Generated Summary
This review article synthesizes findings from numerous prospective cohort studies to examine the long-term health outcomes associated with vegetarian and vegan diets. The research focuses on studies with a high proportion of vegetarian participants, primarily conducted in Western countries. The methodology involves summarizing and comparing data from these studies to assess the impact of vegetarian diets on various health outcomes, including weight, diabetes, cardiovascular disease, cancer, and mortality. The article acknowledges that vegetarian diets vary based on the foods included, which may influence observed associations, and that the findings may not be generalizable to non-Western populations. The primary aim is to provide an overview of the current understanding of the health implications of vegetarianism and veganism, highlighting areas where more research is needed.
Key Findings & Statistics
- Obesity and Weight Gain: Studies consistently show that vegetarians have a lower Body Mass Index (BMI) than non-vegetarians, with vegans generally having the lowest BMI.
- BMI Differences: Differences in BMI are typically in the range of 1-2 kg/m² across age groups.
- Obesity Prevalence: Lower prevalence of obesity among Western vegetarians.
- Diabetes: The risk of type 2 diabetes is strongly linked to obesity. Data from the Adventist Health Study-2 (AHS-2) cohort showed a lower risk of self-reported diabetes in semi-vegetarians, lacto-ovo-vegetarians, and vegans compared to non-vegetarians.
- Diabetes Risk Reduction: Risks in vegetarian groups are approximately half those of non-vegetarians after adjusting for BMI.
- Cardiovascular Disease: Plasma total cholesterol is lower in vegetarians compared to non-vegetarians, primarily due to a reduction in LDL cholesterol.
- CHD Incidence: Differences in cholesterol concentration suggested that the incidence of Coronary Heart Disease (CHD) may be 24% lower in lifelong British vegetarians and 57% lower in lifelong vegans than in meat-eaters.
- Hypertension: Data from the EPIC-Oxford study showed differences in the prevalence of self-reported hypertension across four diet groups.
- IHD Mortality: IHD mortality was 24% lower (95% CI 6, 38) in vegetarians than in non-vegetarians.
- IHD Risk Reduction: The risk of hospitalization or death from IHD was 32% (95% CI 19, 42) lower in vegetarians compared with non-vegetarians in EPIC-Oxford.
- Stroke Mortality: Stroke mortality did not differ significantly between vegetarians and non-vegetarians in a collaborative re-analysis.
- Cancer Mortality: The collaborative analysis found no significant difference in death rates between vegetarians and non-vegetarians for cancers of the stomach, colorectum, lung, breast, or prostate.
- Cancer Incidence: Overall cancer incidence rates were significantly lower in both fish eaters (relative risk and 95% CI compared with meat eaters 0.88, 0.80, 0.97) and in vegetarians (0.88, 0.82, 0.95).
- Colorectal Cancer: Colorectal cancer incidence was compared across five diet groups, with the lowest risk found among fish eaters (0.66, 0.48, 0.92).
- Fracture Risk: Vegans had a 30% (95% CI 2, 66) higher risk of fracture compared with meat eaters in the EPIC-Oxford study.
- Bone Mineral Density: Bone mineral density was 4% (95% CI 2, 7) lower in vegetarians than in omnivores at both the femoral neck and lumbar spine.
- Diverticular Disease: The risk of diverticular disease of the colon was 31% (95% CI 14, 45) lower in vegetarians compared with meat eaters.
- Cataract Risk: Vegetarians had a lower risk of eye cataract compared with meat eaters (relative risk 0.74, 95% CI 0.63, 0.86).
- Kidney Stones: Vegetarians had a 31% (95% CI 2, 52) lower risk of kidney stones compared with participants consuming a high meat diet.
- All-Cause Mortality: In a pooled analysis of five prospective studies, the death rate ratio in vegetarians compared with non-vegetarians was 0.95 (95% CI 0.82, 1.11).
- Mortality in EPIC-Oxford: No significant difference in all-cause mortality between vegetarians and non-vegetarians (death rate ratio and 95% CI 1.05, 0.93, 1.19).
- Mortality in AHS-2: Analyses in both AHS and AHS-2 have shown lower mortality in vegetarians and semi-vegetarians combined than in non-vegetarians.
- Metabolic Syndrome: Vegetarians had a significantly lower risk of having metabolic syndrome compared with non-vegetarians (OR 0.44, 95% CI 0.30, 0.64).
Other Important Findings
- Vegetarians generally have a lower prevalence of overweight and obesity.
- Vegetarians may have a lower risk of ischemic heart disease (IHD) compared to non-vegetarians.
- The risk for stroke did not significantly differ between vegetarians and non-vegetarians.
- Overall cancer mortality is similar for vegetarians and non-vegetarians, but there is some evidence of a lower risk for specific cancers in vegetarians.
- Vegetarians have lower risks for diabetes, diverticular disease, and eye cataracts.
- Vegetarian diets have been linked to lower LDL cholesterol levels.
- Vegetarians may have lower blood pressure.
- Findings suggest a lower risk of hypertension.
- The study mentions that people who eat fish but not meat may have a lower risk of colorectal cancer.
- Data suggests that vegetarians, particularly vegans, may be at a higher risk of low bone mineral density and fractures.
- Vegetarians may have a lower risk of kidney stones.
- Vegetarians showed a lower prevalence of degenerative arthritis and soft tissue disorders.
- Vegetarians showed a lower prevalence of hyperthyroidism.
Limitations Noted in the Document
- The findings are primarily based on studies conducted in Western countries, and the conclusions may not apply to vegetarians in non-Western settings.
- Vegetarian diets vary in composition, which can affect observed associations between diet and health outcomes.
- The paper is based on conference material and is not a full systematic review, which means there may be limitations in the scope of included studies.
- The studies included were designed to recruit a high proportion of vegetarians.
- There is a need for more data, particularly for vegans, to draw stronger conclusions.
- The study notes that vegetarian diets are defined by what foods they exclude, therefore the included foods may vary substantially.
- The study acknowledges that the findings for certain diseases and conditions are inconclusive.
Conclusion
The long-term health of vegetarians appears generally good, and for some diseases, it may be better than that of comparable omnivores. The analysis highlights that the risk of IHD and obesity is lower in vegetarians, and the stroke mortality has not been shown to differ between vegetarians and non-vegetarians. The data suggest that overall cancer rates may be slightly lower in vegetarians. The authors point out that more research is needed, especially concerning the long-term health impacts on vegans. Key findings include a lower risk for specific cancers, such as colorectal cancer for those who eat fish but not meat. It also highlights a lower risk for diabetes, diverticular disease, and eye cataracts. The authors acknowledge, however, that the data is not extensive enough to draw firm conclusions for all outcomes, particularly for vegans, emphasizing the need for further investigation in specific areas like stroke subtypes, bone health, and diseases not yet studied. They suggest that the quality of health among both vegetarians and non-vegetarians in the EPIC-Oxford study points to the benefits of making healthy lifestyle choices. In sum, the current data is insufficient to draw any strong conclusions and much more research is required.