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Q: Is it true that you can damage your metabolism by yo-yo dieting?

A: There's no conclusive evidence that yo-yo dieting does permanent harm to your metabolism, Hill says. But you will experience a temporary drop (5-10 percent) in RMR whenever you significantly reduce calories to lose weight.

Q: What are the best workouts for raising my metabolism?

A: Experts agree that weight training is the most effective way to build and preserve lean muscle, though most seem to concur that the influence of muscle on metabolism is rather slight. Each pound of muscle can raise your RMR up to 15 calories per day, says researcher Gary Foster, Ph.D., associate professor at the University of Pennsylvania School of Medicine in Philadelphia.

In terms of cardio, a high-intensity workout that really raises your heart rate will blast the most calories and provide the biggest short-term metabolic boost--though it won't have a permanent effect on your RMR. (a cardio workout will boost your metabolism anywhere from 20-30 percent, depending on intensity.) After your workout, your metabolism will return to its resting level over several hours but you'll continue burning extra calories in the meantime.

Q: Can the kinds of nutrients you eat affect your metabolism?

A: Most of the scientific data show that food choice has no significant impact on RMR. In other words, fats, proteins and carbohydrates seem to affect metabolism similarly. "The temporary metabolic increase from protein may be slightly higher, but the difference is negligible," Foster says. What does matter is how much you eat. Your metabolism is programmed to decrease whenever you slash calorie intake below what is needed to sustain your basic physiological functions--your body's way of conserving energy when food is in short supply. The more calories you cut, the lower your RMR will drop. For example, an extremely low-calorie diet (fewer than 800 calories a day) could cause your metabolic rate to plunge by more than 10 percent, Foster says. The slowdown is likely to kick in within 48 hours of starting your diet. So to keep your metabolism from nose-diving, you're better off reducing calories in a healthy, moderate way. For safe, lasting weight loss, the average woman shouldn't dip below 1,200 calories a day, Foster adds. To lose a pound of body fat a week, you need to create a deficit of 500 calories per day. The best way to do so, and avoid a major metabolic drop, is through a combination of exercise and diet (rather than through cutting calories alone). For example, you could eliminate 250 calories from your diet, while adding enough activity to burn an extra 250.

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