Fat tissue is considered by the body to be a reserve to save. Before the modern era, man had to resist famines in order to survive and he then drew energy from this precious fabric in case of famine. So that when the level of fat decreases (whatever its initial level), the fat cells send messages to the brain to ask it to do everything to recover the lost fat. The brain runs: it then decreases energy expenditure and causes a increased feeling of hunger. This phenomenon makes it possible to stop losing weight after a certain time: we always eat in the same way, but as energy expenditure has fallen, the weight stabilizes. It is enough then that we eat a little more for the weight to start again on the rise!

When energy intake suddenly increases (this is the case for example after stopping smoking or following a psychological disorder leading to eating more), weight follows the same path. But, very quickly, the body adapts. The increase in weight leads to an increase in active cell mass, and therefore, in the same way, basic energy expenditure (the minimum for the body to continue to function). Expenses and contributions are then balanced again, which marks thestopping weight gain. This is why we always gain weight in stages! A further increase in food intake or a decrease in physical activity then again leads to weight gain.