Running a few times a week at a moderate pace is the best way to improve health, say scientists, as they warn against overdoing it.
Too much exercise really can kill you, scientists have discovered, after finding runners who speed along at 7mph or more are doing themselves more harm than good.
A study of 1,098 runners found that those who ran the fastest were nine times more likely to die prematurely within 12 years than those who enjoyed a more sedate pace of around 5mph for two or three times a week.
In fact, strenuous runners were as likely to die as those who did no physical activity.
"There may be an upper limit for exercise dosing that is optimal for health benefits," said Peter Schnohr, MD, DMSc, a researcher from the Copenhagen City Heart Study, Frederiksberg Hospital in Copenhagen, Denmark.
"It is important to emphasize that the pace of jogging corresponds to very vigorous exercise.
"When performed for decades, this activity level could pose health risks, especially to the cardiovascular system.
"If your goal is to decrease risk of death and improve life expectancy, jogging a few times a week at a moderate pace is a good strategy. Anything more is not just unnecessary, it may be harmful."
Researchers looked at 5,048 healthy participants in the Copenhagen City Heart Study and questioned them about their activity.
They identified and tracked 1,098 healthy joggers and 413 healthy but sedentary non-joggers for 12 years.
Jogging from 1 to 2.4 hours per week was associated with the lowest mortality and the optimal frequency of jogging was no more than three times per week.
Overall, significantly lower mortality rates were found in those with a slow or moderate jogging pace, while the fast-paced joggers had almost the same mortality risk as the sedentary non-joggers.
Researchers registered 28 deaths among joggers and 128 among sedentary non-joggers. In general, the joggers were younger, had lower blood pressure and body mass index, and had a lower prevalence of smoking and diabetes.
Maureen Talbot, Senior Cardiac Nurse at the British Heart Foundation, said: “This study shows that you don’t have to run marathons to keep your heart healthy.
“Light and moderate jogging was found to be more beneficial than being inactive or undertaking strenuous jogging, possibly adding years to your life.
“National guidelines recommend we do 150 minutes of moderate intensity activity a week. It may sound like a lot, but even brisk walking is good exercise and if you’re bit of a couch potato this is a good place to start.”
The research was published in the Journal of the American College of Cardiology.