Even though the body starts using leptin better to burn fat during extreme dieting, it also makes more of a protein (PTP1B) that normally blocks leptin—like the body is both stepping on the gas and the brake at the same time.

Scientists put people on a very low-calorie diet with lots of exercise, then gave them normal food again. They found that a protein called PTP1B went up by 18% during the diet and 45% after, just like the claim said — even though another signal (leptin) was also getting stronger.