies of women who consumed =>300mg caffeine day -1 and smoked 15 or more cigarettes per day was significantly lighter (206 g less) than that of babies whose mothers consumed less caffeine.
Contradictory results were found by Vlajinac et al.(1997): that caffeine intake had an effect only in non-smokers.

Among non-smokers, women whose daily caffeine intake was 71–140mg day -1 had infants weighing 116 g less than the infants of women whose caffeine consumption was 0–10mg day -1.

For those whose caffeine intake was 5 140 mg day 1, the decrease in birth weight was 153 g. The authors suggested that the effect of smoking is more powerful than that of caffeine, so  that  caffeine  intake  does  not produce any noticeable effect in women who smoke.

It is difficult to establish the cause of the inconsisten- cies in the results of studies investigating the associa- tion between caffeine consumption and foetal growth. They may have resulted from recall bias, particularly in retrospective studies, incomplete information on amounts and sources of caffeine consumption, mis- classification of caffeine exposure, inadequate control for confounders or simply unknown study bias.

In two studies (Olsen et al. 1991, Larroque et al. 1993), investigators combined non-users and users (consuming <400 mg caffeine day 1 in Larroque et al. 1993) and used them as the control group. If, for example, exposure to caffeine at dose levels <400 mg day  1  is associated with reduced birth weight, then  comparing this control group with heavier users may obscure any positive association.

Despite inconsistencies in the results, the persistent association between caffeine consumption during pregnancy and low birth weight observed in eight original studies strongly suggests that caffeine may adversely affect foetal growth. This conclusion is supported by a meta-analysis study incorporating seven original studies and involving a total of 64 268 pregnancies, which reported a statisti- cally significant increase in the risk for low birth weight babies in pregnant women consuming >50 mg caffeine  day  1  (Fernandes  et al. 1998).

It should   be indicated that due to the nature of data presentation in individual studies used in meta-analysis, the authors were unable to adjust for potential confounders (ma- ternal age, smoking, alcohol intake or other confoun- ders) that may have contributed to the final result.

Based on the above evaluated data, despite inconsis- tencies in the results, it is concluded that caffeine consumption during  pregnancy  at  dose  levels  of  => 300 mg day 1 may interfere with foetal growth (decrease in birth weight or intrauterine growth retardation), particularly in smokers or heavy alcohol drinkers.

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/10957470


More information: Maternal caffeine consumption and pregnancy outcomes: a narrative review with implications for advice to mothers and mothers-to-be, BMJ Evidence Based MedicineDOI: 10.1136/bmjebm-2020-111432

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