Today, as an estimated 20,000 runners begin their mad dash from Hopkinton to Boston, Dr. Malissa Wood, a cardiologist, four-time marathoner, and codirector of the Massachusetts General Hospital Women's Cardiovascular Health Center, will be setting up shop in the corner of the medical tent at the finish line.
As soon as they're finished, about 25 amateur runners will stroll or hobble over to Wood's corner to let her take a sample of their blood. They will also get a noninvasive test to see how well their hearts are working after the stress of running for about four hours. And, as they have done every year since 2003, Wood and her Mass. General colleagues will compare these postrace test results to the prerace exams done two weeks earlier.
The Mass. General findings on Boston marathoners -- three published papers to date and two pending -- are sobering and lend support to the idea that while moderate exercise is perhaps the most important thing a person can do for health, taking it to extremes, like a marathon, may be dangerous.
Among marathon runners, the biggest cardiac risk seems to arise in people who train the least. People who worked up to a marathon by running at least 45 miles a week for at least three to four months ''were golden. They didn't get into any trouble at all," said Wood. ''If they trained less than 35 miles a week, they were in big trouble."
Translated for the rest of us, this means that for people who are not in peak shape ''sudden, strenuous activity can trigger a heart attack," said Dr. Arthur Siegel, a 20-time marathoner and director of internal medicine at Harvard's McLean Hospital in Belmont.
Roughly 450,000 Americans run in marathons every year. And 325,000 do triathlons, which involves swimming, biking, and running, according to USA Triathlon, the sport's organizing body. Many of these are not well-trained athletes but newcomers who race to raise money for charities. That means, said Siegel, that in many such events, participants ''are getting older and slower. That's where the cardiac risk comes in, especially for middle-aged men with previously silent heart disease."
Moderation is the key, said Dr. Harvey Simon, an avid runner, former marathoner, Mass. General internist, and author of ''The No Sweat Exercise Plan," which advocates very moderate exercise -- even as moderate as gardening and housework -- instead of extreme exertion like marathoning.
''The greatest hazard of exercise is not doing it," said Simon.
''I used to preach 'No pain, no gain,' but now I say, ''No pain, big gain,' " he said. The whole ''aerobics doctrine" that a person needs a lot of strenuous exercise ''inspired the few but discouraged the many," he said. Even walking at the extremely leisurely pace of half an hour per mile has benefits.
Study after study has shown that moderate, regular exercise can indeed reduce the risk of heart disease, diabetes, stroke, hip fracture, and some kinds of cancer.
But exercising moderately takes patience and persistence. If you have not been exercising regularly, you should work up over several weeks to walking 45 minutes a day at least five days a week, said exercise physiologist Kerry Stewart at Johns Hopkins School of Medicine.
At first you may have to stop every few minutes and rest, he said. That's fine -- just start up again. If you get chest pains or severe shortness of breath, of course, stop and call your doctor. If you have heart disease or have had a heart attack, check with your doctor before starting or substantially increasing your workouts.
You shouldn't underdo exercise, but you shouldn't overdo it, either. Chronic fatigue, trouble sleeping, muscle tiredness, nagging congestion or sore throat, persistent aches and pains, and depression are common signs that you may be working out too hard, said Siegel. To avoid this, try not to increase your exercise duration or intensity by more than 10 percent over any two-week period.
The easiest way to gauge whether you're working hard enough is to use the ''sing/talk test": Work hard enough that you can't sing but can talk.
Experienced athletes ''know how delicate the balance is between training to obtain optimal performance and overtraining to the point where muscle function begins to deteriorate," said Dr. Christopher Cooper, an exercise physiologist at UCLA. But for amateurs, finding that balance point can be hard.
As for marathoners, Wood and her Mass. General colleagues have found that running 26.2 miles can lead to clear signs of cardiac stress. They have found that cardiac troponin, a chemical that shows up in blood tests only when heart muscle is damaged, rises in 60 percent of runners, and in some, it rises so high that ''if you had just looked at these scores, these people would have been admitted to the hospital for heart attacks," Wood said.
They've found that another chemical, BNP, or brain natriuretic peptide, another red flag for cardiac dysfunction, goes up after a marathon in 60 percent of runners. Platelets also become activated and more likely to form the clots that can trigger heart attacks, according to a just-published paper by Siegel and Alexander Kratz, director of the hematology lab at Mass. General. And, as shown on echocardiograms, the heart's ability to relax after each beat remains impaired for at least several weeks in most marathoners.
Bottom line? You don't have to run a marathon to get into good shape. Just put on comfortable shoes, get out, and walk. Moderately. And consistently.
Judy Foreman is a freelance columnist who can be contacted at email@example.com