As 20,000 runners began their mad dash to Boston on Monday, Dr. Malissa Wood, a cardiologist, four-time marathoner and co-director of the Massachusetts General Hospital Women's Cardiovascular Health Center, was setting up shop in the corner of the medical tent at the finish line.
As soon as they finished, 25 amateur runners strolled (or hobbled) over to Wood's corner to let her sample their blood. They also got a noninvasive test to see how well their hearts worked after the stress of running for four hours.
As they have done every year since 2003, Wood and her MGH colleagues will compare these post-race test results with the pre-race exams performed two weeks earlier.
The MGH findings on Boston marathoners - three published papers to date and two pending - are sobering. They support this idea: while moderate exercise may be the most important thing a person can do for health, taking it to extremes, as in running a 26.2-mile marathon, may not be.
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," Wood said.
"They didn't get into any trouble at all. If they trained less than 35 miles a week, they were in big trouble," she said.
Translated for the rest of us, this means that "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.
About 450,000 Americans run in marathons every year. And 325,000 do triathlon, which involve swimming, biking and running, according to USA Triathlon, the sport's organizing body.
Many of these are not well-trained athletes, but rather newcomers who race to raise money for charities. That means, Siegel said, 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."
The key to healthy exercise, in other words, is moderation and consistency, especially if you are new to a sport.
Moderate exercise is unarguably good for you. "The greatest hazard of exercise is not doing it," said Dr. Harvey Simon. Simon is an avid runner, former marathoner, MGH 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 such as marathoning.
Study after study has shown that moderate, regular exercise can 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 the 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, 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.
To gauge whether you are working hard enough, you can take your pulse or use a strap-on heart monitor, available at many sports stores. If you are healthy, the goal is to work out at about 70 percent of your maximum heart rate, which can be determined by a stress test in a doctor's office.
You can also estimate your maximum heart rate by taking the number 220 and subtracting your age; if you are 60, your maximum heart rate is 160, so your "target" heart rate during exercise should be about 112 beats per minute.
You can also use the subjective "perceived exertion" scale, which runs from 6 (no effort) to 20 (your absolute max). The goal is to have your perceived exertion be about 12 to 14. An even simpler way is to use the "sing/talk test:" Work hard enough that you can't sing but can talk.
Moderation is the key, said Simon of MGH. "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 just walking at the extremely leisurely pace of half an hour per mile has benefits.
In other words, you shouldn't under-do 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, Siegel said.
To avoid this, try not to increase your exercise duration or intensity by more than 10 percent over any two-week period.
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 the University of California, Los Angeles. But for amateurs, finding that balance point can be hard.
As for marathoners, Wood and her MGH 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 only shows up in blood tests 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 have found that another chemical, BNP (for brain natriuretic peptide), also a red flag for cardiac dysfunction, also 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 MGH.
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.
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