Indirect Calorimetry

Indirect calorimetry is a method used to determine how many calories a person is using during different physical conditions. This is a useful technique because the amount of calories a person uses will vary from one person to the next based upon physical characteristics such as body weight and percent body fat. Indirect calorimetry works by measuring the amount of oxygen a person consumes and carbon dioxide the person produces. Additionally, the nitrogen content in the urine is calculated to determine the total number of calories utilized. Indirect calorimetry differs from direct calorimetry, which measures the amount of heat released from the body. Scientists convert information about the heat that is released from the body to calories based upon several principles from physics. Direct calorimetry is difficult to measure; it is both expensive and laborious and thus indirect calorimetry is the preferred method to measure how many calories are used by certain activities. Other measurements to assess the caloric needs of an individual are based upon equations that estimate an individual's age, height, weight, and gender. These assumptions are less accurate than indirect calorimetry.

For research purposes, indirect calorimetry can be used to measure how many calories are used during sleeping, resting, response to exercise, or a 24-hour period. These different terms apply to different types of calories utilized. Sleeping indirect calorimetry measurements can estimate how many calories are utilized while a person is sleeping. Resting (sometimes referred to as basal, fasting, or postabsorptive) indirect calorimetry measurements can estimate that amount of calories used by the body to maintain all essential life processes (i.e., breathing and heart rate). Response to exercise indirect calorimetry measurements can estimate the amount of calories utilized during physical activity (i.e., running or swimming). Finally, the 24-hour indirect calorimetry measurements can estimate the total amount of calories a person uses during the course of a day (all activities).

Indirect calorimetry collects breath samples to determine the oxygen consumption and carbon dioxide production. The breath samples may be collected through several different methods including a facemask for calories used during exercise, a large plastic hood for calories used during resting conditions, and a human respiratory chamber (sometimes called a metabolic chamber) to measure sleeping or 24-hour energy expenditure. Each of these methods has its advantages and disadvantages. The use of a facemask will most accurately assess how many calories a person is using during exercise. However, a person must wear this mask while performing his or her exercise and needs to be near an analyzer to determine oxygen and carbon dioxide levels. This can limit the testing to riding a stationary bike or running on a treadmill, and cannot be used for hiking or swimming.

Additionally, some people find the mask uncomfortable. The large hood for measuring calories during resting conditions is much more comfortable than the mask used during exercise, but this test is limited to a person lying in bed. Therefore, it cannot be used in many experimental settings. A human respiratory chamber is an effective way to measure the total amount of calories utilized over a certain time period, usually 24 hours. These chambers are sealed off from the outside environment and a person remains [Page 399]inside for an entire test period. The chamber is normally equipped with all of the items a person needs for a 24-hour period including a bed, toilet and sink, exercise equipment, telephone, and television. Patients are normally instructed to follow certain protocols while inside a chamber. They spend a certain amount exercising, resting, eating, and doing light activities (i.e., reading or watching television). Analyzers in the chamber measure the amount of oxygen and carbon dioxide in the room. This test reflects more free living than the other methods described and may better calculate the total number of calories used over the course of a day. These chambers are very expensive and laborious to maintain and can be found only in research institutions.

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