A systematic review and meta-analysis of 13 papers — based on 14 studies — published between January 1980 and May 2016 was conducted by Yeungnam University; eight of the studies “indicated that vegetarians had a lower prevalence and incidence of diabetes than omnivores, while no significant associations between a vegetarian diet and diabetes risk were observed” in the other five studies.
One of the reasons given for the supposed anti-diabetic effects of a vegetarian diet was vegetarians’ generally lower BMI when compared to that of omnivores. Vegetarians also tended to have higher insulin sensitivity, which was “negatively associated with the duration of adherence to a vegetarian diet”.
The consumption of foods such as whole grains, vegetables, fruits, and dairy products, as opposed to red and processed meat and sugary drinks, has been credited to some extent for diabetes prevention.
The dietary fibres, beta-carotene, vitamin C and magnesium in leafy greens help to lower diabetes risk, while nitrosamine and advanced glycation end-products in processed meats could contribute to the risk.
The different types of vegetarian diets (lacto-ovo, lacto, pesco and vegan) also have varying effects on diabetes risk. Pesco-vegetarians were found to be the only vegetarian subgroup whose diet was not associated with a decreased diabetes risk, possibly because of the heterogeneity of their cooking methods, the kind of fish or seafood they ate, or this particular type of vegetarian diet simply having less efficacy.
The review also stated the consumption of fish or seafood and the risk of type 2 diabetes was negatively correlated in Asians, who tended to eat more raw or steamed fatty fish, but positively correlated in Europeans and North Americans who tended to eat more deep-fried white fish.
The quality of a vegetarian diet also matters. For instance, vegetarians in the US tend to be more health-conscious than those in South Asia, who typically consume dairy-rich foods that are also high in fats and sugars. As such, the link between a vegetarian diet and diabetes can be weakened or even have an "opposite directional trend”.
The duration of a vegetarian diet is another important factor, with a long-term vegetarian diet more likely to result in lower obesity or chronic disease rates than an omnivorous one.
Despite the beneficial effects of a vegetarian diet on diabetes risk, the review stated that “self-reported information should be carefully interpreted because it is often inconsistent with real intake”.
Additionally, the review said that since most of the studies in the meta-analysis were cross-sectional, "a cause-effect relationship could not be assessed”.
It concluded that while it was possible for a vegetarian diet to protect against diabetes risk, “well-designed prospective cohort studies from various countries that obtain information on the participants’ motivations for vegetarianism, the duration of adherence to a vegetarian diet, and verification of a vegetarian diet are needed to strengthen these findings”.
“Adherence to a Vegetarian Diet and Diabetes Risk: A Systematic Review and Meta-Analysis of Observational Studies”
Authors: Yujin Lee, Kyong Park