It’s Not Just Body Weight, It’s Body Fat, Too
A similar thing happens in your brain when it comes to how much body fat you’re carrying. “Your brain measures the level of body fat using leptin, a hormone that’s secreted in your bloodstream in proportion to the amount of fat you carry,” says Guyenet. Higher levels of leptin in your bloodstream mean more fat on your body. You may have heard of leptin before, called one of the “hunger” hormones, along with ghrelin. In terms of leptin, it decreases your hunger.
Whenever your weight changes too much, your brain will intervene to push it back to what it thinks is the correct weight for you.
But here’s where things get tricky. Everyone has a certain level of body fat that their bodies are happiest at. “Your brain will defend this amount just like it defends your body temperature,” explains Guyenet. As you lose weight, the amount of leptin in your bloodstream drops — and that’s where the trouble starts. It sends a signal to your brain to help you fight to bring that fat back. Of course, that’s the exact opposite effect that you’re going for. But it’s hard to beat out biology.
Guyenet calls this a classic starvation response. Your brain responds by upping hunger, making those doughnuts in the break room that’ve been sitting out for four hours look actually yummy, and ensures cravings are impossible to ignore. Physiologically, your metabolic rate slows so you can conserve energy and send it right back into building up fat stores, he says. As a 2011 study in the New England Journal of Medicine shows, these “compensatory mechanisms” driving weight regain last for at least one year, a long time to spend battling your body.