10 Truths About Metabolism

One: What is metabolism?

It refers to the amount of energy (calories) your body burns up in a day. Sixty to 70 percent of your daily caloric expenditure goes into your resting metabolic rate (RMR), the energy required to keep your body functioning. People who eat like a horse and never gain weight—said to have fast metabolisms—are born with high RMRs, meaning it takes a lot of food for their particular bodies just to stay alive.

Two: How do you measure it?

The most accurate method uses large, indirect calorimeter machines in labs or hospitals under strictly controlled conditions. New handheld models are less precise, but they're an improvement over the widely used Harris-Benedict formula, which calculates your metabolism based on age, height, weight, gender and activity level. The Harris-Benedict is less accurate the more overweight you are.

Three: How much can one person's metabolism differ from another's?

If age, gender, height and body composition are the same, the variation in resting metabolism is likely to be less than 3 percent, according to Steve Smith, M.D., an Assistant Professor of Endocrinology at the Pennington Biomedical Research Center of Louisiana State University. For example, if two equally active 38-year-old women are both 5'5" and weigh 130 pounds, one might have a daily RMR of 1,800 calories and the other, 1,854 calories—an increase that only buys the second woman about two walnuts per day.

Four: Do overweight people have slower metabolisms?

No. The opposite is usually true, because the more you weigh, the harder your body has to work, says Pamela Peeke, M.D., Assistant Clinical Professor of Medicine at the University of Maryland and author of Fight Fat After Forty.

Five: Can you crank up your RMR?

Ah, the $64,000 question. Although the idea has been challenged, our experts agree that replacing fat with lean muscle does speed your RMR—just not that much. In fact, the effect of strength training may not be as dramatic, or attainable, as fitness coaches suggest. If you don't want to push yourself that hard, the good news is that to lose weight you really just need to get out and move—walk the extra mile, play softball, swim. Regularly.

Six: Do the supplements that promise to speed up your metabolism deliver?

The boosters du jour—caffeine drinks and green tea—will increase RMR by tiny amounts, Smith says, but even the controversial herb ephedra, which has caused dangerous side effects, including death, will give only a five to eight percent spike, maybe allowing you an extra third of a candy bar.

Seven: Does metabolism slow as you age?

Yes, says Jeanine Albu, MD, chief of the Metabolism and Diabetes Clinic at St. Luke's-Roosevelt Hospital Center. "For women, there's a big drop with menopause." Miriam Nelson, Ph.D., Director of the Physical Center for Activity and Nutrition, estimates that between ages 40 and 80, an average person's RMR slows down by 25 to 30 percent.

Eight: Does yo-yo dieting permanently lower metabolism?

No, says Rudolph Leibel, M.D., Chief of the Division of Molecular Genetics, Department of Pediatrics, at Columbia University College of Physicians and Surgeons. "You can succeed and fail as many times as you like without permanently affecting your metabolism."

Nine: How does dieting in general affect it?

Well...nature takes a cruel turn here. Programmed to withstand starvation, our systems slow to conserve energy when we're not getting enough fuel. "As soon as you restrict your calories, your RMR goes down," says Nelson. Then, to add insult to evolutionary injury, as your body gets lighter, it requires less energy to move and maintain itself.

But don't give up: Instead, get up and go. Increasing your activity level seems to decrease appetite, says Nelson. And even modest resistance training can keep your RMR from dropping as you lose weight by building, and helping you hold on to, your lean tissue mass. You don't have to heft huge hunks of iron; reasonably strenuous training with machines or bands, or activities like Pilates, should work just fine.

Ten: Does metabolism really matter?

As far as weight loss goes, "the role of metabolism, RMR in particular, has been overrated," Smith says. Richard Weil, Exercise Physiologist at St. Lukes Roosevelt Hospital Center in New York, agrees: "People are just not expending enough energy. If you drive through McDonald's and eat your French fries in the automatic car wash while you call a guy on your cell phone to mow the lawn for you, you are never, ever going to lose weight." Rather than getting hung up on the rate of your metabolism, concentrate on making it work for a living. Ladies, start your engines!