A pill that helps the body to burn excess fat could one day "replace the treadmill", scientists claim.
They discovered how to trigger a molecule that can turn "bad" white fat cells into "good" energy-burning brown fat cells.
People have two types of fat tissue. White fat, which circulates in the blood to fuel muscle, is the type we associate with chubby stomachs.
Alternatively, brown fat is used by the body to generate heat. As we become colder, levels of this fat drop so it is easier to get rid of.
Researchers at Harvard University in the US discovered that a molecule can turn white fat into brown fat and have learnt how to stimulate its production.
When the body takes in excess energy it is stored as lipids in white fat cells. When there are too many calories coming in, and not enough being burnt, adult stem cells in the body produce more white fat cells, which leads to a flabby physique.
Increasing levels of the molecule which the scientists identified, tells the body to make more brown cells, rather than white. Although Harvard has merely shown proof of a concept, it believes that a pill could be created to produce the same effects.
A drug called tofacitinib, which triggers the molecule, is already being marketed for rheumatoid arthritis, and could also be an effective treatment for obesity.
"It's the first step toward a pill that can replace the treadmill," said Chad Cowan, an associate professor in Harvard's Department of Stem Cell and Regenerative Biology unit, who co-authored the study.
"What we saw here was a stable conversion of the white fat cells to brown cells."
Prof Cowan has been working with fat stem cells for more than seven years, looking for ways to prevent disease and obesity. He is in discussions with several pharmaceutical companies about producing a drug, and clinical trials have already begun in Germany.
A quarter of adults in England are obese - a figure that is on course to climb to 60 per cent of men and 50 per cent of women by 2050.
As well as helping people to lose weight, a drug that turns white cells into brown cells would reduce the chance of developing Type 2 diabetes and protect against heart disease.
The findings were reported in Nature Cell Biology.