Two clinical trials examining probiotic supplementation (one with Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 for 8 weeks, and the other with Bifidobacterium longum R0175 for 6 weeks) showed no significant impact on plasma BDNF levels in individuals with low mood and IBS (Romijn et al. 2017; Pinto‐Sanchez et al. 2017). This evidence concerns the gene BDNF and irritable bowel syndrome.