A study published in The Journal of Nutrition suggests that eating walnuts daily as part of a healthy diet may increase certain bacteria in the gut that can help promote health.

For the randomized, controlled trial, the researchers recruited 42 participants, aged 30–65, who were overweight or obese. Before the study began, participants were placed on an average American diet for two weeks. After this “run-in” diet, the participants were randomly assigned to one of three study diets, all of which included less saturated fat than the run-in diet. The diets included one that incorporated whole walnuts, one that included the same amount of alpha-linolenic acid (ALA) and polyunsaturated fatty acids without walnuts, and one that partially substituted oleic acid (another fatty acid) for the same amount of ALA found in walnuts, without any walnuts. In all three diets, walnuts or vegetable oils replaced saturated fat, and all participants followed each diet for six weeks with a break between diet periods.

To analyze the bacteria in the gastrointestinal tract, the researchers collected fecal samples 72 hours before the participants finished the run-in diet and before each of the three study diet periods.

The researchers found that the walnut diet enriched a number of gut bacteria—Roseburia, Eubacterium eligens, and Butyricicoccus—that have been associated with health benefits in previous research. They also found that after the walnut diet, there were significant associations between changes in gut bacteria and risk factors for heart disease. Eubacterium eligens was inversely associated with changes in several different measures of blood pressure, suggesting that greater numbers of Eubacterium eligens was associated with greater reductions in those risk factors.

Additionally, greater numbers of Lachnospiraceae were associated with greater reductions in blood pressure, total cholesterol, and non-HDL cholesterol. There were no significant correlations between enriched bacteria and heart disease risk factors after the other two diets.

“The findings add to what we know about the health benefits of walnuts, this time moving toward their effects on gut health,” said study author Penny Kris-Etherton, distinguished professor of nutrition at Penn State, in a university press release. “The study gives us clues that nuts may change gut health, and now we’re interested in expanding that and looking into how it may affect blood sugar levels.”

The California Walnut Commission helped support this research, as well as the Penn State Clinical and Translational Science Institute through the National Center for Advancing Translational Sciences’ Clinical and Translational Science Award program.

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