Women gain weight more easily than men because their brains are wired differently.
Researchers at Aberdeen University say hormones responsible for regulating appetite, physical activity and energy expenditure work differently in the sexes.
The discovery could change the way obesity is tackled through more effectively targeted medication.
The study, involving mice, was carried out alongside teams from the University of Cambridge and the University of Michigan.
Researchers were able to transform obese male mice with increased appetite and reduced physical activity into lean, healthy mice but the same transformation did not occur in the female mice.
Professor Lora Heisler, of Aberdeen University’s Rowett Institute of Nutrition and Health, who led the study, said: “The World Health Organisation reports higher rates of obesity in women worldwide, reaching twice the prevalence of men in some parts of the world.
“Currently there is no difference in how obesity is treated in men and women.
“However, what we have discovered is that the part of the brain that has a significant influence on how we use the calories that we eat is wired differently in males and females.
“Cells in this brain region make important brain hormones called pro-opiomelanocortin (POMC) peptides that are responsible for regulating our appetite, physical activity, energy expenditure and body weight.
“We found that the POMC peptides that control appetite in males also control physical activity and body weight.
“But in females they are wired differently.”
The research, published in the journal Molecular Metabolism, could lead to the development of new sex-specific medications.
Prof Heisler added: “More than half of people in the UK are overweight and one in four are clinically obese.
“This is an enormous percentage of the population and given the links established between obesity and serious medical illnesses including cancer, heart disease and diabetes, it is essential that we strive to find new methods to tackle this epidemic to improve our health.
“This study reveals that a sex difference in physical activity, energy expenditure and body weight is driven by a specific source of brain POMC peptides.
“This could have broad implications for medications used to combat obesity, which at present largely ignore the sex of the individual.”