WE ALL KNOW SOMEONE LIKE JIM, a guy in his late twenties who exercises hard and, as a result, thinks he can get away with eating anything he wants. He chases a four-hour mountain-bike ride with a pepperoni pizza and a pitcher of beer. With his concrete quads and thoroughbred lungs, Jim is hyperfit, which is why he doesn't hesitate to order a slab of carrot cake for dessert. "I'll burn it off tomorrow," he smirks.

But he's wrong—perhaps dangerously wrong. Like many other outdoor athletes of his generation, Jim is overlooking a tenacious enemy that will linger inside him long after he sweats away the calories. One that, without a change in diet, will grow even more worrisome as he exercises into his thirties and forties. And someday, when Jim appears to be in prime middle-age condition, it just might kill him. The culprit is arterial plaque—a by-product of high levels of low-density lipoprotein (LDL) cholesterol, which lurks in foods rich in saturated fat. Over years of après-sport cheeseburgers and Saturday-morning doughnut runs, this plaque builds up on the walls of arteries and hardens. Left unchecked, it will gradually strangle the flow of blood to the heart—or even break off during a routine workout and fatally block an artery.

Rogue plaque is the leading cause of exercise-related coronaries for athletes.

That's exactly what happened to Ed Burke. Three years back, Burke—the renowned Colorado Springs–based exercise physiologist who consulted on the training programs of cycling legends Greg LeMond and Lance Armstrong—dropped dead, at 53, from a plaque-induced heart attack during a lunchtime bike ride. But plaque isn't the only exercise-related heart stopper. Last May, 47-year-old Ed Sheehan, former world-class marathoner and onetime Harvard track coach, died from an arrhythmia-related coronary while running. Frankly, there wasn't much Sheehan could've done. Whereas Burke, by heeding his symptoms—mysterious fatigue and chest pain—and seeking treatment, might have prolonged his life.

Burke's condition was first identified in a 1975 study published in the medical journal Angiology. It took subsequent studies, published over the last ten years in The Journal of the American Medical Association, among others, to confirm that rogue plaque is the leading cause of exercise-related coronaries for athletes and the general population. Unfortunately for baby boomers, the first generation to embrace lifelong exercise, this is a frightening irony. Most of us assume that workouts lead to healthy hearts. But for a heart plagued by plaque, exercise itself can be deadly.

There are no statistics to precisely track incidents like the one that killed Burke, but cardiologists make it clear that his fate is not unique—and that thousands of athletes are at risk.

"I predicted this would be a problem back in the 1980s," says cardiologist Paul D. Thompson, head of the Athletes' Heart Program, at Connecticut's Hartford Hospital. "With more people continuing to exercise later and later in their lives, the danger increases."

And it's not just middle-aged jocks who should worry. According to Thompson, anyone over 30 is liable to face a blocked artery if they don't eat right. Pair that with hereditary factors and the risk of an untimely demise jumps, no matter how fit you feel.

But facing facts now can give you the best chance of running strong into old age. It's never too late to lower your risk of a heart attack. Done right, you can actually improve your odds of avoiding heart disease. "If you aggressively address the situation," says Thompson, "you can lessen the damage."