Dietary characteristics of vegan diet and vegetarian diets

Dietary characteristics of vegetarian diets and vegan diet

Vegetarians are defined as individuals who do not eat any meat, poultry or fish. Vegetarians are sub-classified as lacto-ovo vegetarians who eat dairy products and/or eggs and vegans who do not eat any animal  products. Since vegetarian diets are defined by what they do not contain, the actual composition of  vegetarian diets can vary widely. For the purpose of the present review the discussion is largely based on the observed average diet of vegetarians as described in published reports, and mostly for individuals living in affluent Western countries.

Nutritional status of Vegetarians

 

Nutrient intake

Reviews by expert groups such as the American Dietetic Association and Dietitians of Canada (2003) have concluded that a well-planned vegetarian diet is adequate for all stages of the life cycle including infancy, childhood and adolescence. Comparisons of the food intakes of vegetarians and non-vegetarians show that vegetarian diets generally provide relatively large amounts of cereals, pulses, nuts, fruits and vegetables. 

Together with the differences in intakes of animal foods, these differences in food intake result in several characteristic differences in nutrient intake. Vegetarian diets are usually rich in carbohydrates, n-6 fatty acids, dietary fibre, carotenoids, folic acid, vitamin C, vitamin E and Mg, and relatively low in protein, saturated fat, long-chain n-3 fatty acids, retinol, vitamin B12 and Zn.

Vegans can have particularly low intakes of vitamin B12 and low intakes of Ca. These differences in nutrient intake might have favourable or unfavourable effects on the nutritional status and health of vegetarians. Current knowledge of nutrition suggests that the relatively high intakes of dietary fibre, folic acid, vitamin C, vitamin E and Mg and low intake of saturated fat might all have benefits.

In particular, vegetarians are more likely than non-vegetarians to reach targets for fruit and vegetables and the dietary reference values for saturated fatty acids of 10% total energy and NSP of 18 g/d. For example, mean saturated fat intakes expressed as percentage energy are estimated as 10.7 and 10.4 in male and female meat-eaters, 9.4 and 9.3 in male and female vegetarians and 5.0 and 5.1 in male and female vegans.

 For NSP, mean intakes (g/d) are 18.7 and 18.9 in male and female meat-eaters, 22.7 and 21.8 in male and
female vegetarians, and 27.7 and 26.4 in male and female vegans. The impact on health of the relatively high intakes of carbohydrates and n-6 fatty acids and relatively low intakes of protein, retinol and Zn in vegetarians is unclear; mean intakes of these nutrients in vegetarians are probably close to or above the recommended intakes, where applicable, and these nutrients are not discussed further in the present review. For n-3 fatty acids and vitamin B12, however, there is substantial recent research relevant to the status of vegetarians in relation to these nutrients and this aspect will be discussed further, together with Fe status.

Dietary characteristics of vegan diet

 n-3 Fatty acids

Plant foods can provide a-linolenic acid but are devoid of the long-chain n-3 fatty acids EPA and DHA (marine algae can provide EPA and DHA and are acceptable to vegetarians but are not considered here as a plant food); eggs and to a lesser extent dairy products contain low levels of EPA and DHA that vary according to the feeding of the animal. Studies of plasma levels of n-3 fatty acids have consistently shown that vegetarians have lower levels of EPA and DHA than meat-eaters, with lower levels in vegans than in lacto-ovo-vegetarians. Recently, it has been shown that plasma levels of EPA and DHA in vegans are not related to the duration of adherence to the diet over periods of 20 years, suggesting that the endogenous production of these fatty acids in vegetarians and vegans may result in low but stable plasma concentrations. 

Studies using labelled a-linolenic acid have shown that conversion of this fatty acid to EPA and DHA can occur in human subjects but that the rate of conversion is low in females and very low in males. Vegetarian diets are usually rich in linoleic acid, which may reduce the conversion of a-linolenic acid to EPA and DHA, but there is no direct evidence that plasma levels of EPA and DHA in vegetarians can be substantially increased by following a diet low in linoleic acid and high in a-linolenic acid.

It is unknown whether the low levels of EPA and DHA in the plasma of vegetarians have effects on health. In a small uncontrolled trial in which supplements of EPA and DHA were given to ten vegetarians. Mezzano et al. (2000) observed an increase in the plasma levels of these fatty acids and a reduction in platelet aggregation, a possible risk factor for CVD, but more research is needed to explore this possible effect.

Vitamin B12

Food plants do not contain vitamin B12; therefore, the only reliable sources of vitamin B12 for vegetarians are dairy products and eggs, fortified foods and dietary supplements. It has been claimed that some plant foods such as seaweed and tempeh might provide true vitamin B12, but this claim has not been established and much or all the material in these foods that is detected by assays for vitamin B12 may be vitamin B12 analogues that are either inactive or may antagonize true vitamin B12. Early studies of vitamin B12 status in vegetarians have shown that, as would be expected, dietary intake and plasma concentrations are lower than those in meat-eaters.

Clinical evidence of vitamin B12 deficiency has been reported in some vegans but is apparently uncommon. Recent research using more sensitive indicators of vitamin B12 status (plasma methylmalonic acid, homocysteine and holotranscobalamin II) has shown that substantial proportions of vegans and even of vegetarians have suboptimal vitamin B12 status according to these criteria, both among affluent Western vegetarians and in other countries including India, China and Taiwan. Dietary vitamin B12 deficiency is also now recognized to be a serious problem in non-vegetarian populations with a low intake of meat because of poverty. Recent work shows that vegetarians must be careful to ensure that they do consume adequate amounts of vitamin B12. The current dietary reference values for vitamin B12 were derived partly from studies of vegetarians and vegans that have suggested that intakes of vitamin B12 of approximately 0.3 mg/d are sufficient to prevent anaemia and macrocytosis. However, intakes of 0.3 mg/d may be probably associated with moderate elevation in homocysteine that might be detrimental to health.

Iron

The Fe content of vegetarian diets is typically quite similar to that of non-vegetarian diets, but the bioavailability of the Fe is lower because of the absence of haem-Fe. Vegan diets are usually higher in Fe than lacto-vegetarian diets because dairy products are low in Fe. For example, in EPIC-Oxford estimated Fe intakes among 43 582 women are 12.6, 12.8, 12.6 and 14.1 mg/d for meat-eaters, fisheaters, lacto-vegetarians and vegans respectively (Davey et al. 2003). Studies of Fe status have consistently shown that serum ferritin is lower in vegetarians than in non-vegetarians and that Hb levels are similar or slightly lower in vegetarians than in non-vegetarians. Low Fe status is not common in men, but is moderately common among premenopausal women throughout the world, and young female vegetarians need to ensure that their diet includes good sources of Fe with vitamin C to help absorption and that they take supplements if needed.