Will a slow metabolism make you fat? Can spicy foods spike it? Could a pill speed it up? SHAPE answers your most burning questions.

By Stacy Whitman

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.

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.

Q: Can't spicy foods, such as chili peppers and curry, boost metabolism?

A: Yes — but unfortunately not enough to have an effect on weight loss. "Anything that increases your body temperature will temporarily raise your metabolic rate to a certain degree," Peeke says. But with spicy food, the increase is so small and short-lived that it doesn't have an impact that will show on the scale.

Q: What will happen to my metabolism if I lose weight?

A: As you lose weight, your RMR will slow down because you have less body mass to support. As a result, your body requires fewer calories to sustain its vital functions. Consequently, you won't need to eat as much to feel satisfied and to fuel your exercise. If you don't further modify your eating and exercise habits, you'll eventually hit a weight-loss plateau. To get past the plateau and continue shedding pounds, if that's your goal, consume fewer calories (without dropping too low) or increase the intensity or duration of your workouts.

Q: What about supplements and other products that promise to elevate metabolism and melt fat?

A: Don't believe them! No pill, patch or potion can magically raise your metabolism enough to help you lose weight, Peeke says. If you want a quick metabolic boost, you're better off hitting the gym or going for a brisk walk.

Q: Can certain medications slow my metabolism?

A: Some drugs, like those used to treat depression and bipolar disorder, have been shown to lower metabolism. If you're taking a medication that causes weight gain, ask your doctor if there is an alternative drug you can try.