People who ate a mostly plant-based diet reduced their risk of diabetes by 23%, the study found.
The association was even stronger - a 30% drop in the risk of type 2 diabetes - for people who ate healthy plant-based foods, including veggies, fruits, legumes, nuts, and whole grains. These foods contain fiber, vitamins, minerals, antioxidants, and other beneficial ingredients.
So, what isn't an especially healthy plant food?
Processed foods and foods with added sugar. Think foods like white bread, white pasta, breakfast cereal, chips or cookies. The researchers also didn't include starchy vegetables, such as potatoes, in their healthy-choices list.
"A plant-based diet is very healthful in reducing the risk of type 2 diabetes," said the review's senior author, Dr. Qi Sun. He's an associate professor at the Harvard T.H. Chan School of Public Health in Boston.
And the more healthy plant foods, the better, Sun said. But "you should be picky about what types of foods you count as plant-based," he added.
Sun also explained that a diet doesn't have to be strictly vegan or vegetarian to be healthy. He said it's a good idea to minimize animal protein, but choices like fish, chicken, and yogurt can still be part of a healthful diet.
The study didn't spell out exactly why a mainly plant-based diet appeared to lower type 2 diabetes risk. The researchers controlled the data to account for weight, but Sun said people who eat more plant-based foods may maintain a healthier weight, leading to lower diabetes risk.
He said it's also possible that beneficial compounds, like antioxidants and beneficial plant oils, might help promote insulin sensitivity or reduce inflammation. If you're eating more plant foods, you're probably eating fewer animal products. And that reduces the amount of potentially harmful substances you consume, such as cholesterol, saturated fat, and sodium.