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The shortbread cookies, eggnog, turkey and dressing are just mouth-watering memories by
now—except that you feel their effects when you try to zip your jeans. If you’re like most
Americans, you’ve gained about a pound over the holidays. Doesn’t sound like much, except
that next year you’ll add another. And the year after that, (you catch my drift).
But don’t just jump on the diet bandwagon. Turn to exercise instead. Ever wonder whether
exercise can “rev up” your metabolism, so that you burn calories around the clock instead
of only during exercise? In other words, will exercise increase your basal metabolic
rate?
What Is BMR?
Basal metabolism, also referred to as “resting energy expenditure,” represents the calories
needed to support life under resting conditions—to keep your heart beating, brain
functioning, lungs inflating, kidneys and liver working, and so on.
How Is BMR measured?
Because BMR reflects energy expenditure under resting conditions, it’s measured while:
- lying down (so muscles don’t have to work to support the body)
- after a 12-hour fast (so the body isn’t working to digest and absorb food)
- completely relaxed (so nervous energy isn’t increasing the heart rate, for example)
- at a comfortable temperature (so energy isn’t used to keep warm or cool down)
To measure BMR, one breathes through an apparatus that measures how much oxygen their body
uses in a given time period, and that number is converted to calories burned.
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Can I Estimate my BMR? |
Most of us don’t have access to a lab to measure oxygen consumption, so equations
have been developed to help you predict your BMR. These
equations aren’t completely accurate for everyone, but, for healthy adults, they
are usually within 10% of the true value.
Estimating Your BMR (Calories/day)
Men: BMR = (10 x Weight in kg) + (6.25 x Height in cm) - (5 x Age in years) + 5
Women: BMR = (10 x Weight in kg) + (6.25 x Height in cm) - (5 x Age in years) - 161
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Pounds divided by 2.2 equal kilograms
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Inches times 2.54 equal centimeters
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For example, Jeff is a 30-year-old who weighs 176 pounds (80 kg) and is 5’11"
tall (180 cm). His BMR would be: (10 x 80) + (6.25 x 180) - (5 x 30) + 5, or
about 1,780 Calories per day.
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How Much Energy Is Needed for BMR?
A surprisingly large amount! On average, BMR represents about 60% of daily energy
expenditure. So if you’re maintaining your weight (meaning that energy intake equals
expenditure), and your usual intake is 2,500 Calories, about 1,500 Calories are used for
basal metabolism.
What Affects BMR?
In general, the bigger, taller, and younger a person is, the higher their BMR. Men have
higher BMRs than women. The difference is related in part to men’s lower average body fat
levels. Because fat is less metabolically active than muscle, if two people had the same
height, weight and age, but differed in relative fatness, the leaner person would have a
slightly higher BMR.
This fact has led some people to state that BMR is related to lean body mass, implying that
building muscle mass increases BMR a lot. The relationship is not that simple, though. When
BMR is being measured, your skeletal muscles are just lying there, using very little energy.
It’s primarily your organs—brain, liver, heart, and others—that expend energy. Looking at
the example in the BMR Estimation Equations, you can see that of the 1,780 calories Jeff
uses for BMR, his vital organs use over 70%. While exercise can increase your muscle mass,
it doesn’t do much for the size of your liver or brain!
Can I Change My BMR?
Referring to the equations, you’ll see that the major factors
affecting BMR—sex, height, and age—are not easily changed. That leaves weight, and more
specifically, fat versus muscle. Which leads to the next question...
Does Exercise Change BMR?
The answer to this question requires looking at two other issues:
- Does metabolism stay elevated after a single exercise bout? The good news is that
after finishing a hard exercise session, your metabolic rate does remain elevated, even
though you’re no longer exercising. The bad news? The increase is short-lived, and usually
uses only a few calories. To estimate how long your metabolism is increased, you can
monitor how long it takes your heart rate to return to normal after exercise—and the
fitter you are, the shorter the time.
- Does exercise increase BMR, so that even if you haven’t exercised for a day or two,
you’ll still be burning calories at a higher rate than if you were inactive? Numerous
studies tried to answer this question. Here’s what their results show:
- Aerobic exercise does not increase BMR. However, it does increase your RMR, or your
metabolic rate during a period of rest immediately following exercise, but this is short
lived. (Aerobic exercise is an excellent way to burn calories and build cardiovascular
fitness.)
- Although the results are mixed, strength exercise may increase BMR slightly (about
5%), especially if muscle mass increases and fat decreases. The “resting” metabolic rate
of muscle is not large, but it is higher than fat.
- Strength training may reduce the decrease in BMR that occurs with low-calorie dieting.
The Take Home Message?
The possibility of small, but real, increases in BMR are yet another reason to add strength
training to your routine. Don’t expect magic—you won’t get it. But remember that most
adults gain weight very slowly throughout life, reflecting small excesses of food intake
over energy expenditure. So small increases in BMR just might offset that gradual weight
gain.
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