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Too many women are quick to blame their metabolism when those extra pounds refuse to come off. Not so fast. The idea that a low metabolic rate is always responsible for excess weight is just one of a number of misconceptions about metabolism, says researcher James Hill, Ph.D., director of the Center for Human Nutrition at the University of Colorado Health Sciences Center in Denver. And even if you do have a slower-than-average metabolism, it doesn't mean that you're destined to be overweight.

Because the whole subject can be so confusing, Shape went to the experts to dispel some common myths about metabolism. From pills to chili peppers to pumping iron, read on for the real scoop on what does and doesn't rev up your resting metabolic rate (RMR) to help you shed those extra pounds forever.

Q: We hear about metabolism all the time, but what is it exactly?

A: In simple terms, metabolism is the rate at which your body breaks down the nutrients in food to produce energy, Hill explains. a person with a "fast" metabolism, for example, utilizes calories more quickly, in some cases making it easier to stave off excess pounds.

Q: What are the factors that determine metabolism?

A: Body composition is the primary factor that determines your RMR, or the number of calories your body burns at rest. According to Hill, the more total fat-free mass you have (including lean muscle, bones, organs, etc.), the higher your resting metabolic rate will be. That explains why the average man has a 10-20 percent higher metabolism than the average woman. Likewise, the RMR of a plus-sized woman (whose total body mass, including both fat and fat-free mass, is significantly greater) could be up to 50 percent higher than that of a thin woman. Heredity and hormones such as thyroid and insulin are the other important factors that dictate metabolism--though stress, calorie intake, exercise and medications also can play a role.

Q: So are we born with either a fast or a slow metabolism?

A: Yes. Studies of identical twins suggest that your baseline metabolism is determined at birth. But if you have a naturally slow metabolism, weight gain is by no means inevitable and though it may be harder to shed body fat, it's nearly always possible, says weight-loss expert Pamela Peeke, M.D., M.P.H., an assistant professor of medicine at the University of Maryland in Baltimore. You may never burn calories as rapidly as, say, Serena Williams, but you can raise your RMR to a certain extent by exercising and building lean muscle.

Q: When I was much younger, I could eat whatever I wanted. But over the years, my metabolism seems to have slowed. What's happened?

A: If you can't eat as much as you used to without gaining weight, not enough exercise is probably the culprit. After age 30, the average woman's RMR decreases at a rate of 2-3 percent per decade, mainly due to inactivity and muscle loss, Hill says. Fortunately, some of that loss can be prevented or reversed with regular physical activity.

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