Mortality In Vegetarians And Nonvegetarians: Detailed Findings From A Collaborative Analysis Of 5 Prospective Studies1-3

Abstract

We combined data from 5 prospective studies to compare the death rates from common diseases of vegetarians with those of nonvegetarians with similar lifestyles. A summary of these results was reported previously; we report here more details of the findings. Data for 76172 men and women were available. Vegetarians were those who did not eat any meat or fish (n = 27808). Death rate ratios at ages 16-89 y were calculated by Poisson regression and all results were adjusted for age, sex, and smoking status. A random-effects model was used to calculate pooled estimates of effect for all studies combined. There were 8330 deaths after a mean of 10.6 y of follow-up. Mortality from ischemic heart disease was 24% lower in vegetarians than in nonvegetarians (death rate ratio: 0.76; 95% CI: 0.62, 0.94; P 5 y. Further categorization of diets showed that, in comparison with regular meat eaters, mortality from ischemic heart disease was 20% lower in occasional meat eaters, 34% lower in people who ate fish but not meat, 34% lower in lactoovovegetarians, and 26% lower in vegans. There were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined. Am J Clin Nutr 1999;70(suppl):516S-24S.

Generated Summary

This study presents a collaborative analysis of five prospective cohort studies, aiming to compare mortality rates from common diseases between vegetarians and nonvegetarians with similar lifestyles. The research involved a large-scale investigation, combining data from 76,172 men and women, with vegetarians defined as those who did not consume any meat or fish. The methodology employed Poisson regression to calculate death rate ratios, adjusted for age, sex, and smoking status, and a random-effects model was used to calculate pooled estimates across all studies. The primary focus was to assess the association of a vegetarian diet with the risk of death from ischemic heart disease and cancers of the colorectal, breast, and prostate. The study investigated whether the duration of the current diet influenced the mortality rates among vegetarians. The study categorized diets into five groups: regular meat eaters, occasional meat eaters, fish eaters, vegetarians (who ate dairy products, eggs, or both), and vegans (who did not eat any animal products), aiming to provide a more detailed description of the results.

Key Findings & Statistics

• The study included 76,172 participants, with 8330 deaths recorded after a mean follow-up of 10.6 years.
• Mortality from ischemic heart disease was 24% lower in vegetarians compared to nonvegetarians (death rate ratio: 0.76; 95% CI: 0.62, 0.94; P < 0.01).
• The lower mortality from ischemic heart disease among vegetarians was more pronounced at younger ages and was restricted to those who had followed their diet for over 5 years.
• Compared to regular meat eaters, the mortality from ischemic heart disease was 20% lower in occasional meat eaters, 34% lower in fish eaters, 34% lower in lacto-ovo-vegetarians, and 26% lower in vegans.
• No significant differences were found between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined.
• In the Adventist Mortality Study, the death rate ratio for all-cause mortality varied between studies from 0.80 to 1.17, and the test for heterogeneity between studies was highly significant (P <0.0001). The all-studies death rate ratio for all causes of death was 0.95 (95% CI: 0.82, 1.11).
• The all-studies death rate ratios for causes of death affecting men and women for vegetarians compared with nonvegetarians were shown in Table 4. The only death rate ratios that were significantly different from 1 were for ischemic heart disease: P < 0.001 for men and P < 0.05 for women.
• The all-studies death rate ratios for vegetarians compared with nonvegetarians by age at death were shown in Table 5.
• After a mean of 10.6 years of follow-up, there were 8330 deaths before age 90 (3910 men and 4420 women). The death rate ratio for ischemic heart disease varied between studies from 0.45 to 0.97, and the all-studies ratio was 0.76 (95% CI: 0.62, 0.94; P <0.01).
• The death rate ratios for all-cause mortality varied between studies from 0.80 to 1.17, and the test for heterogeneity between studies was highly significant (P <0.0001). The all-studies death rate ratio for all causes of death was 0.95 (95% CI: 0.82, 1.11).

Other Important Findings

• The study found no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined.
• The association of vegetarianism with lower mortality from ischemic heart disease was greater for deaths at younger ages; death rate ratios for ischemic heart disease were 45% (P < 0.01), 31% (P<0.01), and 8% (NS) lower in vegetarians than nonvegetarians at ages <65, 65-79, and 80-89 y, respectively (P < 0.05 for trend).
• The all-studies estimate of all-cause mortality was 5% lower in vegetarians than in nonvegetarians, consistent with the reduction in mortality from ischemic heart disease; however, the CIs for this estimate were relatively wide because of the substantial heterogeneity between the studies.
• The study found that the lower mortality from ischemic heart disease was confined to subjects who had followed their current diet for >5 y.
• The study showed that the occasional meat eaters and fish eaters within the nonvegetarian category also had lower mortality from ischemic heart disease than did the regular meat eaters.

Limitations Noted in the Document

• The study acknowledged that information on alcohol, education, exercise, and body mass index was not available for all subjects, particularly in the Health Food Shoppers Study.
• Variations in smoking rates across the studies may have introduced some confounding.
• The study was unable to determine the exact duration of the vegetarian diet for all participants.
• The heterogeneity between studies, especially in the death rate ratios for ischemic heart disease, indicates that the findings may not be universally applicable and that the effect of a vegetarian diet on mortality might vary depending on the population and study setting.
• The study notes that the results for breast cancer should be interpreted cautiously, as it did not adjust for established reproductive risk factors due to data unavailability.

Conclusion

The collaborative analysis of five prospective studies strongly suggests that a vegetarian diet is associated with a significant reduction in mortality from ischemic heart disease. The all-studies death rate ratio for ischemic heart disease was 0.76 (95% CI: 0.62, 0.94; P < 0.01), highlighting a substantial benefit for vegetarians in this regard. The lower mortality from ischemic heart disease was particularly evident among younger age groups and those who had adhered to a vegetarian diet for over 5 years, underscoring the importance of long-term dietary habits. However, the study found no significant differences in mortality from other major causes of death, including cerebrovascular disease, cancers of the stomach, colon, breast, prostate, or all other causes combined. This indicates that while a vegetarian diet appears to be beneficial for heart health, its impact on overall mortality is less pronounced and more specific to certain diseases. The finding that lower mortality was largely associated with lower serum total cholesterol concentration in vegetarians as compared to nonvegetarians, provides one explanation for the observed lower mortality from ischemic heart disease. It suggests the lower mortality from ischemic heart disease could have been due largely to their lower total serum cholesterol concentrations. The results also point to other potential mechanisms, such as reduced oxidation of LDL cholesterol or changes in blood clotting, as contributors to the observed health benefits. Further research is needed to address the limitations of the study. The heterogeneity between studies and the lack of comprehensive data on potential confounding factors suggest the need for further investigation to fully understand the relationship between vegetarian diets and mortality. Despite these limitations, this study offers valuable insights into the health benefits of vegetarianism, particularly in relation to cardiovascular health, and supports the importance of dietary choices in disease prevention.