Metabolism Basics

When it comes to watching your weight, metabolism is a hot topic -- it's also one of the most confusing concepts around. Essentially, metabolism is the rate at which your body burns calories and is comprised of three things. For starters, there's your resting metabolic rate (RMR), or the number of calories spent to keep you alive (i.e. breathing). "Everyone has a different RMR, which is usually determined by genetics," says Pamela M. Peeke, M.D., M.Ph., F.A.C.P., clinical professor of medicine and Pew Foundation Scholar in nutrition and metabolism at the University of Maryland in Bethesda and author of "Fit to Live."

Then there's your lean body mass, or specifically, your muscle mass. "The more muscle you have, the higher your metabolism," Peeke says. Why? One pound of muscle burns 35 to 50 calories a day, compared to a pound of fat, which burns only five to 10 calories a day.

Finally, your thyroid plays into metabolism, which is why Peeke recommends that everyone over age 30 talk with their physicians to have a simple blood test to make sure it's working properly.

Measuring Metabolism

To determine your RMR, you can get tested with a device called the Body Gem, which calculates it through your breath. Registered dietitians, personal trainers and health clubs may offer this service.

Or you can get a rough estimate of your daily metabolic needs with a simple calculation using an average RMR. In general, RMR for the average-sized man, who's not an elite athlete, is about 1,800 calories a day. For the average-sized woman, RMR hovers around 1,400 calories a day.

Now multiply that RMR by a physical activity factor. If you're getting at least 30 minutes of moderate activity every day, multiply by 1.4. If you're not very active, multiply by 1.2 or 1.3. If you're super active -- logging more than 30 minutes of moderate to vigorous intensity exercise daily -- multiply by 1.5. (Elite athletes might even multiply by as much as 2 or above.)

You now have an approximate number of calories your body needs each day to maintain your current weight.

The Age Factor

Metabolism tends to decline with age. But the decrease is mostly due to loss of muscle. A woman, for instance, might drop 10 pounds of muscle mass between ages 35 and 50 due to lack of physical activity. (Men, however, drop less through disuse.) If each pound of muscle burns 35 to 50 calories, that woman will then burn 350 to 500 calories less each day. And if she doesn't adjust her eating to reflect that change, she'll gain weight.

Peeke notes, though, that by the seventh decade, everybody drops some muscle mass due to sarcopenia -- a normal part of aging. If, however, you can maintain as much muscle as possible throughout life, you can minimize these effects.

Can You Change Your Metabolism?

Yes, and the top strategy is strength training. "By adding muscle to your frame, you'll heat up your body," Peeke says. Do strength training two to three times a week.

But don't stop there. Get at least 30 minutes of daily aerobic exercise and then find ways to be more active in your daily routines. For instance, fidget frequently, take hourly stretch breaks or walk as you chat on the phone. "Any movement will help stoke your metabolic drive," Peeke says.

She also recommends eating small meals every three to four hours. "Otherwise, your body will glaciarize, thinking it's going into a famine, and your metabolism will slow," Peeke says. And don't pay attention to supplements or gimmicks that claim to stoke your metabolism. The only burn you'll feel is in your wallet. Peeke says, "Increase your activity, decrease your portion sizes, and you'll naturally boost that metabolism."