The landmark faecal transplant experiments by Jeffrey Gordon's group at Washington University demonstrated that obesity can be transferred from obese mice to germ-free mice through gut microbiome transplantation alone, without any other variable. This finding established that the gut microbiome causally affects body composition, not merely correlates with it. Subsequent research has begun to identify the specific mechanisms through which gut bacteria affect energy metabolism, fat storage, and appetite regulation.
Energy Extraction and the Firmicutes-Bacteroidetes Ratio
Obese individuals and animals consistently show a higher ratio of Firmicutes to Bacteroidetes compared to lean individuals. Firmicutes species are more efficient at extracting energy from food, particularly from dietary fibre. This means that two individuals eating identical diets can extract meaningfully different numbers of calories from that diet depending on their microbiome composition. Dietary patterns, particularly fibre intake, are the primary determinant of this ratio.
Satiety Hormones and the Gut Microbiome
Short-chain fatty acids produced by gut bacteria from fibre fermentation activate L cells in the gut epithelium to produce GLP-1 and PYY, the primary satiety hormones. These hormones signal fullness to the brain and reduce appetite. A high-fibre diet that supports robust short-chain fatty acid production therefore produces better appetite regulation than a low-fibre diet, independent of the calorie content of the food eaten.