Even when obese mice don’t feel hungry, turning on their hunger neurons with light makes them eat normally again—meaning the problem is the brain isn’t turning on the hunger signal, not that the body can’t respond to it.

In obese mice, turning on hunger neurons just before eating doesn’t make them eat much more—but turning them on while they’re eating does. This suggests their brain’s feeding system becomes less sensitive to normal hunger signals.

When mice get obese from eating lots of fat, their brain's hunger neurons stop responding as strongly to fat in the stomach—even after they lose weight—but still respond normally to sugar or protein.

Obese mice don’t get as excited (in brain terms) when they see food—but when they lose weight, this brain response comes back, unlike their response to fat or hormones.

When mice become obese, their brain’s hunger neurons don’t respond as strongly to ghrelin—the 'hunger hormone'—so they don’t eat as much even when given extra ghrelin, and this doesn’t fix itself after they lose weight.