You may recall the sad story of Grete Waitz, the nine-time champion of the New York City marathon and Olympic medal-winner, whose career as a runner may have played a huge hand in her continuing struggle to fight cancer. A recent study that monitored runners in the Boston Marathon shows how the 26.2-mile run may put a patient's heart health at risk.

The 60 runners tracked by scientists during Boston Marathons in 2004 and 2005 were given echocardiograms before and after their races and their blood was analyzed for any signs of cardiac problems. Before their runs, patients showed no signs of any health problems. Afterward, however, was a different story...

For one, most patients experienced notable changes to their heart rhythms. And, perhaps more importantly, 20 minutes after completing their marathons, 60 percent of the runners had elevated levels of troponin, a protein found in cardiac muscle cells that can serve as a marker for heart trauma. In fact, the level of troponin was so high as to signal the destruction of heart muscle cells in 40 percent of the runners.

Although all the indicators disappeared within a few days, scientists found the effect of marathon running was enough to "stun" a patient's heart.

As someone who enjoys running, it took me while to fully appreciate that it's possible to overdo your exercise program. What patients really need: a combination of endurance and anaerobic sprinting or strengthening exercises that help to increase the instantaneous and dramatic demands on your cardiovascular system and prevent heart attacks.

Just one more reason, among many, for treating exercise like a drug that must be prescribed precisely to do the most good.