Vegetarians appear to have a healthier biomarker profile than meat-eaters


Vegetarians appear to have a healthier biomarker profile than meat-eaters, and this applies to adults of any age and weight, and is also unaffected by smoking and alcohol consumption, according to a new study in over 166,000 UK adults, being presented at this week’s European Congress on Obesity (ECO), held online this year.

Biomarkers can have bad and good health effects, promoting or preventing cancer, cardiovascular and age-related diseases, and other chronic conditions, and have been widely used to assess the effect of diets on health. However, evidence of the metabolic benefits associated with being vegetarian is unclear.

To understand whether dietary choice can make a difference to the levels of disease markers in blood and urine, researchers from the University of Glasgow did a cross-sectional study analysing data from 177,723 healthy participants (aged 37-73 years) in the UK Biobank study, who reported no major changes in diet over the last five years.

Participants were categorised as either vegetarian (do not eat red meat, poultry or fish; 4,111 participants) or meat-eaters (166,516 participants) according to their self-reported diet. The researchers examined the association with 19 blood and urine biomarkers related to diabetes, cardiovascular diseases, cancer, liver, bone and joint health, and kidney function.

Even after accounting for potentially influential factors including age, sex, education, ethnicity, obesity, smoking, and alcohol intake, the analysis found that compared to meat-eaters, vegetarians had significantly lower levels of 13 biomarkers, including: total cholesterol; low-density lipoprotein (LDL) cholesterol – the so-called ‘bad cholesterol; apolipoprotein A (linked to cardiovascular disease), apolipoprotein B (linked to cardiovascular disease); gamma-glutamyl transferase (GGT) and alanine aminotransferase (AST) – liver function markers indicating inflammation or damage to cells; insulin-like growth factor (IGF-1; a hormone that encourages the growth and proliferation of cancer cells); urate; total protein; and creatinine (marker of worsening kidney function).

However, vegetarians also had lower levels of beneficial biomarkers including high-density lipoprotein ‘good’ (HDL) cholesterol, and vitamin D and calcium (linked to bone and joint health). In addition, they had significantly higher level of fats (triglycerides) in the blood and cystatin-C (suggesting a poorer kidney condition).

No link was found for blood sugar levels (HbA1c), systolic blood pressure, aspartate aminotransferase (AST; a marker of damage to liver cells) or C-reactive protein (CRP; inflammatory marker).

“Our findings offer real food for thought”, says Dr. Carlos Celis-Morales from the University of Glasgow, UK, who led the research. “As well as not eating red and processed meat which have been linked to heart diseases and some cancers, people who follow a vegetarian diet tend to consume more vegetables, fruits, and nuts which contain more nutrients, fibre, and other potentially beneficial compounds.

These nutritional differences may help explain why vegetarians appear to have lower levels of disease biomarkers that can lead to cell damage and chronic disease.”

The authors point out that although their study was large, it was observational, so no conclusions can be drawn about direct cause and effect. They also note several limitations including that they only tested biomarker samples once for each participant, and it is possible that biomarkers might fluctuate depending on factors unrelated to diet, such as existing diseases and unmeasured lifestyle factors.

They also note that were reliant on participants to report their dietary intake using food frequency questionnaires, which is not always reliable.

Vegan and vegetarian diets have gained popularity over the past decade. Characteristically, vegans do not consume any animal-derived products including eggs, dairy, meat, and fish. Vegetarians exclude meat and fish but may consume milk and eggs. A subclass of the vegetarian diet may consume fish but not meat, termed pescatarians. Despite these definitions, varying levels of strictness and adherence to dietary restriction exist at the level of the individual [1].

The prevalence of vegetarian and vegan diets differs globally. In developing regions, meat-free diets are traditionally adopted owing to religious, social, ecological, or economic constraints as opposed to personal choice [2]. In industrialised countries, most individuals are afforded the choice of food consumption and level of dietary restriction.

Populations in developed countries may adopt these diets for environmental, ethical, religious, health beliefs or social reasons. Presently, well-planned vegan and vegetarian diets have been regarded by the British Dietetic Association and other organizations in industrialised countries to be suitable throughout the lifespan, inclusive of infancy and pregnancy [3,4,5]. However, concerns have been raised regarding the ability of these diets to adequately provide essential micronutrients, such as iodine [6].

Iodine is an essential micronutrient, required in trace quantities, which is vital for the synthesis of thyroid hormones—triiodothyronine (T3) and thyroxine (T4) [7]. The thyroid hormones are crucial for the regulation of metabolism, growth, and neurological development [8]. Iodine deficiency presents as a spectrum of clinical disorders termed ‘iodine deficiency disorders’ (IDD’s) which occur when recommended intakes are not achieved (150 µg day−1) [8].

These include hypothyroidism, goitre abnormal thyroid nodular pathology, and cretininism in infants born to mothers with a low iodine status during pregnancy and lactation [7,9]. Low iodine intake may be a risk factor for thyroid nodule formation, particularly in females [10,11].

Most nodules are harmless; however, some may result in thyroid dysfunction or malignancy [11]. Excessive iodine intake (>1000 µg day−1) may lead to hyperthyroidism in individuals with preexisting thyroid disease or iodine deficiency [12].

Iodine deficiency is not limited to developing countries—mild–moderate deficiency exists in industrialised nations including Europe, UK, Australia and select populations in the USA [9]. In 2011, iodine nutrition was highlighted as a significant public health concern following estimates indicating that 2 billion people globally were deficient [13].

Recent data collected by the WHO show a global decline in iodine deficiency between 1993 and 2019, suggesting that less than 8.5% of the world’s population are affected [14]. However, subgroups of European populations are still at increased risk of iodine deficiency [6].

Iodine deficiency traditionally was assessed by monitoring the prevalence of visible goitre in populations [7]. After the development of newer methods for measuring iodine status, it was recognized that low-level deficiency may be present in industrialised populations not displaying obvious thyroid enlargement [15].

Various biomarkers can be used to estimate population iodine status and intake [15]. Urinary iodine concentration (UIC) is the most common and practical marker [16]. This is because >90% of the iodine ingested from dietary sources is readily excreted in the urine [17]. Spot UIC and 24 h measures can be used to detect and monitor iodine adequacy and deficiency. However, these estimates only correspond to recent intake [16].

Additionally, thyroid function tests are required routinely to detect iodine adequacy in vulnerable populations such as pregnant and/or lactating women and infants [16]. Dietary iodine can be estimated indirectly by UIC or by common dietary assessment methods [15]. Limitations of the methods used must be considered. Biomarkers of status and dietary intake methods are not always the same between studies which adds to the challenge of reliably comparing iodine amid populations [18].

Individuals residing in developing countries, who are reliant on plant-based foods in their diet, have a higher prevalence of iodine deficiency [19]. The bioavailability of iodine from plant sources has been suggested to be determined by rainfall and water collection on crop leaves with much of the iodine within plants not being bioavailable [20].

In industrialised countries where people consume a ‘Western diet’, the key dietary sources of iodine are bread fortified by iodised salt, cow′s milk, and dairy products [21]. Seafood, eggs, and seaweed are also iodine rich but are not regularly consumed [22].

Water and salt iodination strategies are present in most states in the US and select countries in Europe [23,24]. Countries such as the UK have yet to establish a mandatory salt fortification program and despite regular manufacturing of iodised salt, it is not widely available for public purchase [25].

For this reason, individuals who consume diets excluding iodine-rich food, principally dairy, eggs, and/or fish, have increased risk of iodine deficiency [26]. Further complicating this issue is the growing availability and acceptance of plant-based food ‘alternatives’, regularly consumed by vegans and vegetarians, that naturally have negligible iodine content and are not regularly fortified [27,28]. The size of the plant-based ‘alternatives’ food market has been reported to have almost doubled between 2014 and 2017 in the UK [29].

Currently, two reviews exist investigating iodine in the diets of vegans and vegetarians, one in 2005, which was updated in 2009 by the same authors [30,31]. The most recent review included eight studies, covering a period between 1981 and 2003, with the conclusion that strict vegans and vegetarians living in Europe have iodine values below recommended levels and are at risk of deficiency.

In the years since publication, these diets have become more widely accepted and it is likely that food consumption practices have changed considerably since this last assessment of iodine intake in adults following vegan and vegetarian diets. Given the potential health consequences of iodine deficiency, it is important to re-examine whether adults following either a vegan or vegetarian diet are still at risk of iodine deficiency.

Thus, the aim of this review is to assess the iodine intake and status in adults following a vegan or vegetarian diet in industrialised countries across time. The objectives included (1) evaluation of the methods used to assess iodine; (2) determination of the iodine intake and food consumption in vegan and vegetarian adults; (3) assessment of the iodine status and prevalence of iodine deficiency using urinary iodine concentration (UIC); (4) comparison of the iodine intake, status and prevalence of deficiency between vegans, vegetarians and omnivores; and (5) consideration of gender differences in estimates of iodine nutrition.

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More information: This story is based on poster presentation EP3-33 at the European Congress on Obesity (ECO).


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