Extreme endurance exercises may cause structural changes in the heart, says a new study.

The review published in the Mayo Clinic Proceedings looked at studies that had gathered data about the medical effects of excessive endurance exercise like ultra-marathons.

"Physical exercise, though not a drug, possesses many traits of a powerful pharmacologic agent. A routine of 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 pharmacologic agent, a safe upper dose limit potentially exists, beyond which the adverse effects of physical exercise, such as musculoskeletal trauma and cardiovascular stress, may outweigh its benefits," says Dr. James H. O'Keefe of Saint Luke's Hospital of Kansas City, lead author of the study.

Researchers say that physically active people are healthier but excess of such exercises may not really make a heart healthy.

In March this year, an ultra-marathoner Micah True died suddenly during routine training.

"Physically active people are much healthier than their sedentary counterparts. Exercise is one of the most important things you need to do on a daily basis. But what this paper points out is that a lot of people do not understand that the lion's share of health benefits accrue at a relatively modest level. Extreme exercise is not really conducive to great cardiovascular health. Beyond 30-60 minutes per day, you reach a point of diminishing returns," he said.

Not all people respond the same way to an exercise regime. A study published in PLoS ONE pointed out that regular exercise may increase heart attack risk in some people. It is usually advised that people must talk with their doctor before starting any new exercise training.

Many experts believe that a moderate level exercise of 30 minutes a day 5 times a week is an ideal way to stay fit and to delay onset of type-2 diabetes. For children, about 60 minutes of physical activity is recommended.