'Fat gene' progress makes mice skinny

However, experts say that the chances of finding something that could make diet and exercise redundant is remote.

In some people, eating too much is far more likely to result in obesity - while others remain rake-thin despite eating a high-calorie diet.

They stay thin, say some experts, because their bodies "burn off" extra fat in their food rather than storing it.

Researchers at the Howard Hughes Medical Institute in the US have discovered a body chemical called an enzyme which helps process fat in the liver.

It appears to be key, say the researchers, in whether the body chooses to store the fat - or burn it off.

They found that a special breed of obese mice altered so that they lacked the enzyme were thinner.

The enzyme is closely connected with a hormone called leptin, which also plays a role in human appetite.

Mice genetically altered so they produced neither the enzyme, called SCD-1, or leptin, were, they said "markedly less obese" than the "normal" obese mice.

Mystery remains

However, although the researchers say that the effect is genuine, they do not fully understand why a liver enzyme - or the lack of it - could have such a pronounced effect on the whole body.

However, they are not convinced that a drug to alter SCD-1 activity would be safe.

Professor Jeffrey Friedman, who led the research, said: "Mice lacking the enzyme have abnormalities of glands in the skin and eyes.

"A key question is whether a partial reduction of SCD-1 activity - rather than a complete loss of the enzyme's activity - could alter metabolism without incurring unwanted side-effects.

"There are no guarantees that the benefits would outweigh potential side-effects."

Health threat

Millions worldwide, particuarly in developed countries, are classed as obese, which puts them at risk of diabetes, heart disease and other health problems.

Paul Trayhurn, Professor of Obesity Biology at Liverpool University, said that the findings were "exciting", as they revealed a previously-unknown way by which the body regulates fat.

However, he said: "We are still some way off a way of pharmacologically assisting people who need to lose weight.

"Diet and exercise remain the best ways of doing that."