Abstract
Vegetarians, who do not eat any meat, poultry or fish, constitute a significant minority of the world’s population. Lacto-ovo-vegetarians consume dairy products and/or eggs, whereas vegans do not eat any foods derived wholly or partly from animals. Concerns over the health, environmental and economic consequences of a diet rich in meat and other animal products have focussed attention on those who exclude some or all of these foods from their diet. There has been extensive research into the nutritional adequacy of vegetarian diets, but less is known about the long-term health of vegetarians and vegans. We summarise the main findings from large cross-sectional and prospective cohort studies in western countries with a high proportion of vegetarian participants. Vegetarians have a lower prevalence of over- weight and obesity and a lower risk of IHD compared with non-vegetarians from a similar background, whereas the data are equivocal for stroke. For cancer, there is some evidence that the risk for all cancer sites combined is slightly lower in vegetarians than in non- vegetarians, but findings for individual cancer sites are inconclusive. Vegetarians have also been found to have lower risks for diabetes, diverticular disease and eye cataract. Overall mortality is similar for vegetarians and comparable non-vegetarians, but vegetarian groups compare favourably with the general population. The long-term health of vegetar- ians appears to be generally good, and for some diseases and medical conditions it may be better than that of comparable omnivores. Much more research is needed, particularly on the long-term health of vegans.
Generated Summary
This document summarizes the long-term health of vegetarians and vegans based on prospective cohort studies, focusing on the findings from studies with a high proportion of vegetarian participants, primarily in Western countries. The review aims to consolidate and update previous research, providing insights into the health outcomes associated with vegetarian and vegan diets. The methodology involves analyzing data from large-scale studies, including the Adventist Health Study-2 and EPIC-Oxford, to assess the prevalence of various health outcomes such as obesity, cardiovascular disease, cancer, and mortality. The scope is limited to studies conducted in Western countries, with a recognition that findings may not be directly applicable to vegetarians in non-Western populations due to dietary and lifestyle differences. The research emphasizes the need for further investigation, particularly regarding the long-term health effects of vegan diets and the potential impact of dietary factors on various health outcomes.
Key Findings & Statistics
- Vegetarians have a lower prevalence of overweight and obesity and a lower risk of IHD compared with non-vegetarians from a similar background, whereas the data are equivocal for stroke.
- For cancer, there is some evidence that the risk for all cancer sites combined is slightly lower in vegetarians than in non-vegetarians, but findings for individual cancer sites are inconclusive.
- Vegetarians have also been found to have lower risks for diabetes, diverticular disease and eye cataract.
- Overall mortality is similar for vegetarians and comparable non-vegetarians, but vegetarian groups compare favourably with the general population.
- The studies include a total of nearly 280,000 participants, including over 100,000 vegetarians.
- Studies of western vegetarians have consistently shown that vegetarians have a lower BMI than otherwise comparable non-vegetarians, with differences typically in the region of 1-2 kg/m² across all adult age groups, vegans generally having the lowest BMI.
- Using data from the Adventist Health Study-2 (AHS-2) cohort, Tonstad and colleagues showed a lower risk of self-reported diabetes in semi-vegetarians, lacto-ovo-vegetarians and vegans compared with non-vegetarians, risks in the vegetarian groups being approximately half those of the non-vegetarians overall, after adjusting for BMI.
- A collaborative re-analysis in 1999 of data from five prospective studies showed that IHD mortality was 24% lower (95% CI 6, 38) in vegetarians than in non-vegetarians.
- More recently, analyses in EPIC-Oxford included non-fatal as well as fatal end points and the risk of hospitalization or death from IHD was 32% (95% CI 19, 42) lower in vegetarians compared with non-vegetarians.
- A collaborative analysis of data from the first five studies, published in 1999, found no significant difference in death rates between vegetarians and non-vegetarians for cancers of the stomach, colorectum, lung, breast or prostate.
- Colorectal cancer incidence was compared across five diet groups; the lowest risk was found among fish eaters (described as pesco-vegetarians in the study), with an adjusted relative risk for all colorectal cancers of 0.57 (95% CI 0.40, 0.82) compared with non-vegetarians.
- In the analysis of mortality data from the EPIC-Oxford study, standardised mortality ratios relative to the UK population for all causes of death were 52% for both vegetarians and non-vegetarians.
Other Important Findings
- Vegetarians generally have lower plasma total cholesterol, primarily due to a reduction in LDL cholesterol.
- Vegetarians have a lower average BMI than non-vegetarians, which may contribute to lower risks of obesity-related diseases.
- The risk of type 2 diabetes is lower in vegetarians than in non-vegetarians.
- There is strong evidence that the risk of IHD is lower in vegetarians.
- Overall cancer incidence rates were significantly lower in both fish eaters and vegetarians.
- The risk for cancer at all sites combined is slightly lower in vegetarians.
- In the EPIC-Oxford study, vegetarians were at lower risk of eye cataract.
- High meat consumption was associated with a higher prevalence of degenerative arthritis and soft tissue disorders.
- Vegetarians were found to have a lower risk of diverticular disease.
- Vegetarians had a significantly lower risk of metabolic syndrome.
- There was no difference in all-cause mortality between vegetarians and non-vegetarians in the EPIC-Oxford study.
- Bone fracture rates may be higher in vegans.
Limitations Noted in the Document
- The conclusions may not apply to vegetarians in non-western countries.
- The foods included in vegetarian diets can vary substantially, and therefore the associations observed may differ between studies.
- There is a lack of data on the long-term health of vegans.
- Few studies have been of sufficient magnitude and duration to examine the long-term health of vegetarians and vegans.
- The results are based on the material presented at a conference and is not a full systematic review.
- Some studies were not designed to account for all variables.
- Data on stroke subtypes and bone health are limited.
- More research is needed for diseases and conditions not yet studied.
- The ‘healthy volunteer effect’ may influence some findings.
Conclusion
The findings suggest that vegetarian diets are associated with several positive health outcomes, including a lower risk of cardiovascular diseases, type 2 diabetes, and certain cancers. The analysis supports the idea that a well-planned vegetarian diet can supply all the nutrients required for good health. The strong evidence indicates that the risk of IHD is lower in vegetarians, and there is some evidence that the risk for all cancer sites combined is slightly lower in vegetarians. The consistent findings of low BMI in western vegetarians may not apply in non-western populations. The document emphasizes that more research is needed, particularly regarding the long-term health effects of vegan diets and the potential impact of dietary factors on various health outcomes. It also highlights the need for more data on stroke subtypes, bone health, and diseases and conditions not yet studied. The research suggests that the health of vegetarians is generally good and may be better than that of comparable omnivores for some diseases and medical conditions. This is evident in the lower risk of diabetes, diverticular disease, and eye cataract among vegetarians. The authors conclude that the current data are insufficient to draw any strong conclusions about the health of vegans and that much more research is required. Overall mortality is similar for vegetarians and comparable non-vegetarians, but vegetarian groups compare favourably with the general population. Further investigation is needed to understand the complex relationship between diet and health fully.