“We found that walnuts in the diet increased the diversity of bacteria in the gut, and other non-related studies have associated less bacterial diversity with obesity and other diseases like inflammatory bowel disease,” says Byerley. “Walnuts increased several bacteria, like Lactobacillus, typically associated with probiotics suggesting walnuts may act as a prebiotic.”
https://www.sciencedaily.com/releases/2017/07/170728100832.htm
The Colitist 