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