Mediterranean diet reduced risk for diabetes, other adverse health outcomes

Women who adhered to a more Mediterranean-like diet had a 30 percent lower rate of type 2 diabetes than women who did not, according to new research by Brigham and Women's Hospital published in the journal JAMA Network Open.

For the study, researchers examined outcomes for more than 25,000 participants in the Women's Health Study, a longitudinal cohort study that followed female health professionals for more than 20 years. The team examined several biomarkers to look for biological explanations for these results, finding key mechanisms including insulin resistance, body mass index (BMI), lipoprotein metabolism and inflammation.

The Women's Health Study enrolled female health care professionals between 1992 and 1995, collecting data through December 2017. It was designed to evaluate the effects of vitamin E and low-dose aspirin on risk of heart disease and cancer. Additionally, participants were asked to complete food frequency questionnaires (FFQs) about dietary intake when the study began and answer other questions about lifestyle, medical history, and demographics. More than 28,000 women provided blood samples at the beginning of the trial, according to the study.

The researchers said they leveraged data from the FFQs and blood samples to investigate the relationship between the Mediterranean diet, type 2 diabetes, and biomarkers that might explain the connection. To do so, they assigned each participant a Mediterranean diet intake score from 0 to 9, with points assigned for higher intake of fruits, vegetables, whole grains, legumes, nuts and fish, moderate intake of alcohol, and lower intake red meat or processed meat. The researchers measured a range of biomarkers, including traditional ones such as cholesterol, and more specialized ones that can only be detected using nucleic magnetic resonance. These included lipoproteins and measures of insulin resistance, the researchers said.

Of the more than 25,000 participants in the Women's Health Study, 2,307 developed type 2 diabetes. Participants with higher Mediterranean diet intake at the beginning of the study, scores greater than or equal to 6, developed diabetes at rates that were 30 percent lower than participants with lower Mediterranean intake, scores less than or equal to 3. This effect was seen only among participants with a BMI index greater than 25, overweight or obese range, and not among participants whose BMI was less than 25, normal or underweight range.

Biomarkers of insulin resistance appeared to be the biggest contributor to lower risk, followed by biomarkers of BMI, high-density lipoprotein measures and inflammation.

One of the strengths of the study was its length. However, the authors note several limitations, including that study participants were predominantly white and well-educated, and all were female health professionals. In addition, dietary intake was self-reported and only examined at the start of the study. Biomarkers were also only measured when participants entered the study.

Samia Mora, MD, MHS, corresponding author of the study from the Brigham and Women's Hospital divisions of Preventive Medicine and Cardiovascular Medicine and an associate professor at Harvard Medical School, said she emphasizes that insights into the biology that explains how the Mediterranean diet may help protect against diabetes could be helpful in preventive medicine and for physicians speaking to patients about dietary changes.

"Even small changes can add up over time," said Mora in a statement. "And there may be many biological pathways that lead to a benefit. One of the best things patients can do for future health is to improve their diet, and now we are beginning to understand why."