Four things you probably didn't know about your metabolism -- but should, if losing fat and staying lean is your goal

My metabolism is slow.

It doesn't matter whether I'm standing in line at Starbucks, eating lunch at the mall food court or working out at the gym, I've overhead the phrase time and again. The sizes and shapes of those who utter it vary -- short, tall, large or small -- but is it just another empty excuse? Can so many of us really be suffering from a slow metabolism?

FALLACY: The amount of bodyfat you carry affects your metabolism

FACT: The amount of muscle you carry in your overall body composition determines your metabolic rate

In general, when someone refers to her metabolism, she's talking about her resting metabolic rate (RMR) -- the amount of calories needed to sustain all the body's operations (maintain temperature, transport nutrients in and out of cells, pump blood, breathe, etc.) at rest. And the strongest predictor of metabolism is your fatfree mass, says David C. Nieman, PhD, FACSM, director of the Human Performance Laboratory at Appalachian State University (Boone, North Carolina). "Fat-free mass is everything but the fat tissue," he explains. "It's predominantly made up of muscle but also includes bone tissue and water contained in the body."

It's the muscle that makes all the difference. For instance, if you were to compare your metabolic rate to that of a sedentary woman weighing the same, you'll likely burn more calories at rest than she will because you have more muscle and she probably has more fat due to her inactivity. "Muscle tends to be very metabolic, in terms of burning calories, compared to fat; fat is not an inert tissue, but it doesn't expend nearly the amount of calories as muscle," says Robert Keith, PhD, RD, FACSM, professor of nutrition and food science at Auburn University (Auburn, Alabama). "When you think about fat's job, it's actually to store energy. It isn't going to be a tissue that burns a lot of calories because that would be counterproductive. So metabolism is very much tied up into body composition, and the more muscle you have, the more likely you are to have a higher resting metabolism."

FALLACY: Most people who are overweight have slow metabolisms

FACT: Overweight people actually have faster metabolisms than average

"A lot of people like to blame their metabolism for their weight gain. But it's interesting: Once you know a person's fat-free mass, there's hardly any variance from person to person," Niemen explains. "In other words, humans are very similar when it comes to the energy it takes to keep a kilo or a pound of fat-free mass alive."

The Human Performance Lab at Appalachian State has tested hundreds of people and, according to Niemen, the correlation between fat-free mass and resting metabolic rate is so high that "it is a myth that people, especially larger people, can blame their obesity on a slow metabolism." In fact, it may be just the opposite.

"As you get bigger and bigger, your metabolism increases; it actually works in favor of those people trying to lose weight. Because they're burning more calories, they keep eating and eating too much, and that's why they gain the weight. The metabolism isn't the issue, it's their eating habits," says Niemen.

When it comes to gaining weight, you actually gain some fat-free mass. For every 20 pounds that the average person gains in weight -- that's without training -- one-fourth is fat-free mass and three-fourths is fat. That one-fourth of fatfree mass is supporting tissue to help the body carry the extra weight. "People who have a lot of bodyfat and are still (mobile) have a fair amount of lean mass because they have to; it's almost like a self-imposed weightlifting regimen," Keith points out. "Because if they're up moving around, climbing stairs, they haul a lot of bodyweight around so they actually do compensate for that with some hypertrophy." Therefore, if you're overweight and active, you're getting some increases in metabolic rate -- just not as much as your less-fat counterparts.