New Study Pinpoints How Mediterranean Diet Reduces Diabetes Risk

The known reduction in the risk of type 2 diabetes associated with adoption of the Mediterranean diet appears specifically attributed to its beneficial effects on some key factors, a new study published online in JAMA Network Open reveals.

While a reduction in BMI may be somewhat obvious, other mechanisms include beneficial effects on insulin resistance, lipoprotein metabolism, and inflammation.

However, the diet's antidiabetes effect does not appear to extend to people whose weight is considered healthy (BMI under 25 kg/m²), according to the findings.

"It is striking to see in these US women how strong the long-term antidiabetic properties of a Mediterranean-type dietary pattern are," senior author Samia Mora, MD, of the Center for Lipid Metabolomics, Brigham and Women's Hospital, Harvard Medical School, in Boston, Massachusetts, told Medscape Medical News.

"While it was known that the Mediterranean diet has many health benefits in particular on metabolism and inflammation, it was not previously known which of these biological pathways may be contributing to the lower risk of diabetes and to what magnitude."

"Our findings support the idea that by improving their diet, people can improve their future risk of type 2 diabetes, particularly if they are overweight or have obesity," she added.

"And it'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."

Mediterranean Diet Reduced Diabetes Riskin Those With BMI ≥ 25 kg/m²

The Mediterranean diet, with an emphasis on healthy olive oil as the predominant source of oil, favors fruits, vegetables, legumes, nuts, and seeds, fish and dairy products, while limiting intake of red and processed meats as well as sweets.

The diet has been linked to as much as a 25% to 30% reduction in the risk of diabetes in previous observational studies.

To investigate the precise mechanisms that underlie the prevention of diabetes, lead author Shafqat Ahmad, PhD, also at Harvard, and colleagues examined data from 25,317 healthy women participating in the Women's Health Study who had baseline assessments between September 1992 and May 1995. They were a mean age of 52.9 years at baseline.

Over the course of the study, 2307 participants developed type 2 diabetes.

With a mean follow-up of 19.8 years, those who had the highest self-reported adherence to the Mediterranean diet (a score ≥ 6 on a scale of 0 to 6) at baseline, had as much as a 30% lower risk of developing type 2 diabetes after multivariate adjustments compared to those with a lower Mediterranean diet score (a score ≤ 3; hazard ratio, 0.70).

The diabetes-related biomarkers that contributed the most to the reduced risk were insulin resistance, accounting for 65% of the reduction, followed by BMI (55.5%), high-density lipoprotein measures (53%), and inflammation (52.5%).

Other factors, though to a lesser degree, included branched-chain amino acids (34.5%), very low-density lipoprotein measures (32.0%), low-density lipoprotein measures (31.0%), blood pressure (29.0%), and apolipoproteins (23.5%).

Differences in A1c levels only had a limited effect on the risk (2%).

Notably, a subgroup analysis looking at effects of the diet according to baseline BMI showed the reductions in type 2 diabetes associated with higher intake of the Mediterranean diet only extended to those with an above normal weight (BMI ≥ 25 kg/m²).

Mora noted that, as this was not a prespecified analysis, these findings should be viewed as hypothesis-generating, but are consistent with the well-known increase in diabetes risk seen with a higher BMI.

"[The finding] fits with the biology and pathogenesis of type 2 diabetes that is driven in large part by excess weight, in particular for visceral adiposity and its resulting metabolic dysregulation and inflammation," she said.

"We know from other studies, such as the Nurses' Health Study, that the risk for type 2 diabetes in women increases even at BMI levels below 25 kg/m², but the risk goes up exponentially at around a BMI of 25 and higher."

Strong Role of Insulin Resistance a Surprise

The strong role of insulin resistance was a surprise, Mora added.

"We were surprised that insulin resistance, measured by a simple blood biomarker, would have the strongest mediating effect — even stronger than BMI — for the Mediterranean diet on risk of diabetes," she noted.

"This could represent an opportunity to intervene earlier and more intensively on improving insulin resistance through dietary approaches such as the Mediterranean diet, especially [because] insulin resistance can precede by years and decades the overt hyperglycemia and clinical diagnosis of diabetes."

Yet another surprise was that A1c had no substantial mediating effect on the reduction of diabetes risk with the Mediterranean diet.

"This could suggest that the cat is out of the bag by the time the A1c rises," Mora observed.

A study limitation is that the Women's Health Study consisted of well-educated US women who were health professionals and predominantly White, so the results may not be generalizable to men or individuals of other races or ethnicities.

In addition, BMI was self-reported and participants were not uniformly screened for diabetes, therefore surveillance bias could be possible.

However, the findings suggest that "even a small increase in adherence to the Mediterranean diet has substantial benefits over many years in preventing diabetes, among many other health benefits such as lowering insulin resistance and inflammation, improving lipid metabolism, and lowering blood pressure," Mora said.

"And of course, the more the adherence, the more the benefit."

The study received support through grants from the National Institutes of Health, the National Heart, Lung, and Blood Institute, the National Institute of Diabetes and Digestive and Kidney Diseases, the American Heart Association, and the Molino Family Trust. A coauthor is listed as a coinventor on patents held by Brigham and Women's Hospital related to the use of inflammatory biomarkers in cardiovascular disease (licensed to AstraZeneca and Siemens).

JAMA Network Open. Published online November 19, 2020. Full text

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