Stress does not stay in your head: it reaches the gut through the gut–brain axis. Chronic stress triggers the release of cortisol and other hormones that alter gut motility, protective mucus production and, according to animal studies and some human studies, the permeability of the intestinal barrier. These changes affect the environment in which your bacteria live and may favour opportunistic species over beneficial ones.
The relationship works both ways, so sleeping well, exercising and practising relaxation techniques benefit both your mind and your microbiome. Your gut–brain axis report shows your microbiome's potential to produce compounds such as GABA or tryptophan derivatives; it does not measure your stress or anxiety levels.