Training very hard 'as bad as no exercise at all'
Training very hard may be as bad for you as not putting on your running shoes at all, a report in the Journal of the American College of Cardiology says.
Scientists studied more than 1,000 healthy joggers and non-joggers over a 12-year period.
The study suggests jogging at a steady pace for less than two and a half hours a week was best for health.
UK guidance says adults should aim for 150 minutes of exercise each week.
The Danish study asked participants to register how frequently they jogged, for how long and at what intensity - as well as requesting details of their health.
The scientists concluded the ideal pace to jog was about 5mph (8km/h) - and that it was best to jog no more than three times a week or for 2.5 hours in total.
Researcher Jacob Louis Marott, from the Frederiksberg Hospital in Copenhagen, said: "You don't actually have to do that much to have a good impact on your health.
"And perhaps you shouldn't actually do too much.
"No exercise recommendations across the globe mention an upper limit for safe exercise, but perhaps there is one."
This is a small study, particularly when it comes to the people in the most active groups - only 36 were classified as "strenuous" joggers and just two of this group died.
So experts caution this makes it harder to detect and be confident of the differences between each group.
But the paper does build on previous research, such as this study in mice, which suggested exercise affected heart rhythm, and other work which has suggested long-term strenuous endurance exercise can damage the heart.
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."
Update 6 February 2015: This story, which was first published on 3 February, has been amended to give more detail of the scale of the study.