One of the best diets for health keeps getting better.

A new study that tracked more than 25,000 women for a quarter century found that the more their eating patterns were in sync with the Mediterranean diet, the less likely they were to die during that period. The relationship held up even when researchers accounted for other factors that influence longevity, including age, exercise habits and smoking history.

The findings were published Friday in the journal JAMA Network Open.

The Mediterranean diet is heavy on fruits, vegetables and whole grains. Extra virgin olive oil, which is rich in antioxidants, is the main source of fat. Protein comes from lean sources like beans, legumes and nuts as well as fish, poultry, eggs and low-fat or fat-free dairy. Wine is welcome in low to moderate amounts, while red and processed meats, butter and sweets are eaten sparingly or not at all.

The diet is a longtime favorite of doctors, nutritionists and weight-loss programs. Studies consistently show that it helps people slim down, reduce cholesterol and lower their blood pressure, all of which reduce the risk of cardiovascular disease. It also helps people manage their blood sugar and stave off Type 2 diabetes.

How, exactly, does the Mediterranean diet pull this off? That’s what Shafqat Ahmad, who studies cardiovascular disease development at Sweden’s Uppsala University and Harvard Medical School, and his colleagues wanted to find out.

They turned to the Women’s Health Study, which enrolled tens of thousands of female health professionals who were at least 45 years old. When the women joined the study in the mid-1990s, they answered 131 questions about the foods they ate.

The researchers used those answers to give each woman a score between 0 and 9 that reflected the degree to which they were following the Mediterranean diet. If they were above the median when it came to consumption of vegetables, fruits, nuts, whole grains, legumes or fish, they got one point. Ditto if they were above the median on their ratio of monounsaturated (which are good) to saturated (which are bad) fatty acids.

If the women were below the median for consumption of red and processed meats, they earned another point. And if they consumed between 5 and 15 grams of alcohol per day — the equivalent of a typical glass of wine or a can of beer — they got a point as well.

Those with total scores between 0 and 3 were categorized as having “low” adherence to the Mediterranean diet. A total of 4 or 5 was classified as “intermediate,” and a sum between 6 and 9 was considered “high.”

The Women’s Health Study ended in 2004, but researchers kept checking in with the participants once a year. Ahmad and his colleagues focused on the 25,315 women who had both diet data and a host of biomedical measurements from when they entered the study.

By November 2023, 3,879 of the women had died. But the risk of being among them wasn’t the same for everyone.

Compared to the women in the low adherence group, those in the intermediate group were 16% less likely to die during the study period, while the risk of death for those with the highest fidelity to the Mediterranean diet was 23% lower, according to the study.

When the researchers controlled for smoking behavior, physical activity, alcohol intake and menopausal factors, women in the intermediate group had an 8% lower risk of death, and those in the high group had an 11% lower risk of death.

In addition to a reduced risk of death from any cause, following the Mediterranean diet was associated with a lower risk of dying of cancer or cardiovascular disease, said Dr. Samia Mora, a cardiologist at Brigham and Women’s Hospital in Boston and the study’s senior author.

As for why the Mediterranean diet seemed to protect against premature death, the most influential factor — among the roughly 40 biomarkers the researchers could test — was a group of metabolites that appeared to explain 14.8% of the benefit. Ahmad and his colleagues called particular attention to higher levels of a useful amino acid called alanine as well as lower levels of another amino acid called homocysteine that is elevated in people with heart disease.

Inflammation accounted for 13% of the mortality benefit enjoyed by those with high adherence to the Mediterranean diet. Chronic inflammation is associated with a variety of health problems, including cardiovascular diseases, diabetes and some cancers.

A woman’s body mass index and a measure of how well her body processes triglycerides were each responsible for 10.2% of the reduced risk of death, and insulin resistance accounted for 7.4%.

The study suggests that making even modest improvements in these factors could help people get more longevity out the Mediterranean diet, Ahmad said.

But he and Mora added that there must be other biological mechanisms at work that their study wasn’t able to measure. The gut microbiome may be one of them, they said.

Dr. Frank Hu, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health, said the study offers “new insights” into why people who embrace the Mediterranean diet tend to live longer.

“It suggests that the health benefits in reducing mortality are explained by its effects on harmful blood metabolites, inflammation, insulin resistance, and body weight rather than by reducing total and LDL cholesterol,” said Hu, who wasn’t involved in the work.

The study comes with several caveats, including the fact that 96% of the participants were white women. That means the results may not generalize to the population at large.

In addition, the women were asked about their eating habits only once, so there’s no way to know whether their diets changed as they got older.

However, Mercedes Sotos Pietro, a nutritional epidemiologist at the Autonomous University of Madrid, said the findings about the reduced risk of death are in line with research she has conducted using data from the Nurses Health Study and the Health Professionals Follow-up Study that assessed diet multiple times.

Sotos Pietro, who didn’t work on the new study, said the Mediterranean diet is “golden” because it includes a variety of tasty foods and doesn’t forbid anything. That makes it easy for people to stick with it for a long time, she said.

Hu added that the diet’s flexibility makes it adaptable to many cuisines.

“As an example, an Asian individual might use tofu as a protein source and replace white rice with brown rice,” he said. “Individuals can incorporate traditional recipes from other cultures and locally available foods while maintaining the MedDiet‘s key principles.”