Vegetarian diets

Vegetarian diets have been eaten by some ethnic and religious groups for centuries for ethical reasons, and millions of people throughout the world cannot afford to eat meat. More recently, vegetarianism has been advocated as a diet that can potentially reduce the risk of chronic diseases while providing recommended nutrient intakes. So are vegetarian diets that large numbers of people adhere to particularly beneficial in their effects on health?

In 1954 it was reported that cholesterol concentrations were lower in vegetarians than in meat eaters,1 a finding repeatedly confirmed and refined to show that vegans have appreciably lower concentrations of total cholesterol and low density lipoprotein cholesterol than vegetarians.2 Vegans—who exclude eggs, milk, and dairy products in addition to not eating meat and fish—have lower intakes of saturated fatty acids than do vegetarians.

An analysis of five prospective studies indicated that mortality from ischaemic heart disease was 24% (95% confidence interval 6% to 38%) lower in vegetarians than in meat eaters,3 confirming earlier observations from a prospective follow-up of Seventh Day Adventists. A similar but non-significant difference of 19% has been reported from the 2009 analysis of the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford) study.4 Differences in low density lipoprotein cholesterol, body mass index, and blood pressure between vegetarians and meat eaters seem to account for much of the protective effect.4 An early attempt to identify dietary determinants of the cardioprotection conferred by vegetarianism suggested that low intakes of saturated animal fat and dietary cholesterol may be more relevant than avoidance of meat.5 Furthermore, vegetarian diets include relatively high amounts of nuts, whole grains, unsaturated oils, fruit, and vegetables, all of which may be cardioprotective.

Lower rates of type 2 diabetes and hypertension have been reported in Adventists who follow a vegetarian diet than in those who do not, with highly significantly reduced relative risks for diabetes in vegans and vegetarians of 0.22 and 0.39, respectively, and for hypertension of 0.25 and 0.45. Modest reductions were seen in those eating fish or meat occasionally.6 The low rates of obesity and high intakes of dietary fibre and low glycaemic index foods among vegetarians might explain the reduced rates of type 2 diabetes,7 although the Health Professionals Follow-up Study reported an increased risk of developing diabetes in people with a high intake of processed meat.8

The World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) second report on diet and physical activity as causes of cancer found meat consumption to be the only “convincing” dietary association with colon cancer.9 However, the results of cohort studies comparing vegetarians and meat eaters have not been consistent. Californian Adventists who are vegetarians have a lower total mortality and incidence of colon cancer than non-vegetarian Adventists,10 whereas the EPIC-Oxford study reported no difference between vegetarian and meat eating cohorts.11

The WCRF/AICR report was based on the totality of epidemiological and experimental evidence, including comparisons of cohorts with high and low meat consumption, whereas the differences between the EPIC-Oxford vegetarian and meat eating cohorts may have been insufficient to show differences in disease frequency. The low standardised mortality ratios of both the meat eating and vegetarian cohorts for all causes of death (52%) and for all cancers (70% in meat eaters and 69% in vegetarians) almost certainly reflect the healthy lifestyle of all study participants.

Concern about the nutritional adequacy of vegetarian diets has centred around intakes of vitamin B12, iron, zinc, calcium, iodine, vitamin D, and long chain n-3 fatty acids. The absence of animal protein and the bulky nature of vegetarian diets have been suggested to result in insufficient protein and energy for optimal growth in infants and young children. Although vegetarians, notably young women, do have lower iron stores than those who eat meat, there is little evidence that meat avoidance itself is associated with malnutrition.

However, vegetarianism covers a wide spectrum of dietary practices. Vegetarians who consume eggs, milk, and dairy products in addition to wholegrain cereals, pulses, vegetables, and fruits should have little difficulty in achieving adequate intakes of energy and individual nutrients, although some women may need supplemental iron, especially those with heavy menstrual bleeding. Vegans, in contrast, need nutrient supplements or fortified food products to ensure an adequate intake of vitamin B12, and depending on the balance of their diet, they may also benefit from fortification of foods with additional nutrients. Comprehensive eating guidelines covering all stages of life are readily available in peer reviewed journals and from national and international vegetarian and vegan societies.

Thus, vegetarians who participate in long term prospective studies have appreciably lower mortality ratios than the general population. Much of this benefit is probably because vegetarians tend to be non-smokers and to have a generally healthy lifestyle. When compared with volunteer cohorts of meat eaters of similar socioeconomic status they tend to have lower rates of ischaemic heart disease and obesity and its comorbidities, notably type 2 diabetes. However, little evidence exists that meat avoidance, rather than other attributes of the vegetarian diets, account for these health benefits. Lacto-ovo vegetarians should have little difficulty in achieving adequate intakes of nutrients, although vegans may need fortified foods and nutrient supplements.

Vegetarianism is therefore an acceptable dietary pattern in terms of health outcomes, but insufficient evidence exists at present to claim that it confers greater benefit than Mediterranean, Asian, or “modified conventional Western” dietary patterns—which are characterised by a low intake of saturated fats; a modest intake of animal protein; and a high intake of wholegrains, pulses, vegetables, and fruits—and in which fat intake is derived predominantly from unsaturated vegetable oils. Claims for unique benefits of a vegetarian lifestyle may be more appropriately made on ecological grounds.12