A 'fat controller' in the gut could be the key to preventing obesity, diabetes and heart disease, research suggests.
Scientists have pinpointed an enzyme that determines whether the fat we eat is burnt off as energy or stored in the body.
The breakthrough raises the prospect of a pill being developed which targets the enzyme in people, allowing them to eat without worrying about putting on weight.
Research at the University of California focused on MGAT2, an enzyme found in the intestines of mice and humans.
Mice without the protein were able to eat a high-fat diet while remaining slim and healthy.
The fat they absorbed was burnt off as energy, rather than stored, the journal Nature Medicine reports.
The mice in the experiment also seemed better at processing sugar, cutting their risk of diabetes, and had lower levels of 'bad' cholesterol in their blood.
A pill that targets the enzyme in people could provide a new weapon in the battle of the bulge.
The researchers said: 'Our studies identify MGAT2 as a key determinant of energy metabolism in response to dietary fat and suggest that the inhibition of this enzyme may prove to be a useful strategy for treating obesity and other metabolic diseases associated with excessive fat intake.'
With almost a quarter of men and women obese and children faring little better, such a drug is likely to have mass appeal.
Even more appealing is the prospect of a pill that makes the body fit, as well as keeping it slim.
Last year, US scientists unveiled an experimental drug which fools the muscles into thinking they have worked long and hard, boosting fitness as well as burning off fat.
Mice treated with AICAR for four weeks burned more calories and had less fat than untreated mice and when tested on a treadmill, they could run almost 50 per cent longer.
Researcher Professor Ronald Evans, of the Salk Institute in California, said: 'We have exercise in a pill.
'It is tricking the muscle into "believing" it's been exercised daily.
'It proves you can have a pharmacological equivalent to exercise.'
But obesity experts say such pills are years from the market, and most people would benefit from eating less and exercising more.