A large and comprehensive meta-analysis led by scientists from the Harvard T.H. Chan School of Public Health is offering the most convincing evidence to date that a plant-based diet can significantly lower a person's risk of developing type 2 diabetes. However, not all vegetarian diets are created equal with the researchers suggesting plant-based eaters avoid sugar, refined white flour and processed foods if they really want to best lower diabetes risk.

"Plant-based dietary patterns are gaining popularity in recent years, so we thought it was crucial to quantify their overall association with diabetes risk, particularly since these diets can vary substantially in terms of their food composition," explains Frank Qian, first author on the new research.

The meta-analysis gathered together data from nine studies, comprising over 300,000 subjects in total. As well as finding a direct association between increased plant-based dietary patterns and decreased risk of diabetes, the research suggests not all vegetarian or vegan diets confer the same health benefits.

A more vegan-inclined diet, focusing on fruits, vegetables, whole grains, legumes, and nuts showed the most significant reductions in diabetes risk as opposed to other vegetarian diets that included sugar, refined flour and processed foods. Those with the highest adherence to the more healthy plant-based diet showed a 23 percent decrease in type 2 diabetes risk compared to those eating a more unhealthy, but still plant-based, diet.

A variety of mechanisms may underpin the results found in this research. Nutrition researcher Ian Johnson, who did not work on this current study, suggests a number of different potential explanations.

"The benefits may come from reduced consumption of meat and other animal products, from protective effects of plant constituents such as dietary fibre, from a lower risk of overweight and obesity, or perhaps from a combination of all these effects," says Johnson.

A large array of recent study has revealed how diets can protect against, and even in some cases reverse, type 2 diabetes. However much of that research has focused on low-calorie or ketogenic diets. This new research suggests what you eat could be as important as how much you eat, and just cutting out meat is not enough if you are actively trying to reduce type 2 diabetes risk factors.

"Overall, these data highlighted the importance of adhering to plant-based diets to achieve or maintain good health, and people should choose fresh fruits and vegetables, whole grains, tofu, and other healthy plant foods as the cornerstone of such diets," says senior author Qi Sun.

The new research was published in the journal JAMA Internal Medicine.