Why does lost weight come back?

Summary

This short explainer, based on a 2025 review in the journal Cell, tackles a question that frustrates many people: why does weight come back after all the effort of losing it?

The video describes a weight thermostat in the brain. Over millions of years, the body evolved strong defences against starvation, and genes explain a large part of why some people hold on to weight more than others. When you lose body fat, levels of leptin, a hormone made by fat cells, fall. The brain reads this as a sign of famine.

It responds in several ways at once: hunger signals get stronger, fullness signals get weaker, the body burns a little less energy, and high-calorie foods start to feel more rewarding. The system that stops us from losing weight is much stronger than the one that stops us from gaining it.

The second half explains why this changes the conversation. Newer weight-loss medicines copy gut hormones that are released after a meal, which tells the brain that energy is coming in and quietens these hunger signals. Your doctor can discuss whether such treatment suits you, and what happens if it is stopped.

Read more about weight regain after stopping GLP-1 medicines.