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Diet / Weight Control Infocenter

Weight and Slow Metabolism: Myth or Reality?

by Caroline J. Cederquist, M.D.

Doctors and other health professionals often hear their patients complain of a slow metabolism, certain that they're just not burning calories as efficiently as other people who eat the same amount.

Clinicians tend to think it's more likely that patients are just mistaken about how much they're actually eating. It's hard to blame them if they're a bit skeptical, as there's a good body of research that shows that people both under-report their intake, and also honestly underestimate it.

But in the past, even if physicians suspected a patient really did have a low metabolic rate, it wasn't easy or even worthwhile to try to determine if that was so.

The method for accurately testing metabolism was complicated and cumbersome, and simply too expensive to be practical in an outpatient setting.

But now, with a new instrument recently approved by the Food and Drug Administration, clinicians can get an accurate reading of a patient's resting metabolic rate, or RMR, in 10 to 15 minutes.

Your RMR is the rate at which your body uses energy just to keep its basic functions going when you're at rest, circulation, breathing, digestion. Most people are surprised to learn that 65 to 75 percent of our energy is expended this way, without counting what it takes to walk or talk or work, let alone actually exercise.

This is part of why people who try to lose weight by exercising more, without making dietary changes, typically get very little result for their trouble. Since up to three-quarters of our energy is expended without even moving, an exercise-only approach is dealing with a pretty small proportion of the overall calorie use anyway.

Of course, over time, a good exercise program will speed up the RMR as well, and then people start to see accelerated results in weight loss, but most folks get discouraged and give up long before that happens.

Conversely, people with a slow metabolic rate burn less energy overall, so they will be more inclined to gain weight, even if they're not eating any more than the next guy.

For instance, say Betty has a slow metabolism and Veronica has a normal one. If they go to lunch and have the same food, then go for a walk together, the same meal, the same walk, Veronica burns more calories.

Sounds like a raw deal for Betty, but that's the way it is. And the metabolism does slow down in obese people, and once that happens, they can continue gaining weight even if they don't ever add more to their daily caloric intake.

So for people who seem especially prone to gaining weight, you can see how it would be helpful to find out if there was an abnormally slow metabolism involved to begin with, particularly before it gets to the point of obesity.

But for many years, the only reliable methods for testing metabolism were direct calorimetry, measuring precisely the amount of heat a body released at rest in an atmospherically controlled chamber, or indirect calorimetry, which involved having a patient fast overnight, then capturing their exhaled breaths in plastic bags and measuring the content of the gases. Because the ratio of gases is dependent on metabolic rate, that analyses provided an accurate measure of RMR.

But these tests could really only be conducted in research facilities, because few regular medical hospitals were equipped with the expensive equipment required for them, so various mathematical formulas were developed for estimating metabolic rate based on weight and intake.

Some worked pretty reliably for people of healthy weight and body composition, when they were able to accurately report what were eating. But with the 'portion distortion' caused by the creeping enlargement of serving sizes, even the most conscientious patients nowadays can underestimate their intake.

And for people who are not of healthy weight and body composition, the formulas don't apply. For example, it takes less energy to sustain fat mass than fat-free mass, so when a larger percentage of someone's overall weight is in fat, there goes the equation.

But someone has finally come up with a hand-held device for indirect calorimetry that is practical and affordable enough for doctors to have in their offices. A patient basically just has to sit still and breathe into a tube for 15 minutes, and the device analyzes the exhaled gases and calculates the metabolic rate.

Remember that we said that your RMR level can actually improve. Exercise will do it as you gain muscle, and so will weight loss itself, sometimes to a surprising degree. Using the new indirect calorimeters, we have often found that a patient's RMR is higher after a 20-pound loss of fat mass than it was before she dropped that weight.

That's a quantifiable improvement, and for a weight- loss patient who's been struggling, that kind of numerical proof really helps them keep up the good work.

THROUGH THICK & THIN:

What can cause an abnormally low metabolism?

1. Diabetes or insulin resistance (pre-diabetic condition)

2. Higher percentage of body fat

3. Underactive thyroid function

4. Loss of lean body mass, muscle with age or inactivity

5. Genetics

If you think you have an unusual metabolism, it may be worth getting it tested, now that it's possible to have that done easily and accurately. There's no point in laboring against unseen obstacles.

See Also:

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