Probiotics, the "good bacteria" that people eat (and take in supplements) in order to maintain a healthy gut, don't promote a healthy gut in already healthy people. That's one conclusion of a study done in Denmark, where scientists tested fecal matter to how the "good" bacteria affected the digestive system when probiotics were in the diet. The study concluded that "there is no convincing evidence for consistent effects of probiotics on fecal microbiota composition in healthy adults."
The study notes that microbiota assist in "synthesis of vitamins and amino acids, prevention of pathogen colonization, maturation and regulation of the immune system" and "regulation of brain behavior" among other duties. This particular study focused on healthy individuals. The authors report that "no effects were observed on the fecal microbiota composition in terms of α-diversity, richness, or evenness in any of the included studies when compared to placebo."
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