An obesity pill that transforms 'bad' fat to 'good' could replace exercise, helping people shed pounds and with them their risk of type 2 diabetes, heart disease and cancer.

That is the claim by scientists who believe they have made a breakthrough in the battle against the bulge.

They said the discovery could be 'the first step towards a pill that can replace the treadmill'.

Harvard Stem Cell Institute at Havard and Massachusetts General Hospital have identified two compounds that can turn white or 'bad' fat cells into brown 'good' fat cells in the body.

When a person eats too many calories without burning them off, they are stored as white fat cells by adult stem cells.

A team of scientists believe they have made a breakthrough which could see an obesity pill replace exercise as people try to fight their weight (picture posed by model)

That is what causes people to pile on the pounds.

The new study found two small molecules that convert fat stem cells, which would normally produce white fat, into brown-like fat cells.

These brown fat cells burn excess energy and subsequently reduce the size and numbers of white fat cells.

Associate professor Chad Cowan, at Harvard, said: 'What we were really impressed by is that there are some compounds that have this same kind of effect when they are administered to animals, but when you remove them, the effect goes away.

'But what we saw here was a stable conversion of the white fat cells to brown cells.

'You're constantly replenishing your fat tissue so if you were on a medication to convert the cells, each new fat cell would be more metabolically active and would convert to brown fat over time.'

The researchers said this reduces the chances of a person developing type 2 diabetes, or any of the other conditions related to a build-up of fat.

One of the two molecules is already approved to treat rheumatoid arthritis, but scientists warned a pill is some way off.

They warned the compounds could damage the immune system.

Associate professor Cowan warned these compounds 'target the same molecule, and that molecule plays a role in the inflammatory response'.

He added: 'So if you administered them for a long time, the person taking them could become immune compromised.'

The compounds were found by screening a library of about 1,000 compounds.

They said the discovery could be 'the first step towards a pill that can replace the treadmill'. Harvard Stem Cell Institute at Havard and Massachusetts General Hospital have identified two compounds that can turn white or 'bad' fat cells into brown 'good' fat cells in the body (picture posed by models)

The team said a typical pharmaceutical company library has between 1.5 and 2 million compounds, meaning there could be more similar compounds out there.

A German team is currently carrying out tests on the two compounds on laboratory mice and associate professor Cowan added: 'We expect to have results fairly soon.

'The compounds appear to work the same way in mice, but we don't know what the long term metabolic or immune system effects are.

'The good news and bad news is that science is slow; just establishing proof of concept takes an enormous amount of time.

'We thought that working with stem cells would lead to the discovery of new drugs and therapies, and now it's really starting to happen - a decade of hard basic scientific work is paying off.'

White fat cells store energy as lipids and play a role in the development of obesity, type 2 diabetes, and related conditions - including heart disease

Brown fat has been shown in mice to lower triglyceride levels, reduce the insulin resistance associated with type 2 diabetes, and burn white fat.

However the team pointed out that although any pill might help us stay slim, it would not replace the other benefits of exercise.