Nutrition

4 Things You Probably Didn't Know About Your Metabolism

If losing fat and staying lean is your goal, bear the following metabolic facts in mind.

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Metabolism means how quickly your body burns calories, and that speed varies drastically from person to person. But somehow the term has been pigeonholed into more basic terms. One's metabolism is not a black-and-white issue, it is much more complex. What follows are four myths regarding metabolism that are important to be aware of while coming up with the best course of action to shed excess weight.

Myth 1: The amount of bodyfat you carry affects your metabolism.
Reality: The amount of muscle you carry in your overall body composition determines your metabolic rate.

In general, when one refers to their metabolism, they're 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 fat-free 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."

Myth 2: Most people who are overweight have slow metabolisms.
Reality: 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 fat-free mass is supporting tissue to help the body carry the extra weight. "People who have a lot of body fat 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.

Myth 3: Many people who struggle with their weight are likely suffering from a thyroid disorder.
Reality: Thyroid disorders are fairly rare among the general population.

Just like everything else in our bodies, hormones have an impact on metabolism, too. Thyroid hormones are the principle ones when it comes to influencing your metabolic rate. "People who have high thyroid function have very high metabolic rates, and those who have hypothyroid or underfunctioning thyroids have slow metabolic rates," Keith remarks. But before you can pinpoint your weight problem on hypothyroidism, you should know that it isn't a common ailment. Only about 4% of Americans have some sort of thyroid disorder or disease. The only way to be diagnosed is by your physician, who will probably test you only if you elicit symptoms.

There are other hormones, such as adrenaline (aka epinephrine), that are involved in determining your caloric expenditure. In the case of adrenaline, something as simple as drinking a cup of coffee can cause this hormone to boost your metabolism. "[Caffeine] increases your alertness by increasing adrenaline levels at least to par. It stimulates; the heart rate goes up some," says Keith. "That's the major effect of caffeine for calorie burn and metabolism—its effect on hormones." (He also points out that caffeine can be used to burn some calories and help control weight when used wisely and in moderation.)

Myth 4: When you hit a sticking point during weight loss, it's due to a stalled metabolism
Reality: Plateaus are a sign that your metabolism is doing its job.

When trying to lose weight, most of us have hit the dreaded sticking point—when the scale won't budge, your clothes aren't getting looser and frustrations begin to rise. It's the perfect time to re-evaluate your eating and exercise plan, and consider: How much weight have you lost so far?

"As most people find out, they can lose 10% of their bodyweight before it becomes more difficult to lose another 10%," Keith says. "At about that point, the weight won't come off as fast as it once did."

There are two reasons why: First, you're smaller, so your metabolism is matching the "new" you; second, your body thinks it's starving, and as a result, thyroid hormone levels drop and your resting metabolic rate may drop 10%-25%, so you aren't burning as many calories in a day as you were. And here's what most of us forget: When we lose weight, we become lighter, meaning the amount of calories we burn in everyday activities drops as well. Eventually your metabolism adjusts to your new, smaller body."At some point you come back to energy balance—where your output and input are the same—so you stop losing weight," Keith explains. "That's the sticking point or plateau a lot of people talk about."If losing more weight is your goal, you'll have to either exercise more or restrict your calories more, but be wary—if you're already at a healthy weight, be honest with yourself in analyzing whether you truly need to push to lose even more. There's no viable reason to take your weight loss to the extreme.

Among all these fallacies, there is one truth we've all heard before: As we age, our metabolisms do slow down. "Typically, we use a number like 20%-25% [to describe how much your metabolism decreases] from the time you're 22 to the time you're 75 or so," Keith explains. So if you had a RMR of 1,500 calories at age 20, then at age 75 your RMR may be 1,200 calories. Part of the decline in metabolic rate is hormonal.

"Your hormones aren't as geared up as they once were when you were younger," notes Keith. But if you're able to preserve your muscle mass, that will help offset the drop somewhat. And remember, fat-free mass is the greatest predictor of metabolic rate. Since you're reading muscle & fitness hers, you probably put in some time in the weight room, and your efforts won't go metabolically unnoticed.

All in all, it can be said that metabolism is the great equalizer. The body likes to defend its weight. "If you eat too much and gain weight, your metabolism goes up trying to fight the weight gain," Niemen says. "If you eat too little, the metabolism slows down below normal to help the body conserve what it has so it doesn't lose as much as expected."

"Your body is going to make adjustments to protect itself," adds Keith. "At some point you come back to what we call energy equilibrium—energy intake is equal to your energy output—and you stop losing weight." Ultimately, the metabolism wants to maintain balance and, in the grand scheme of things, that's a function that actually makes a lot of sense.

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