Heavy exercise can have the same harmful side-effects as drugs, it's been claimed.
Researchers claim that instead of pounding the pavements or trekking on the treadmill for hours at a time, small bouts of exercise are more effective for keeping in shape.
Daily exercise can prevent heart disease, high blood pressure, diabetes and obesity.
But marathons and other endurance races could cause structural changes to the heart, leading to heart attacks.
Dr James O'Keefe, of St Luke's Hospital in Kansas city, who reviewed previous research, said: "Physical exercise, though not a drug, possesses many traits of a powerful pharmacological agent.
"Daily physical activity can be highly effective for prevention and treatment of many diseases, including coronary heart disease, hypertension, heart failure and obesity.
"However, as with any pharmacological agent, a safe upper dose limit potentially esists, beyond which the adverse effects of physical exercise may outweigh its benefits."