In a randomized, controlled trial, researchers found that eating walnuts daily as part of a healthy diet was associated with increases in certain bacteria that can help promote health. Additionally, those changes in gut bacteria were associated with improvements in some risk factors for heart disease.
Journal Reference: Alyssa M Tindall, Christopher J McLimans, Kristina S Petersen, Penny M Kris-Etherton, Regina Lamendella. 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. The Journal of Nutrition, 2019; DOI: 10.1093/jn/nxz289
RCT but only 42 participants.