working out leads to weight gain

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The situation: You're putting in more effort at the gym than ever.

Why you're gaining: You're letting your body's natural instincts run the show. Eating more is a normal, often unconscious reaction to an amped-up exercise regimen, says Glenn Gaesser, PhD, a professor of exercise science and health promotion and the director of the Healthy Lifestyles Research Center at Arizona State University. What surprised us is that moving less when you're outside the gym is another typical side effect. "It's a built in self-preservation response," he says. In a study Gaesser led, published in The Journal of Strength and Conditioning, 81 sedentary and overweight but otherwise healthy women started a fitness regimen, and after 12 weeks, even though all subjects were more aerobically fit, nearly 70 percent lost less fat than predicted. A few even gained up to 5 pounds of body fat. Food intake and activity levels outside the gym weren't being monitored, but Gaesser believes the women who gained fat were probably unknowingly compensating for their sweat sessions.

How to stop it: The study found that early changes in weight were predictive of later changes, so if you find that your scale is moving in the wrong direction during the first few weeks of increased exercise, it will likely keep going, so take a hard look at your food intake and activity levels outside the gym.