The Long Term Health of Western Vegetarians

(These notes are from a talk given by Paul Appleby at the World Vegetarian Congress, Edinburgh, Scotland, in July 2002.)

Introduction

Many studies have assessed the nutritional adequacy of vegetarian diets. These have shown that a well-planned vegetarian or vegan diet can supply all of the nutrients required for good health. Many other studies have assessed factors relating to health such as body mass index, blood pressure and serum cholesterol levels in vegetarians, with generally positive outcomes. However, relatively few studies have assessed the long-term health of Western vegetarians.

Such studies must include large numbers of participants who need to be categorised by diet and other lifestyle characteristics (ideally at regular intervals) and ‘followed-up’ for death and disease incidence over several years. These studies are known as ‘prospective’ or ‘cohort’ studies. Several prospective studies have recruited large numbers of Western vegetarians (and comparable non-vegetarians), enabling estimates of the relative mortality and disease incidence of vegetarians to be made.

The table below summarises these studies, the main findings of which will be reviewed here.

(The first five studies listed contributed data to a collaborative analysis of mortality in vegetarians first published in 1998. No data on mortality or disease incidence from the other three studies have been published as yet.)

Mortality and disease incidence rates for the vegetarians in these studies may be compared with rates in the population from which they were recruited by calculating Standardised Mortality/Incidence Ratios (SMR/SIR). However, the SMR/SIR can be misleading because of the ‘healthy volunteer effect’ and because the cohort may not be representative of the reference population in many respects. For example, vegetarians are less likely to smoke than the ‘average’ person.

A better approach is to compare death and disease incidence rates for vegetarians and non-vegetarians within studies. Although the vegetarians and the non-vegetarians within studies may differ with respect to non-dietary lifestyle characteristics, such as smoking habits, it is possible to control for these factors in the statistical analysis by appropriate ‘adjustment’. Thus, results are typically ‘adjusted’ for age, sex, smoking and other non-dietary factors likely to be associated with disease risk.

Main findings of the studies

Adventist Mortality

• all-cause mortality positively associated with consumption of eggs and meat
• meat consumption positively associated with fatal ischaemic heart disease (especially in men)
• no clear association between fatal colorectal cancer and meat and other animal products
• ‘suggestive’ positive associations between fatal breast and prostate cancers and meat intake
• lower prevalence of diabetes among vegetarians

Health Food Shoppers

• all-cause SMR ~ 60% for the cohort as a whole
• no difference in all-cause mortality between vegetarians and non-vegetarians
• non-significantly lower ischaemic heart disease (IHD) mortality in vegetarians (by ~ 15%)
• significantly higher breast cancer mortality in vegetarian women (owing to lower parity?)
• daily consumption of fresh fruit is associated with lower mortality from all causes, IHD, lung cancer and respiratory diseases

Adventist Health

• lower all-cause mortality in vegetarians
• significant positive association between meat (beef) consumption and IHD mortality in men
• significant protective effect of nut consumption for fatal and non-fatal IHD
• colon and prostate cancers more common in non-vegetarians
• higher fruit consumption associated with lower risks of cancer of the lung, prostate and pancreas
• lower prevalence of diabetes, hypertension, rheumatism and rheumatoid arthritis among vegetarians

Heidelberg

• all-cause SMR ~ 50% for the cohort as a whole
• significantly lower all-cause and cancer mortality in ‘long-term’ (>20y) compared with ‘short-term’ vegetarians
• a suggestion of lower all-cause and cancer mortality (but higher IHD mortality) in ‘moderate’ (ate meat or fish ‘occasionally’) compared with ‘strict’ vegetarians

Oxford Vegetarian

• all-cause SMR ~ 50% for the cohort as a whole
• significantly lower all-cause and cancer mortality in ‘non-meat-eaters’ compared with meat-eaters after 12 years follow-up
• however, no difference in all-cause mortality between vegetarians and non-vegetarians after 18 years follow-up
• non-significantly lower IHD mortality in vegetarians (by ~ 15%)
• strong positive associations between animal fats and IHD mortality

Collaborative analysis of mortality in vegetarians

Individual data from these five studies were pooled in 1996 to give data for 76,000 persons of whom nearly 28,000 were vegetarians. There were 8330 deaths between the ages of 16 to 89 after an average of 10.6 years follow-up, allowing the researchers to make accurate estimates of relative mortality for vegetarians compared with non-vegetarians for several common causes of death, as well as for all causes combined.

Death rate ratios (DRR) for vegetarians compared with non-vegetarians, adjusted for age, sex and smoking, were calculated for each of the five studies and then combined to give an ‘all studies’ DRR. The results for all-cause mortality are shown in the following table.

The results of the collaborative analysis suggest that vegetarians may have a slightly lower overall mortality than comparable non-vegetarians (by about 5%), although this result was not statistically significant. In the same analysis, vegetarians were found to have a significantly lower mortality for ischaemic heart disease (by about 25%), but not for other common causes of death, as shown in the following table.

Mortality in British vegetarians

A recent re-analysis of data from the two British studies found no differences in overall death rates between vegetarians and non-vegetarians. The researchers concluded that the low mortality of British vegetarians compared with the general population “may be attributed to non-dietary lifestyle factors such as a low prevalence of smoking and a generally high socio-economic status, or to aspects of the diet other than the avoidance of meat and fish”.

Life expectancy of vegetarians

A recent analysis of data from the Adventist Health Study predicted life expectancy in Seventh-day Adventists following different behaviour patterns. The researchers found that a combination of different lifestyle choices could influence life expectancy by as much as 10 years. Among the lifestyle choices investigated, a vegetarian diet was estimated to confer an extra 1½ to 2 years of life.

The researchers concluded that “the life expectancies of California Adventist men and women are higher than those of any other well-described natural population” at 78.5 years for men and 82.3 years for women. The estimated life expectancies of vegetarian California Adventists were 80.2 years for men and 84.8 years for women. Other beneficial lifestyle choices included high nut consumption, a high level of exercise and maintaining a moderate body weight.

Conclusions

Vegetarians

• have low mortality and low chronic disease rates compared with the general population
• have lower mortality from ischaemic heart disease than non-vegetarians (by ~ 15-25%)
• may enjoy an additional 1-2 years of life (at least among US Adventists)

However

• much of the benefit is attributable to non-dietary lifestyle factors such as the avoidance of smoking and a high socio-economic status
• vegetarians have similar mortality to comparable non-vegetarians for all causes of death combined, stroke and the most common cancers
• a vegetarian diet is not a panacea

Of course, life expectancy and chronic disease rates are not the only measures of health status, and other studies have suggested that vegetarians may enjoy a number of health benefits compared with non-vegetarians, including having

• lower body mass index (by ~ 1 kg/m2)
• lower blood pressure (by up to 5 mm Hg systolic)
• lower total cholesterol (by ~ 0.5 mmol/L)

So, should you be a vegetarian for health reasons? When asked this question, the Nobel-prize winning author Isaac Bashevis Singer replied: “Yes, for the health of the chicken!”

Selected References:

• Key et al. Mortality in vegetarians and non-vegetarians:detailed findings from a collaborative analysis of 5 prospective studies. Am J Clin Nutr 1999; 70(suppl): 516S-24S.
• Appleby et al. The Oxford Vegetarian Study: an overview. Am J Clin Nutr 1999; 70(suppl): 525S-31S.
• Fraser. Associations between diet and cancer, ischaemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists. Am J Clin Nutr 1999; 70(suppl): 532S-8S.
• Key et al. Health benefits of a vegetarian diet. Proc Nutr Soc 1999; 58: 271-5.
• Fraser & Shavlik. Ten years of life - is it a matter of choice? Arch Intern Med 2001; 161: 1645-52.
• Appleby et al. Mortality in British vegetarians. Public Health Nutr 2002; 5: 29-36.

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This page created 7 July 2002 by Paul Appleby.