Two studies in the journal Heart suggest that health benefits may be
curtailed in people who exercise very frequently or very intensely.

In a Swedish study, researchers analyzed data from exercise
questionnaires and hospital records of nearly 45,000 men. Men who
exercised intensively more than 5 hours a week at the age of 30 were
more likely to develop atrial fibrillation (an abnormal heart rythm)
than men who exercised less than 1 hour a week. Their risk was even
higher if they subsequently quit exercising later in life.

In the second study, researchers followed more than 1000 patients with
coronary heart disease. Overall, patients who exercised strenuously 2–
4 days a week had the lowest risk for death and cardiovascular events.
But there was an increase in risk in groups that rarely exercised and
in those who exercised vigorously every day.

Editorialists speculate that intensive exercise may have a
proinflammatory effect that may be especially harmful in some people
with atherosclerotic disease.

CardioExchange.