Scientists have found out why the brain prevents maintaining lost weight

For decades, people have been told that weight loss is a matter of willpower: eat less, move more. But modern science refutes this approach — it's far more complicated than that, researchers say.

This was reported by The Conversation.

For humanity's ancient ancestors, fat reserves were a matter of life and death: too little fat — and starvation could kill, too much — and you became easy prey. Over thousands of years of evolution, the body built a powerful system for protecting energy reserves, deeply embedded in the brain's functioning. The paradox is that in the modern world, where food is available at every turn and physical movement has been reduced to a minimum, these very mechanisms that once saved lives now become the main obstacle on the path to weight loss.

As soon as a person begins to lose weight, the body sounds the alarm as if survival is at stake: hunger hormone production surges sharply, food cravings intensify, and energy expenditure drops. The brain possesses extremely powerful mechanisms for defending body mass — it is capable of "remembering" previous weight and persistently strives to restore it.

For ancient ancestors, this system was a lifesaver: after periods of famine, the body quickly restored its usual mass during more plentiful times. However, in modern humans, the same mechanism becomes a trap — the brain perceives any excess weight gained as a new norm that must be defended. This is precisely why millions of people regain the weight after dieting — and it is not at all about a lack of discipline, but rather that biology is working exactly as evolution intended.

Hope is offered by new-generation weight loss medications — semaglutide and tirzepatide: they mimic gut hormones that signal the brain to reduce appetite. However, they are far from suitable for everyone — some patients experience side effects, while others see no weight loss at all. Moreover, after treatment is discontinued, biology often reasserts itself, and the lost weight returns.

Researchers particularly emphasize: good health is not the same as the "right" number on the scale. Adequate sleep, balanced nutrition, and physical activity significantly improve heart function and metabolism, even if the scale barely budges. Scientists draw special attention to the early period of life — from pregnancy to approximately age seven — when a child's weight regulation system is especially malleable and is shaped by parental nutrition.