For Steve French, a 27-year-old medical school student, the decisive moment came when he couldn't button a new pair of pants that had a 40-inch waist. He had run track in high school, but since then he'd been cooped up with his medical texts. Living on pizza and beer, he packed 226 pounds onto his five-foot-eight frame. What he saw in the mirror was an "obese mesomorph, fat and on my way to dying," he says. French had treated heart attack victims, breathless, terrified guys shocked that their hearts could actually be failing. If he didn't want to be one of them, he decided, it was time for a change.

More Americans die of heart disease than from any other cause, and the damage begins early in life. Several studies, including one of 11,000 men younger than 39, show that obesity, high blood pressure, inactivity, and high cholesterol at a young age set the stage for heart disease later on. And while doctors have more and better tools than ever for treating a broken heart (statins to lower bad cholesterol, noninvasive imaging scans to spot blockages, among others), there's nothing more important than focusing on prevention. Now. "Sure, maybe someday we'll be able to reverse heart disease completely," says Dr. Leslie Chong, director of preventive cardiology at the Cleveland Clinic. "But we already know how to prevent it: Be active, don't smoke, maintain a healthy weight."

As it turns out, the most effective ways to build a stronger heart are also the most fun: weekend hikes, football on the beach, long bike rides in the morning before work (see "A Stronger Heart, Season by Season"). Heart health is about being active, getting outdoors, and eating right. Living this way reduces the likelihood of obesity, hypertension, clogged arteries, and other heart-related diseases -- and it's addictive.

That's because your body adapts quickly to an active life. Six months into a vigorous training program and the left ventricular chamber has already increased as much as 20 percent in volume. The chamber walls get thicker. Your muscle cells' mitochondrial engines learn to consume energy more efficiently, while changes in the heart and extremities boost the amount of oxygen the body can consume per minute by 20 percent.

Choosing an exercise program that gives you no pleasure, however, "is a guaranteed way to fail," says New York cardiologist Dr. Richard Stein. That's why outdoor sports, from hiking to surfing, are perfect for turning your health around. Likewise, people will stick with a healthier diet once it dawns on them that fresh food tastes better than the processed stuff they once ate. A heart-healthy diet isn't a matter of taking "huge, willy-nilly doses" of supplements, says Scott Powers, director of the Center for Exercise Science at the University of Florida. It's about broccoli rabe sautéed with garlic, grilled grouper in lime juice.

And your body wants to eat this way. We're built for a diet rich in fruits and vegetables, yet we consume enormous quantities of starch and dairy instead. Omega-3 fatty acids, found in fish, reduce cardiovascular inflammation; omega-6s, found in fried foods and baked goods, increase it. The ratio of omega-3s to omega-6s in our modern Western diet is one to 20. Our ancestors' ratio? One to one.

Your mental state also plays a role. Research has shown that men who anger quickly under stress could be three times as likely to develop premature heart disease and five times as likely to have an early heart attack. Depression can double your risk of heart attack. And sleeping poorly? Forget it. Getting less than six hours a night on average is a critical predictor of early death.