The Reality Behind Metabolism Myths

Are some people just born to be fat -- even though they follow a low-calorie diet to the letter? Does the body's metabolism slow as we age? And just what is metabolism anyway?

Metabolism is all the physical and chemical processes within the body that create and use energy. Here are some common "beliefs" about metabolism, with comments from the experts to set the record straight.

Myth No. 1

Differences in body metabolism can lead some people to get fat while others stay thin.

For a long time, scientists thought that overweight people were different from others. The belief was that overweight people had an unusually low metabolic rate -- the rate at which the body burns calories. It wasn't that they ate more than anybody else, but simply that they didn't burn it up as fast, according to this theory.

Reality: The theory is untrue. Research with a "calorimeter" at the U.S. Department of Agriculture's Diet and Human Performance Laboratory (DHPL) debunked that myth. The calorimeter is a device that measures virtually every aspect of human life, from the amount of food you eat to the amount of air you breathe.

The research showed that a person could be overweight yet still have a high metabolic rate, or be thin and have a low metabolic rate, says William Rumpler, Ph.D., a research physiologist with the DHPL. Dr. Rumpler has spent more than a decade studying 1,200 adults who have spent 24 hours at a time in the calorimeter, a room-sized box with 80,000 sensors that calculate how many calories the body takes in and gives off. Sensors measure how much heat the body gives off, how much oxygen the body breathes in, how much carbon dioxide it expels. Meanwhile, every morsel of food a test subject eats is measured down to the gram for an exact calorie count.

Myth No. 2

If weight gain isn't a function of metabolism, then it is a matter of diet. In other words, it's your fault you're fat.

Reality: Not quite. "The drive to eat is strong," acknowledges Dr. Rumpler. "But most of us who maintain our weight have a regulatory mechanism that tells us when to stop eating. That's missing with a lot of overweight people. The body isn't sensing when it's satiated. There's some kind of metabolic disorder."

Weight gain occurs when there is a mismatch between the calories you take in and the calories you burn off. "The important thing is to stay physically active," Dr. Rumpler says. "The more active you are, the more the body wants to maintain itself at a lower fat level."

Myth No. 3

Stick with low-fat foods, and you're bound to lose weight.

Reality: There are a lot of good reasons to cut your fat intake. Saturated fat, for example, can cause heart disease. And fat has more than twice as many calories per ounce as carbohydrates and protein.

But you still have to reduce your overall caloric intake if you want to lose weight. Gorging on low-fat snacks won't make you slim. "People have gotten the wrong message," says John Allred, Ph.D., a nutrition professor at Ohio State University. "They think that as long as it's low-fat they can have two servings." Sorry, you've still got to count calories.

Myth No. 4

Your metabolic rate remains the same throughout your adult life, so as long as you eat the same amount and exercise the same amount, you won't gain weight.

Reality: Unfortunately, our metabolic rate begins to slow down in middle age. Even if we consume exactly the same number of calories, and don't deviate from our exercise regimen, we'll put on extra pounds. Our lifestyle changes, too. As a 20-year-old, you might spring up the stairs. At 50, you'll wait for the elevator. The answer: Eat less, exercise more.

"No one knows why the metabolic rate goes down, but between the ages of 40 and 60, you might expect to gain 10 to 15 pounds," says Dr. Allred. "Now, that's not dangerous. People who enter their elderly years a little overweight really might be better off than if they're underweight."

Don't resign yourself to gaining weight, however. You're in charge of how much exercise you get and how much food you eat. Those are far more important influences than aging, Dr. Allred says.

Myth No. 5

To get a true fix on your weight, it's important to weigh yourself daily.

Reality: Losing a pound doesn't mean you've lost a pound of fat. Many circumstances can affect the magic number that appears on your bathroom scale. Some quick-loss diets impose a diuretic-type response; most of the weight lost is just water. In fact, even if you didn't eat for two days, you wouldn't see much of a decline in body weight, says Dr. Rumpler.

Basically, people who weigh themselves every day and panic because they gained two pounds in a single day are getting upset over nothing. "There are very few humans on earth who could eat enough food in one day to gain two pounds of fat," says Dr. Rumpler. "You'd have to eat 7,000 calories in addition to your daily requirement. It's probably water weight. When I do a study, I don't weigh people more than two times a week."

Myth No. 6

If two people both weigh 180 pounds, exercise to the same degree and eat the same amount, their weights will mirror each other.

Reality: That sure seems to make sense, but it's not accurate. Research has found that people who lose weight have to work harder to keep it off than people who are simply maintaining their natural weight. Exercise, however, will allow the dieter to eat more. Think of it in these terms: If you drive your car farther, you need to put more gas in it.