Poor diets are known to promote cardiovascular disease and newer studies have demonstrated they also have negative impacts upon the complement of microbiota present in the intestine. Recent studies have uncovered the potential role of microbiota in the risk of cardiovascular disease. Studies using walnuts as a dietary intervention have yielded results suggesting they are able to reduce cardiovascular risk. Walnuts are composed of polyunsaturated fatty acids (e.g., a-linolenic acid, ALA), fiber, and hydrolysable tannins. The potential contribution of the fatty acids, fiber, or bioactive compounds to changes in the microbiota and how those changes relate to cardiovascular risk is currently not known. A study by Tindall and colleagues reported in the April 2020 issue of The Journal of Nutrition addressed this question.

Subjects (n=42) participating in a randomized, crossover, controlled feeding study at risk of cardiovascular disease provided samples to study microbial populations and cardiovascular risk factors (blood pressure, serum lipids, and lipoproteins). Fecal samples were collected 72 hours prior to the end of the run-in period (2 weeks of a standard Western diet, SWD) and after each of the three diet periods. The experimental diet periods were 6 weeks long and included: 1) walnuts (WD, 47-99 g/d, 2.7% ALA), 2) a fatty acid matched diet devoid of walnuts (WFMD, 2.6% ALA), or 3) a diet where oleic acid replaced ALA (ORAD, 0.4% ALA).

The relative abundance of certain species was affected by consumption of the WD diet or the WFMD diet. For example, whole walnuts caused an increase in Gordonibacter spp., when compared to the WFMD diet. These species have the capacity to metabolize ellagitannins in walnuts to smaller compounds that can be absorbed where they can suppress inflammation and produce other beneficial vascular effects. The WD and WFMD diets caused an increase in the relative abundance of Roseburia spp., which can ferment dietary fiber and have been linked to improved insulin sensitivity. The increase in relative abundance of Lachnospiraceae resulting from WD consumption was inversely related to markers of cardiovascular risk. The authors concluded that some changes in microbiota resulted from the fatty acid composition of the diets, whereas other components in walnuts can also have beneficial impacts on gut microbiota, which lead to improvements in cardiovascular risk profiles.

In a commentary on this article, Holscher points out the importance of the stringent controls used in Tindall’s feeding study, as it provides data that can be used in further studies to evaluate the impact of specific microbiota and their metabolites on health measures. She further describes the importance of using multiple approaches that provide greater taxonomic resolution and details of the specific metabolites being produced. Holscher concludes the commentary by suggesting that this combination of approaches will provide the information needed to create the most effective dietary therapies.

References Tindall AM, McLimans CJ, Petersen KS, Kris-Etherton PM, Lamendella R. 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. Journal of Nutrition, DOI: https://doi.org/10.1093/jn/nxz289.

Commentary provided by Holscher HD. Gut microbes: Nuts about fatty acids. Journal of Nutrition, DOI: https://doi.org/10.1093/jn/nxaa045.

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