AN “exercise pill” that could allow couch potatoes to build muscle and burn fat without getting up from the sofa is being tested on patients.

The drug is being developed by GlaxoSmithKline to treat high cholesterol but has also been shown to reduce weight gain dramatically in mice, even when they are fed a high-fat diet, and turn them into marathon runners.

The drug, known as GW 501516, is being tested on patients to find if it can successfully improve their good HDL cholesterol.

The discovery, made by researchers at the Salk Institute for Biological Studies in La Jolla, California, could lead to a quick fix for obesity as well as preventing associated disorders such as heart disease and type 2 diabetes.

The Californian team discovered that activation of a protein called PPAR-delta speeds up the metabolism, increasing the rate at which the body burns fat.

They produced a genetically engineered mouse endowed with the activated form of PPAR-delta in its muscles. The result was a dramatic increase in muscle cells. Professor Ronald Evans, who led the team, said: “By expressing genes for an activated form of PPAR-delta, we created a mouse that could, compared with normal mice, run marathons.”

By turning on PPAR-delta, the team had produced highly efficient muscle fibres that burned fat more rapidly, making it almost impossible for the mice to gain weight even in the absence of exercise.

“This discovery will make it possible to design treatments that change our muscle make-up and help resist obesity and associated metabolic disorders,” Evans said.

To test the concept, he and his team treated normal mice with the GlaxoSmithKline drug GW 501516. These mice also gained less weight when given a high-fat diet.

Although Evans recognises the potential for abuse by athletes, he believes activating the PPAR protein could prevent physical fatigue and enhance the quality of exercise. He also believes it may lead to a new class of drugs to promote weight loss.

A recent Commons health select committee report estimated that obesity, which is responsible for 9,000 deaths a year, was costing Britain £2 billion a year to treat.

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