Scientists believe that a one-off 30-minute run, swim or strenuous walk significantly boosts longevity by warding off cardiovascular disease.
It is well known that exercise lowers blood pressure and protects the heart.
But the breakthrough study is the first to suggest a more profound and lasting effect from lower levels of activity.
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The American scientists behind the research have shown that even an isolated, moderate workout can protect the heart from ageing.
The findings were last night welcomed by British experts who stressed “a little exercise is better than nothing”.
Professor Sir Nilesh Samani, medical director at the British Heart Foundation, said: “This new research shows exercise may help protect heart cells from ageing more quickly.
The benefits of regular exercise are not in doubt
"We already know regular exercise has enormous health benefits.
"If you are someone who doesn’t do much exercise at the moment, please remember every 10 minutes of physical activity counts when it comes to your heart health.”
Scientists think those who do little exercise have cells that are biologically much older.
The most inactive have shorter telomeres – tiny caps on the ends of DNA strands – which protect cells from deterioration but progressively shorten with age, increasing the risk of disease.
Researchers at the University of Maryland wanted to see what happens inside heart muscle cells after a single workout.
They taught young, healthy female mice how to run on small treadmills and then returned them to their cages for several days.
Other mice remained sedentary.
The mice were put back on treadmills for 30 minutes, a workout designed to simulate moderate exercise in people.
Tissue samples were taken from their hearts either immediately after they had finished running or an hour later and compared with samples from the sedentary group.
Results published in Experimental Physiology showed that immediately after a single, 30-minute jog, the mice’s heart cells were noticeably different than those that had not moved.
Strikingly they showed higher levels of the proteins directly related to telomere length.
Dr Ian Campbell, a GP from Nottingham, said: “The benefits of regular exercise are not in doubt – it helps improve health and increases longevity.
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What this study does is help explain just how that happens.
“The direct effect of a 30-minute workout is easily explained in part by the changes that occur to blood pressure and heart rate.
But here, for the first time, we might be discovering exercise also makes changes at a cellular level.
If the same applies to humans it’s even more evidence to support the benefits of an active lifestyle in promoting good health and wellbeing.”