How Accurate Are Devices That Track Calories Burned?

When accuracy is highly important in determining the number of calories burned with specific activities, a laboratory-based test called indirect calorimetry is often used. This test estimates the amount of heat produced (calories) by measuring the differences in oxygen and carbon dioxide in inspired and expired air. Obviously, this type of testing would be impractical for people to use in a free-living situation. To address this need, a number of devices are available for estimating caloric expenditure during exercise, including heart rate monitors, pedometers, and accelerometer-based activity monitors. While none of them are as accurate as calorimetry for measuring calories burned, each has its own advantages and disadvantages. To a large degree, the device that is best for a particular individual depends on one's budget and preferred type of exercise.

The word pedometer literally means, "counting footsteps". This count is converted into distance by knowing the length of your usual stride. The number of calories that you have burned is estimated from the distance that you have walked. For example, a brisk walk for an average 200-pound adult burns about 100 calories per mile. To get an estimate of the calories burned while walking, the number of miles walked is multiplied by 100. A drawback of the pedometer is that body weight and intensity of walking, important determinants of calories burned, are not taken into consideration. Additionally, pedometers are unable to provide meaningful caloric information related to activities involving "resistance" such a bicycling or weight lifting. Pedometers are the least expensive of the devices used to estimate calories burned and can be perfectly adequate if walking or jogging is the primary form of exercise. A popular campaign, the "10,000 step programs", uses a pedometer to help people achieve the goal of walking 5 miles each day, consuming approximately 500 calories in the process. Heart rate monitors typically consist of a chest belt that transmits heart rate

information to a wrist-worn display.

Heart rate monitors are commonly used by competitive athletes to assist with their training programs, but certain ones can also provide information on caloric expenditure. Polar, a manufacturer of heart rate monitors, developed a feature known as OwnCal that uses personal information (body weight, height, age, etc.), along with heart rate information to calculate the number of calories burned. Unfortunately, the formulas that most heart rate monitors use to measure calories burned have not been compared with more accurate methods, such as indirect calorimetry. Another shortcoming of heart rate monitors is that they are not useful for tracking caloric consumption during activities that do not cause the heart rate to increase, such as many activities of daily living. Additionally, heart rate can be elevated by nervousness or dehydration, leading to inaccurate results. A heart rate monitor could be particularly useful, however, for someone who swims for exercise, since special water resistant models are available.

The most accurate of the commercially available devices to measure caloric consumption utilize an instrument called an accelerometer. Two examples of this type of calorie counter are the bodybugg Calorie Management System and the BodyMedia FIT (also known as the SenseWear System). The bodybugg has gained recognition by being used by contestants on the reality TV show, "The Biggest Loser". Accelerometers are usually worn around the arm and measure the motion of the body in several planes. In addition to measuring movements, both of these products measure skin temperature,heat flux (the amount of heat leaving the body), and galvanic skin response -- a measure of electrical conductivity that changes when you sweat. The device uses all of this information to calculate caloric consumption. Products using accelerometers are the most expensive of devices used to measure caloric consumption but have been well studied scientifically and found to have an acceptably small margin of error (from 2.5% to 10% in clinical studies). Some manufacturers of these products also provide a subscription service that allows you to download your activity information and compare with your food intake.

While none of these devices is capable of determining the exact number of calories burned during various activities, they can encourage regular exercise and be a good motivator for sticking with a weight loss program.

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