(Cologne (Deutsche Presse-Agentur) -- Endurance is important during exercise but the stubborn belief that the body begins to burn fat only after 30 minutes is a myth, according to Ingo Froboese, a professor at the Health Centre of the German Sport University in Cologne.

"The truth is that the body basically has four different energy supply systems that it can draw on depending on the demand and the situation," he said.

Glucose (sugar) metabolism and lipid (fat) metabolism play the biggest role in everyday activities and during exercise. The two systems work almost in parallel, the energy share from each varying depending on the body's requirements.

"Lipid metabolism is the system with the largest energy depot, owing to the gigantic reserves of stored fat," Froboese explained. A man weighing 70 kilograms has about 180,000 kilocalories of stored fat, he said.

Although lipid metabolism is almost always active during exercise, Froboese said, it is much less effective in people who are not physically fit.

"Lipid metabolism is directly involved in providing sufficient energy as soon as exercise begins," Froboese said. But he added that glucose metabolism predominated in unfit people, while fit ones burned more fat than sugar from the very start.

Being the most important energy generator, lipid metabolism should regularly be tapped both in everyday activities and during exercise, Froboese advised. "For this, muscles need enough oxygen in everything they do because muscles can only burn fat when sufficient oxygen is available to them," he said.

Someone who runs without breathing heavily, for example, is attacking his fat deposits. As Froboese put it, "120 million fat cells may resist, but they don't stand a chance against a functioning metabolism."

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