WHEN THE LATEST REPORT ON THE LINK between exercise and good health was published in mid-April, you could practically hear spandex snapping and sneakers being laced. A 26-year study of 17,321 Harvard University alumni by Harvard and Stanford researchers found that “vigorous” activity—defined as activity that burns at lease 1,500 calories a week—lowered their risk of dying by 25 percent. A lower risk of dying? Athletes everywhere high-fived each other. Scientists and physicians sniffed, “We told you so.” The media went into overdrive: The study’s findings were reported on most TV newscasts and made the front page of the New York Times and major Texas newspapers.

About the only person who wasn’t impressed by the study, it seems, was Kenneth Cooper, the Dallas physician who is the nation’s most famous fitness expert. For the past year, the 64-year-old Cooper has been arguing the opposite of what the new study suggests: Vigorous exercise, he contends, is bad for you. In truth, his beef is partly semantic: While the new study describes 45 minutes of brisk walking five times a week as vigorous exercise, Cooper says, “That’s not what I call vigorous.” Such exercise, he insists, is moderate, and he agrees that moderate exercise can be good for you—but only up to a point. Kenneth Cooper believes too much exercise of any kind is dangerous and potentially life-threatening.

The news that exercise may be deadly is shocking enough, but what’s even more shocking is that it’s being delivered by Cooper, the guy who got America exercising in the first place. It was Aerobics, Cooper’s 1968 best-seller, that made the earliest case for getting in shape: “Vigorous activity,” he wrote, “has more and more proved worthwhile both as preventive medicine and as a cure.” In the book’s appendix, Cooper published a slew of charts detailing how a person could achieve the target level of exertion by following his system of aerobics (a word he coined from the adjective “aerobic” meaning “occurring only in the presence of oxygen”). Points were tabulated on the basis of completed distance. A round of golf (“No motorized carts!”) earned you three points, the same as running a mile; running a mile five times got you fifteen points. The weekly ideal, Cooper asserted, was thirty points.

The book was revolutionary, shaking up the sedentary sixties. Before its release only 100,000 eccentrics called themselves joggers, but by late 1968, the nation’s trails were overflowing, and now more than 34 million people run regularly. Also in 1968, heart attack rates peaked and then began to drop steadily. And Aerobics brought instant fame to the unassuming cooper—here and around the world. (To this day, Brazilians call aerobic workouts “Coopering.”) Among other things, its success allowed him in 1970 to open the Cooper clinic and the Cooper Clinic and the Cooper Institute for Aerobics Research. At the complex, which sits on 32 acres on Preston Road, a team of specialists consults with the nearly five thousand patients who come to the clinic each year and pay up to $1,200 for stress and fitness tests, complete physicals, nutritional analyses, psychological workups, and the like. Not surprisingly, many of Cooper’s patients are celebrities in their own right: Ross Perot, Roger Staubach, Tom Landry, Linda Gray, Susan Howard, and even Governor George W. Bush, who often jogged on the clinic’s well-manicured course before moving to Austin.

Yet while the arc of Cooper success has been consistent for nearly three decades, his thinking has not. He began to have doubts about the benefits of exercise in 1984, when his 52-year-old friend Jim Fixx, the author of The Complete Book of Running, dropped dead of a heart attack while jogging. For the next few years, Cooper fixated on Fixx, trying to understand whether his death had anything to do with his sixty-mile-a-week running habit. Eventually, unwilling to concede the dangers of exercise, Cooper wrote a book, Running Without Fear, that linked Fixx’s death to hereditary factors.

Then similar stories began to drift in. There was the woman who spent ten hours a day teaching aerobics until she developed a melanoma on her jaw and was dead within the year. There was the super-athlete, a winner of Ironman competitions, who a deadly melanoma. There were the elite athletes who trained to exhaustion and came down with terminal illnesses: Fred Lebow, the founder of the New York City marathon—dead of a brain tumor; running guru George Sheehan—dead of prostate cancer; master marathoner Werner Tersago—dead of a brain tumor; world record holder Sy Mah, who had run 524 marathons—dead of cancer; Steve Scott, the first American to break a four-minute mile one hundred times—battling testicular cancer; marathoner Mark Conover, winter of the 1988 Olympic trials—now fighting Hodgkin’s disease; and Olympic runner Marty Liquori—now in the advanced stages of leukemia. Today Cooper claims to know of 150 such cases, including 94 athletes with prostate cancer who are patients at his clinic. The anecdotal evidence, he says, is overwhelming. Too much exercise can kill you.

IN HIS SPACIOUS WOOD PANELED OFFICE ON THE clinic grounds, Kenneth Cooper likes to barrage visitors with charts and statistics and endless talk about the science of good health. Seated behind his desk, he does his best to stay calm; and much of the time he has the serious demeanor of someone consumed with living his life right. But often he bounds out of his chair to take a phone call or greet a patient, and he habitually rises up and down on his toes, as if he can’t wait to break into a sprint. For a man near retirement age, he has an unusually athletic frame and impressive energy. Just as he cares for his patients’ well-being, he clearly cares for his own.

And just as he is direct with his patients when diagnosing what ails them, he is refreshingly matter-of-fact when he explains his shift in thinking. “At the time, I knew scientific evidence had established that regular exercise was essential to good health and an effective life,” he says. “But I erroneously assumed that more was better—that the longer you ran, cycled or swam, the healthier you would be. As Cooper is quick to point out, he never claimed to be 100 percent certain about exercise; in the epilogue to Aerobics he wrote, “I’d be the first to admit we still have a long way to go….Many more and larger studies need to be done before conclusive and final data can be obtained. And he has changed his mind before. Early in his career, Cooper opposed weight lifting because he believed bigger muscles put undue strain on the heart, but he today encourages his patients to do strength training. SO when the anecdotal evidence seemed to suggest that there might be a point at which the benefits of exercise cease, Cooper began asking questions.

He first got some answers in 1989, when his own institute released the results of a long-term study that purported to show the relationship between levels of activity and death rates in 13,400 people. Plotted on a graph, the findings represented a reverse J curve: Over five categories of fitness ranging from poor to elite, death rates dropped off the more active the subject was—unless the subject was exceptionally active, in which case death rates rose slightly. Apparently, there was something about “distress” exercise that broke down or destroyed the bodies immune system, but what?

That something, Cooper surmises, is a free radical. A free radical is a molecule of oxygen that has been damaged by some kind of stress, such as cigarette smoke, pesticides, car exhaust, or excessive exercise, until it has lost one of its electrons. It then becomes dangerous, a rogue molecule in search of stability. The avenue to stability is to steal an electron from an adjacent cell, a process that causes oxidative damage in the muscles, liver, blood, and other tissues. As far back as 1928, Cooper later wrote, a study of cyclists showed that free-radical damage could be measured in the amount of pentane—a residue of free radicals—in their breath. Cyclists who exercised for twenty minutes at less than 50 percent of their maximum exertion level had a normal amount of pentane, but those who exercised at 75 percent of their maximum level had a nearly twofold increase. One 1988 study showed that the blood of ultra-marathoners who ran a 50-mile race had an unusually high level of thiobarbituric acid reactive substances, or TBARS—a marker for oxidative stress. By contrast, a 1990 study for runner who ran a 13.1-mile race showed no rise in TBARS.

There are two ways, Cooper believes, to curtail the effects of free radicals. One is to cut back on exercise. “If you are running more than fifteen miles a week,” he is fond of saying, “you are running for some other reason besides your health.” Cooper advises walking two miles in less than forty minutes five times a week, and he more or less follows his own advice: He runs or walks five or six times a week, for a total of twelve to fifteen miles.

The other way to combat free-radical damage, Cooper says, is to employ the only known antidote, antioxidants, a group of vitamins—C, E, and beta-carotene—believed to counteract oxidation in the body. Cooper has so much faith in antioxidants that he wrote a book about them: Antioxidant Revolution, published last year. The only catch is, he prescribes dosages far in excess of the government’s recommended daily allowance. Indeed, Cooper’s antioxidant cocktail—a minimum of 1,000 milligrams of C, 400 units of E, and 25,000 units of beta carotene—is considered massive by most physicians, many of whom take vitamins themselves but are slow to recommend them to their patients. Still, Cooper believes that the weight of the evidence backs him up, and many researchers agree. “His views reflect a growing view in the scientific community,” says Scott Grundy, the chairman of the department of human nutrition at the University of Texas Southwestern Medical Center.

Here too, Cooper follows his own advice: He takes C, E, and beta-carotene every day. And his vitamins aren’t anything fancy: He buys generic pills in bulk from a Dallas grocery. “I want my program to be safe, effective, and cheap,” he says. “It costs almost nothing except discipline.

CLEARLY, KENNETH COOPER IS NO stranger to discipline. The son of an Oklahoma dentist, he earned undergraduate and medical degrees at the University of Oklahoma and a degree in public health from Harvard before signing on as an Air Force doctor at age 26. Within a few years, he won wide acclaim for developing an aerobic conditioning program for astronauts to replace the Royal Canadian Air Force exercises that they had been using, and everyone agreed he was going places. “I was about to be promoted to full colonel,” he says. “I was seven years away from retirement and a nice military pension.” But Cooper wanted out: His dream, he says, was to start a preventive medicine center and do research on aerobic exercise. So in 1970, he quit the Air Force and moved to Dallas with his wife, who was pregnant, and no income. “People thought I was crazy,” he says cheerfully.

Part of what got Cooper through that period was his faith. He was raised in a in a strict Baptist household and embraced the rigor of the upbringing; in fact, at age sixteen, he considered becoming a medical missionary. Even today, Cooper begins every morning with fifteen minutes of Bible study and prayer. The shelves in his office are crammed with books by religious writers such as Charles Swindoll, who is the president of the Dallas Theological Seminary. Cooper counts among his friends Billy Graham and Oral Roberts, whose university in Tulsa houses the only other Cooper aerobics center. And, of course, there’s the way he pleads his case for good health, which can best be described as preaching. When he rattles off the 22 components of a good medical exam or the year a certain study was written up in the Journal of the American Medical Association, it’s the closest thing you’ll hear to a sermon this side of church.

Cooper’s new crusade, as it were, is the high cost of health care, a subject that gives him fits. “The whole concept of health and medicine is fouled up,” he complains. “We should be paying for preventive care, not for the consequences of ill health.” Cooper is so committed to the cause that he traveled to Washington last summer to lobby politicians on behalf of his own recommendations for health care reform: public campaigns to reward healthy behavior, periodic preventive screenings, and tax incentives for preventive care. “Prevention is a low-cost option for health care reform,” he says.

At the same time, Cooper is continuing to focus his energy on the link between exercise and good health. At the moment, he is midway through a study of two groups of men—some who run ten to twenty miles a week—to see whether they show evidence of toxic brain syndrome, which he suspects may be the precursor to the deadly disorders that befall elite athletes. Cooper is committed, he says, to pinpointing the connection between vigorous activity and real body damage. “It kind of gets to me,” he says, “because I’m the one who got all this started in the first place.”