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Counting calories burned is not as easy as 1-2-3

Devices use approximations of a number of factors. 'They can be right on -- or as much as 25% or 30% high or low,' a Wisconsin professor says.

January 04, 2010|By Karen Ravn
  • EXERCISE: Body weight and level of exertion are two factors that determine how many calories a person uses up.
EXERCISE: Body weight and level of exertion are two factors that determine… (Mark Boster / Los Angeles…)

Whether you're trudging on a treadmill, pedaling on an exercise bike or taking on Roger Federer in a Wii game of Grand Slam Tennis, the machine will probably give you a reading of how many calories are going up in sweat.

In fact, no matter what you're doing -- and that includes doing nothing -- you can find a gadget to compute how many calories you're working off.

But should you put a lot of weight on these figures?

"The short answer is no," says Dr. Chris Cooper, director of the UCLA exercise physiology research laboratory. "They're all estimates based on a number of assumptions."

"They can be right on -- or as much as 25% or 30% high or low," says John Porcari, a professor of exercise and sport science at the University of Wisconsin in La Crosse.

Arriving at a true count of how many calories you're burning, i.e., your energy expenditure, is not as simple as 1-2-3. To get a direct count while, say, you're ellipting along on an elliptical, you would have to be in a special room where precise measurements could be made of the heat released from your body. This method (based on the fact that burning calories releases heat) is possible but usually not practical.

The machines or gizmos that purport to count the calories you burn only estimate them based on factors related to those calories: weight, heart rate, oxygen consumption. So the numbers they yield are approximations -- scientifically based approximations. Some approximations, of course, are more approximate than others.

Measuring oxygen

One of the higher-tech calorie-counting methods -- indirect calorimetry -- measures how much oxygen a person consumes and how much carbon dioxide he or she produces when exercising at different intensity levels, as indicated by heart rate. Scientists have developed equations for converting these measurements into calories burned, i.e., equations to calculate how many calories you're burning when your heart is beating at a given rate.

Some, but not all, gyms and health clubs are equipped to measure clients this way. The measurement is too cumbersome and complicated for day-to-day calorie counting, but once you've been tested, you can use the results to count calories whenever and wherever you please. You simply need to load your personal data into a suitable heart rate monitor, and the monitor can then convert heartbeats to calories burned.

The conversion isn't 100% accurate. A number of factors unrelated to calorie burning can influence your heart rate -- environmental temperature, your own temperature, your anxiety level, etc. Your heart might start racing when you're simply sitting there, not moving a muscle, listening to your boss give you a raise -- or a pink slip.

The conversion also depends on the sort of exercise you're doing. New Leaf, a health and fitness company whose technology is used by many fitness centers, accounts for this variable by loading two sets of data into your heart monitor -- one for weight-bearing exercise (such as walking or jogging) and one for non-weight-bearing exercise (such as riding a stationary bike).

If clients exercise regularly, the relationship between heart rate and calories burned will change over time. New Leaf recommends rechecking your profile after exercising for 12 weeks to see if you need to enter new numbers into your heart monitor. "But eventually you'll plateau out," says Terry Kapsen, senior vice president of marketing and business development at New Leaf. That is, you won't keep getting fitter and fitter forever.

Using averages

The more common type of calorie counters compute how many calories an individual burns according to average measurements for people who are similar to the individual in certain ways, such as age, gender, weight and fitness level.

Life Fitness, a leading manufacturer of exercise equipment headquartered in Schiller Park, Ill., takes that approach with its machines. You enter your weight and wear a heart rate monitor. The counter then calculates how many calories you burn according to what an average person of that weight would burn while exercising at the intensity indicated by the monitor, at the level of difficulty associated with the training program you're using. Life Fitness has 38 programs, with a different calorie-counting equation for each.

If you choose not to enter your weight on a Life Fitness machine, it will assume you weigh 150 pounds. In general, this will overestimate calorie use if you weigh less than 150 and underestimate it if you weigh more, especially on weight-bearing exercises.

How well averages work for calibrating calorie counters depends to some extent on the type of exercise you're doing, says William Haskell, an exercise physiologist and professor of medicine at Stanford University.

* Treadmills (as long as they ask for your weight) and exercise bikes (where weight is less important because you're sitting down) offer fairly good accuracy.

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