A growing body of research suggests that gut bacteria influence weight. One recent study found that obese people have a less diversity in their gut flora than lean people. Other studies have suggested that an increase in a group of gut bacteria called Firmicutes, and a decrease in a group of gut bacteria called Bacteroidetes, are linked with obesity.
Research done on animals may provide clues about how gut bacteria affect weight gain. One recent study found that mice that received a "gut bacteria transplant" from an obese person gained more weight and fat mass than those who received bacteria from a lean person.
What's more, the transplant altered the metabolism of the mice: animals that received gut bacteria from an obese person had metabolic changes linked with obesity in humans (such as increased production of compounds called branched-chain amino acids); while those that received gut bacteria from a lean person had metabolic changes linked with reduced body weight (such as increased breakdown of carbohydrates).https://www.livescience.com/39444-gut-bacteria-health.html