utritionists are interested in vegetarian diets because of their compositional differences when compared to normal omnivorous diets. The obvious difference is absence of meat and fish, but other more subtle differences are also present and are interesting to consider with relation to health. Vegan diets are more restrictive still, and the absence of animal products can further change dietary intakes and long-term health consequences. Evidence suggests that mortality is reduced in vegetarians when compared to non-vegetarians, although some studies have found no difference between the two. However, it should be noted that vegetarians also often have differences in their lifestyles which may explain the difference in mortality. Lifestyle for example might explain the improvements in longevity found in vegetarian religious groups such as Seventh Day Adventists. The actual longevity advantage of vegetarianism is therefore controversial.
A number of studies have looked at the nutrient intakes of vegetarians and compared them to those of omnivores. Generally vegetarian diets are healthy if enough effort is expended in obtaining adequate essential nutrients. Obvious macronutrient differences are highlighted in studies, with vegetarians eating less protein and more carbohydrate as might be expected. Iron deficiency anaemia is more common amongst vegetarians than meat eaters largely because the most readily available source of iron in the human diet is red meat. However, in Western nations the iron status of vegetarians is less problematic than in developing nations because of supplement use and overall better quality nutrition. Fortified cereals for example are present in most Western diets and these are often a source of iron. Because vitamin B12 is only available from foods of animal origins, vegans will develop deficiencies unless dietary supplements are taken.
Adequate intakes of omega 3 fatty acids can also be a problem for vegetarians, because the plant derived α-linolenic acid (ALA, C18:3 (n-3)) is not metabolised at a high rate to the longer chain eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)). Alcohol can increase flux through this pathway, which explains some of its beneficial health benefits, but vegetarians that do not drink alcohol likely have sub-optimal plasma levels of EPA and DHA. Marine algae has recently become available commercially in capsule form and this is able to supply vegetarians with DHA, which can then be retro-converted to EPA. Vitamin D is also present mainly in foods of animal origin including eggs and fish, and as such vegans are unlikely to obtain adequate intakes when the necessary sun exposure is not available. However, supplements are available and are likely required by non-vegetarians in any case.
Evidence also suggests that vegetarians and especially vegans, are leaner than their omnivorous counterparts. However, some studies have shown that although vegetarians are closer to their idea weight, vegans can actually be underweight, which may detrimentally affect health. The reason for the lower weight of vegetarians and vegans is not entirely clear, however it is interesting to speculate. One explanation suggests that vegetarians have a consciously more healthy lifestyle when compared to omnivores, which seems plausible. The desire to become a vegetarians may also demonstrate dietary discipline which may lead to a decreased intakes of non-healthy weight promoting processed and refined foods. However, it might also be the fact that food sources are limited for vegetarians, and as such much of the processed foods which contain weight promoting ingredients also contain animal products and are therefore off limits.
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