It’s been suggested that the microbiome may impact body fat gain and obesity risk through various potential mechanisms, including calorie extraction from food, inflammatory signaling, and lipid oxidation pathways.[1] However, available evidence indicates the microbiome may not actually be an important (or, at the very least, modifiable) determinant of body weight in humans.
Several clinical trials have examined whether a process called a fecal microbiota transplant (FMT) can impact body weight. With FMT, the microbiota of a healthy (or, in this case, lower body weight) individual is transferred into the gut of another individual with the intention of colonizing their microbiome with the donor’s microbes. In most cases, these trials have found FMT does not result in weight loss.[2][3][4][5] Oral probiotics, meanwhile, have been shown to result in weight loss, but the effect seems too small to matter — in one meta-analysis of 15 randomized controlled trials, probiotics led to an average weight loss of only 0.6 kg (1.3 pounds).[6]