The belly fat in mice makes a lot of a heat-boosting chemical when it’s cold—but when the mice are inflamed or have this bacteria, that fat stops making it, while other fat types don’t change as much.

The study found that a specific gut bacterium and a toxin called LPS reduce a helpful fat chemical (15-HETE) mainly in one type of belly fat in mice, making it harder for them to stay warm in the cold — other fat types didn’t show the same clear pattern.