The Mediterranean diet reduces the risk of diabetes by 30% in women who are overweight or obese, according to a new analysis of the Women’s Health Study.

Investigators collected health data from healthcare professionals over 25 years starting in 1993. Food frequency questionnaires and blood samples showed that participants who consumed more foods from the Mediterrenean diet early in the study had a 30% lower rate of type 2 diabetes than women who did not, said investigators from Brigham and Women’s Hospital, Boston.

But this effect was seen only among women with a body mass index greater than 25, a threshold indicating that someone is clinically overweight or obese. It was not seen in participants whose BMI was clinically normal or underweight, reported Samia Mora, M.D., and colleagues.

The results of blood sample analyses further suggest that the diet reduced diabetes risk in participants by improving insulin resistance, lipoprotein metabolism and inflammation. 

The results show that the protective effects of diet can occur over many years, the researchers said.

“[I]t’s important to note that many of these changes don’t happen right away. While metabolism can change over a short period of time, our study indicates that there are longer-term changes happening that may provide protection over decades,” the authors said.

The Mediterreanean diet is high in olive oil, fruits, vegetables, legumes, nuts and seeds. Although much evidence already demonstrates that the diet reduces the risk of heart disease, type 2 diabetes and other adverse health outcomes, the current study was unique in length; many previous studies that have looked only at the diet’s short-term effects, the authors noted.

Full findings were published in JAMA Network Open.