Measurement of the Whole-Body Metabolic Rate
Direct Calorimetry Measures Heat Liberated from the Body.
Because a person ordinarily is not performing any external work, the whole-body metabolic rate can be determined by simply measuring the total quantity of heat liberated from the body in a given time.
In determining the metabolic rate by direct calorime-try, one measures the quantity of heat liberated from the body in a large, specially constructed calorimeter. The subject is placed in an air chamber that is so well insu-lated that no heat can leak through the walls of the chamber. Heat formed by the subject’s body warms the air of the chamber. However, the air temperature within the chamber is maintained at a constant level by forcing the air through pipes in a cool water bath. The rate of heat gain by the water bath, which can be measured with an accurate thermometer, is equal to the rate at which heat is liberated by the subject’s body.
Direct calorimetry is physically difficult to perform and is used only for research purposes.
Indirect Calorimetry—The “Energy Equivalent” of Oxygen.
Because more than 95 per cent of the energy expended in the body is derived from reactions of oxygen with the different foods, the whole-body metabolic rate can also be calculated with a high degree of accuracy from the rate of oxygen utilization. When 1 liter of oxygen is metabolized with glucose, 5.01 Calories of energy are released; when metabolized with starches, 5.06 Calories are released; with fat, 4.70 Calories; and with protein, 4.60 Calories.
Using these figures, it is striking how nearly equiva-lent are the quantities of energy liberated per liter of oxygen, regardless of the type of food being metabo-lized. For the average diet, the quantity of energy liber-ated per liter of oxygen used in the body averages about 4.825 Calories. This is called the energy equivalent ofoxygen; using this energy equivalent, one can calculate with a high degree of precision the rate of heat libera-tion in the body from the quantity of oxygen used in a given period of time.
If a person metabolizes only carbohydrates during the period of the metabolic rate determination, the cal-culated quantity of energy liberated, based on the value for the average energy equivalent of oxygen (4.825 Calories/L), would be about 4 per cent too little. Con-versely, if the person obtains most energy from fat, the calculated value would be about 4 per cent too great.
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