INS and Impaired glucose tolerance: Given that R. gnavus is a primary gut microbe that is capable of decarboxylating tryptophan and phenylalanine into tryptamine and phenethylamine as shown by our previous findings and other studies13,16, elevated fecal tryptamine and phenethylamine levels were found along with impaired glucose tolerance and insulin sensitivity in germ-free mice colonized with R. gnavus (p < 0.01 in all cases, Fig. 1H, I).