The main finding was that vegan and lacto-ovo vegetarian diets were associated with a nearly one-half reduction in risk of type 2 diabetes compared with the risk associated with nonvegetarian diets after adjustment for a number of socioeconomic and lifestyle factors, as well as low BMI, that are typically associated with vegetarianism. Pesco- and semi-vegetarian diets were associated with intermediate risk reductions: between one-third and one-quarter. These data indicate that vegetarian diets may in part counteract the environmental forces leading to obesity and increased rates of type 2 diabetes, though only vegan diets were associated with a BMI in the optimal range. Inclusion of meat, meat products, and fish in the diet, even on a less than weekly basis, seems to limit some of the protection associated with a vegan or lacto-ovo vegetarian diet. These findings may be explained by adverse effects of meat and fish, protective effects of typical constituents of vegan and lacto-ovo vegetarian diets, other characteristics of people who choose vegetarian diets, or a combination of these factors.
The notion that animal protein stimulates insulin secretion and possibly insulin resistance was proposed decades ago (19
). However, a number of other dietary constituents are associated with protection against diabetes in observational studies or influence insulin sensitivity in food trials (6
). Vegetarian diets are rich in vegetables and fruits, foods that reduce oxidative stress and chronic inflammation. The vegan group consumed ~650 g/day of fruits and vegetables, which is about one-third more than the amount consumed by nonvegetarians (data not shown). Observational evidence has shown that these dietary constituents are associated with a reduction in type 2 diabetes of ~40% (6
). Vegetarian diets contain substantially less saturated fat than nonvegetarian diets, and saturated fatty acids have been shown to reduce insulin sensitivity, though a recent review concluded that some of the data supporting this idea was flawed (20
). The vegetarian diet typically includes foods that have a low glycemic index such as beans, legumes, and nuts. We did not calculate the glycemic load of the diets. Though low-glycemic-response diets are associated with less prevalence of type 2 diabetes, cohort studies have not consistently found a relation between dietary glycemic index or load and risk of diabetes (21
); furthermore, whether the glycemic response causes diabetes is not established.
Protection against type 2 diabetes associated with vegetarian diets is partly due to the lower BMI of vegetarians (), where the effects of diet when not adjusted for BMI were greater yet. Disentangling the effects of diet on insulin sensitivity independent of lower adiposity among vegetarians may be difficult. Only sparse data have investigated whether vegetarians matched to nonvegetarians with regard to adiposity differ in insulin resistance or sensitivity. In a study that matched vegetarians and nonvegetarians, nonvegetarians had higher insulin, glucose, and homeostasis model assessment values than vegetarians (23
). Whether vegetarians and nonvegetarians were matched with regard to abdominal girth was not reported. The protective effect of vegetarianism in the current study was evident in individuals with BMI below or above 30 kg/m2
, further strengthening the notion that independent effects of the diet are present.
Church attendees tend to have higher body weight than nonattendees (24
), and increasing trends in BMI in the general population have also been observed among Adventists (data not shown). Vegans were the only church members whose mean BMI was <25 kg/m2
. Previous studies have reported a difference of ~2 BMI units between vegans and meat eaters (4
). In the current study, the difference of 5 BMI units may indicate greater protection in current environments where a variety of high-energy dense foods are available. Some evidence indicates a temporal relationship between initiating plant-based diets and leanness (2
), though a randomized study found that a vegetarian diet did not improve long-term weight loss (25
). As with most dietary trials, the participants' compliance to the diet declined substantially over time.
The present cohort is likely to be more homogenous than general populations regarding nondietary factors allowing comparisons between dietary groups to be less affected by other differences. This may be true regarding smoking and alcohol use, which are practices strongly discouraged by the church. One of the major confounders of diet and disease associations in observational studies is cigarette smoking. As the participants were almost exclusively nonsmokers, the confounding effects of smoking on body weight and risk of type 2 diabetes were avoided. The cohort exhibited an unusually wide range of dietary exposures and included one of the largest numbers of vegans studied in any sample. The results are likely to be generalizable given that we found expected relationships between diabetes and age, ethnicity, sex, BMI, physical activity, sleep, and television watching.
Our data are cross-sectional and do not allow causal inferences to be made. However, reverse causation is unlikely in that subjects diagnosed with diabetes would be less expected to differentially change their diet from vegetarian to omnivorous than subjects without diabetes. We were unable to assess physical activity for about one-sixth of the cohort because responses to one or more of the questions required for the calculation of MET units were missing. Food-frequency questionnaires involve a certain degree of measurement error; however, the ability to allocate subjects into a broad dietary pattern is probably very strong. All variables were self-reported; however, our calibration study found evidence for good validity for the diagnosis of diabetes. Diabetes may have been underreported in the vegan and other vegetarians because of their lower BMIs; however, this is unlikely to affect the study conclusions substantially given the association we observed between diet and diabetes in individuals with BMI both below and above 30 kg/m2.
The cohort was not representative of the general population; i.e., participants were church attendees. Members who choose vegetarianism are likely to be more compliant with other church tenets and to differ from nonvegetarians with regard to major determinants of type 2 diabetes. This was indeed the case with regard to some factors; e.g., nonvegetarian diets were more associated with black ethnicity, less education, more television watching, and fewer hours of sleep than were vegetarian diets. On the other hand, nonvegetarians were younger and reported more physical activity and alcohol consumption, which are all established protective factors against type 2 diabetes. Nevertheless, the association between diet and type 2 diabetes remained strong after adjustment for these factors.
In conclusion, this study showed that all variants of vegetarian diets (vegan, lacto-ovo, and pesco- and semi-vegetarian) were associated with substantially lower risk of type 2 diabetes and lower BMI than nonvegetarian diets. The protection afforded by vegan and lacto-ovo vegetarian diets was strongest.