Energy expenditure can be determined directly, by measuring heat output from the body. This requires a thermally insulated chamber, in which the temperature can be controlled so as to maintain the subject's comfort and in which it is possible to measure the amount of heat produced — for example by the increase in temperature of water used to cool the chamber. Calorimeters of this sort are relatively small, so that it is only possible for measurements of direct heat production to be made for subjects performing a limited range of tasks, and only for a relatively short time. Most estimates of energy expenditure are based on indirect measurements — either measurement of oxygen consumption and carbon dioxide production (indirect calorimetry; section 5.1.1) or indirect assessment of carbon dioxide production by use of dual isotopically labelled water (section 5.1.2). From the results of a number of studies in which energy expenditure in different activities has been measured, it is possible to calculate total energy expenditure from the time spent in each type of activity (section 5.1.3).
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