By Stephen Smith, Globe Staff, 4/1/2003

t is hailed as the zenith of human performance, an event testing the true mettle of athletes -- the marathon, a 26-mile, joint-jolting measure of running prowess and emotional endurance.

But just how good is it for the human body to be pushed so far for so long?

''I don't think anybody can tell you what the optimal level of physical activity is to get all of the health benefits that we all want,'' said Dr. Miriam Nelson, a nutrition and physical activity specialist at Tufts University's Friedman School. ''But the reality is, it's not the marathon.''

But if you do take feet to pavement for this year's Boston Marathon, it's caveat runner.

There are the minor injuries -- pulled muscles, blisters, shin splints. And then there are the potentially life-threatening medical crises -- heat stroke, water intoxication, heart failure.

''It is a double-edged sword,'' said 1976 Boston Marathon champion Jack Fultz, now coach of the Dana-Farber Marathon Challenge team and an adviser to Nike. ''Training for and running a marathon can be a very beneficial thing for one's physical stature, but it can also be very detrimental.''

To be sure, most runners in this year's Boston Marathon will finish the race without incurring major complications. But, as the death last year of runner Cynthia Lucero revealed, marathoning is no trivial pursuit.

Lucero's demise was attributed to drinking excessive fluids during the Boston race, a phenomenon known as hyponatremia -- water intoxication. For runners accustomed to the maxim that too much water is never enough, the death of Lucero demonstrated the perils of downing extreme amounts of liquid. The result -- a profound dilution of sodium in the blood, which in turn can cause lethal brain swelling.

More broadly, researchers have come to understand that the water intoxication of marathoners reflects the body's systemic response to being driven to such extremes. In fact, Dr. David Greenes of Children's Hospital Boston, who is presiding over a major study of runners' health, refers to a marathon as ''a scheduled disease.''

Last year, the research team cobbled together by Greenes tracked 484 Boston Marathon runners. While their findings await formal release, Greenes did reveal that a substantial chunk of the runners had dangerously low sodium levels -- even if they had not complained of symptoms related to hyponatremia.

''There are, no doubt, some health benefits to training as vigorously as people do for marathoning,'' Greenes said. ''But I think there are also clearly some risks. We as a medical community need to understand what the risk factors are for the rare but unfortunate outcomes that we see.''

That is precisely what Dr. Arthur Siegel has been doing after hanging up his own marathon shoes in the late 1980s. His work revolves around the notion that marathoning produces such acute injury to the architecture of the body that a massive inflammatory response ensues.

One manifestation of that, he said, can be found in the blood of runners. After examining samples drawn from 80 marathoners, Siegel concluded that the morning after a race, the balance between blood thickening and blood thinning factors was out of whack. Clotting and inflammatory factors were elevated, putting runners at risk of heart attacks.

The kidneys of runners can be inflamed, too, Siegel reports, further evidence of what happens when asking the human body to cover 26 miles in a few hours. A marathon ''is an overdose of a good thing,'' said Siegel, internal medicine chief at McLean Hospital in Belmont. ''But the point is not to scare people out of doing it. The point is helping them do it more safely.''

That's why Siegel recommends that marathoners consider taking an aspirin to give them an edge against potential blood clotting.

''What I tell people is that a marathon is a very special event,'' said Dr. Lyle Micheli, who has treated injuries for three decades at the Boston Marathon. ''Don't think of it as part of your daily health regimen.''

Stephen Smith can be reached at stsmith@globe.com.

This story ran on page C4 of the Boston Globe on 4/1/2003.
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