For those who cannot exercise, the benefits of fitness training could be delivered in a tablet. Researchers believe an "exercise pill" could transform the lives of people who are unable to exercise because of obesity or physical disabilities.
Hopes for such a pill emerged last week when scientists found that an experimental drug allowed mice to run on a treadmill for 270 minutes before exhaustion set in. Mice that went without the drug lasted only 160 minutes.
The endurance boost was accompanied by other apparent benefits. Mice which had the drug for eight weeks put on less weight and had better control of blood sugar levels, suggesting that such a pill might also help people with diabetes.
Scientists led by Dr Ronald Evans at the Salk Institute in San Diego made the discovery when they set out to explore what endurance meant on the molecular level. He said: "If we really understand the science, can we replace training with a drug?"
The scientists turned to a drug known as GW501516, which had previously been shown to improve stamina and burn fat faster.
Through a series of tests with mice on treadmills, Dr Evans found that the drug changed the activity of nearly 1,000 genes. Many of the genes that became more active were involved in the burning of fat. But other genes were suppressed.
Writing in the journal Cell Metabolism, the scientists described how the findings might explain why athletes can "hit the wall" when they push themselves hard.
The drug makes the body burn fat faster, but burns sugar more slowly. On the drug, the drop in blood sugar level that is responsible for the hitting-the-wall feeling happens much later than normal.
"In endurance sports such as cycling, marathons and race-walking, 'hitting the wall' is a dramatic demonstration of sudden and complete exhaustion," the scientists wrote.
The drug, which achieves its effects via muscle proteins called PPARD, "is sufficient to dramati- cally improve endurance capacity".
Dr Weiwei Fan, the paper's first author, said: "Exercise activates PPARD but we're showing that you can do the same thing without mechanical training. It means you can improve endurance to the equivalent level as someone in training, without all of the physical effort."
It means you can improve endurance to the equivalent level as someone in training, without all of the physical effort.
DR WEIWEI FAN, the paper's first author, on how the drug can improve endurance capacity without mechanical training, such as the use of treadmills.
The compound was originally developed in the 1990s to treat metabolic and cardiovascular disease. It was later abandoned, apparently after some studies found that high doses might cause cancer. But despite the drug being dropped commercially, scientists continued to study the compound.
A decade ago, tests in animals showed that it could potentially boost stamina. The finding spawned a black market for the drug and its subsequent abuse by some athletes in the 2008 Beijing Olympics. The World Anti-Doping Agency then banned the drug and warned that it was not safe.
Dr Ali Tavassoli, professor of chemical biology at Southampton University, who was not involved in the latest study, said any "exercise pill" could potentially be abused, not only by athletes, but also by horse trainers and others.
He said: "But there are people who, for one reason or another, cannot exercise. A pill might allow them to get to a place where they can start to exercise for real."