Indeed, stepping off the treadmill isn't an issue for most Americans; according to the International Health, Racquet and Sportsclub Association, only about 14 percent of us have gym memberships, and of those, says Medical News Today, fewer than 20 percent use them. But among this urban, fitness-minded minority, Cardiello sees lots of guys tip into exercise compulsion when they start to gain notoriety as a resident Ironman. "They become a celebrity at the gym," he says. "So why go out in a social setting?" The guys also stick together for safety; their slacker friends are liable to tempt them with alcohol and greasy food. "Those people aren't supporting their habit," Cardiello says. "Drug users only hang out with other drug users."

During the height of his exercise dependency, Baccaro quit drinking with his friends. "It just didn't feel as important," he says. "I wanted to get myself in shape instead. Not that I don't appreciate them, but I can find things to do other than sitting at the bar getting fat."

Though compulsive exercise can lead to health problems, gym junkies have a hard time believing that they may be harming their bodies; after all, what they're doing is supposed to be good for them. This notion is reinforced by the high they get after a grueling workout—the same one you get from snorting cocaine. Both affect the mesolimbic dopamine system in the brain's pleasure center, which rewards behaviors (like, say, running) that contribute to survival as well as those (like inhaling nicotine) that do not. When the body rewards an action by making you feel good, you feel compelled to repeat that action. However, the more you get used to exercise, the less dopamine you produce and the more exercise you need to do to reach that high. "The same thing happens when you're a cocaine addict," Adams says. And if you keep at it long enough, you become increasingly prone to injury and emotional instability. "People who exercise tend to be less anxious, less depressed, less stressed," Adams says. "When you exercise obsessively, you get the opposite of that."

Mark Sisson, a former marathon runner and triathlete and the author of the exercise guide The Primal Blueprint, was constantly pursuing that rush, logging more than 100 miles a week. "I was always thinking, 'My God, when is this run going to end? I'm going to feel so good when it's over,'" he says. He eventually quit chasing the dragon. Then, about three years ago, he had an epiphany. "I realized that at no point, from the time the gun went off to the time I crossed the finish line, was I having fun," he says. "Guilt drives a whole lot of people to the gym, and that's unfortunate." As for Baccaro, he now logs only 10 or 11 miles a week, but he's having a hard time accepting that. "I hate to even admit how little I run," he says.

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