Eating 2-3 ounces of Walnut a day good for gut health, heart health says Journal of Nutrition study

USA: Eating two to three ounces of walnuts a day as part of a healthy diet could be a good way to improve gut health and reduce the risk of heart disease, a recent study has suggested.

According to the study, published in the Journal of Nutrition, walnut consumption promotes gut microbiota that may contribute to its heart health benefits in adults at cardiovascular risk. It found that consuming walnuts daily as a part of a healthy diet increases certain bacteria that promote health. Those changes in gut bacteria were associated with improvements in some risk factors for heart disease. 

Cardiovascular disease is the leading cause of mortality and morbidity worldwide which has been linked to poor diet quality. Recent evidence has suggested an important role of gut microbiota in cardiovascular risk. Gut microbiota is also critical for human health. The microbes present in the lower gastrointestinal tract are involved in the extraction and metabolism of nutrients not fully digested in the small intestine. Diet largely affects the functionality and composition of bacteria present in the large intestine. 

Previous research has shown that walnuts, when combined with a diet low in saturated fats, may have heart-healthy benefits. For example, previous work demonstrated that eating whole walnuts daily lowers cholesterol levels and blood pressure. However, It is unclear whether the favorable effects of walnuts on the gut microbiota are attributable to the fatty acids, including α-linolenic acid (ALA), and/or the bioactive compounds and fiber.

Penny M Kris-Etherton, The Pennsylvania State University, University Park, PA, USA, and colleagues examined between-diet gut bacterial differences in individuals at increased cardiovascular risk following diets that replace SFAs with walnuts or vegetable oils.

"Replacing your usual snack -- especially if it's an unhealthy snack -- with walnuts is a small change you can make to improve your diet," said Kristina Petersen, assistant research professor at Penn State. "Substantial evidence shows that small improvements in diet greatly benefit health."

For the study, researchers recruited 42 participants with overweight or obesity who were between the ages of 30 and 65. 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 each of the three study diet periods.

Key findings include:

"The walnut diet enriched a number of gut bacteria that have been associated with health benefits in the past," Petersen said. "One of those is Roseburia, which has been associated with protection of the gut lining. We also saw enrichment in Eubacteria eligens and Butyricicoccus."

The researchers 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," Kris-Etherton said. "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 study, "Walnuts and Vegetable Oils Containing Oleic Acid Differentially Affect the Gut Microbiota and Associations with Cardiovascular Risk Factors: Follow-up of a Randomized, Controlled, Feeding Trial in Adults at Risk for Cardiovascular Disease," is published in the Journal of Nutrition. 

DOI: https://doi.org/10.1093/jn/nxz289

Provided by: Penn State University