Fat Bacteria, Thin Bacteria

Twin studies revealed something startling. When gut bacteria from an obese twin were transplanted into germ-free mice, those mice gained significantly more fat than mice given bacteria from the lean twin — even though both groups ate identical diets and identical portions throughout the experiment.

An obesity-associated microbiome can pull more calories out of the same food, harvesting extra energy from meals that another person's bacteria would let pass. Researchers estimate the daily difference at roughly 150 calories — small per meal, but a steady thumb on the scale over time. Your bacteria help decide how much energy your body takes from everything you eat.