Exercise may not always protect you against ill health and may even be harmful to the hearts of some people, researchers say.
A new study analysed data from six exercise studies involving 1,687 regular exercisers to find out the effects of -regular workouts on the heart.
The results shocked and confused the scientists - in almost one in ten people tested, exercise seemed adversely to affect blood pressure, insulin levels or `good` HDL cholesterol levels, the Daily Mail reported.
And in seven percent of people, not just one but two of these risk factors for heart disease were worse as a result of exercise.
According to lead author Claude Bouchard from Louisiana State University, there is no clear explanation for this effect on a small portion of the population but `it`s not a good sign`.
Bouchard suspects that it is down to genetics and advises exercisers to have their blood pressure, cholesterol and glucose checked regularly.
Regular amounts of aerobic exercise like running are known to slow the unhealthy changes to the cardiovascular system that occur with age.
However, intense and prolonged endurance training for many years - for instance, for marathons or triathlons - could be damaging to the heart.
Greg Whyte, author of the study from Liverpool John Moores University, said the research found that 50 percent of long-term endurance and ultra-endurance runners, some of whom had been training for 43 years, showed signs of heart damage.
The study has been published in the journal PLoS One.