Exercise can alter an inactive person's DNA within minutes of them working up a sweat, Swedish researchers say.

Scientists from the Karolinska Institute in Stockholm found that DNA molecules in muscles are chemically modified after just 20 minutes of hard pedaling on an exercise bike.

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The study focused on experiments in which healthy volunteers who did not workout regularly experienced a burst of exercise. Afterwards, a small amount of the participants' muscle tissue was analyzed for chemical changes.

The researchers said that while the underlying genetic code in the volunteers' DNA remained the same, the DNA molecules within their muscles changed in order to "turn on" genes that let the muscles to adapt to exercise.

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Professor Juleen Zierath, the study leader, said, "Our muscles are really plastic. We often say 'You are what you eat,' well, muscle adapts to what you do."

She added, "If you don't use it, you lose it, and this is one of the mechanisms that allows that to happen."

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Writing in the journal Cell Metabolism, the team said further test tube experiments showed that caffeine mimicked the muscle contraction that comes with exercise in a similar way.

However, Zierath warned that drinking coffee was not a substitute for exercising as it would require an almost "lethal" dose to replicate the effects on muscles seen in lab tests.

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