Eating Disorders and Obesity: A Comprehensive Handbook

By Christopher G. Fairburn; Kelly D. Brownell | Go to book overview

24
Measurement of Energy Expenditure

ANDREW M. PRENTICE

Human energy expenditure can be estimated using a wide range of techniques of differing sophistication and cost. It is a general truism that the more complicated and expensive methods provide the most accurate and precise estimates. With the exception of the relatively new doubly labeled water method (DLW), it is also true that the most accurate methods are least representative of real life because they require the subject to be restrained or encumbered. Thus, compromises must generally be made, and it is most important to match the technique selected to the question at hand.


DIRECT CALORIMETRY

Direct calorimetry measures heat loss (as opposed to indirect calorimetry, which measures heat production; see below). The difference between these is simply one of time: Heat loss must follow heat production, and the delay is proportional to the specific heat capacity of the body. Thus, over periods of more than 1 or 2 hours, the two methods should give identical results. This prediction was shown to be true over a century ago and has since been confirmed. Indirect calorimetry, technically much easier and cheaper, provides additional estimates of substrate utilization. Therefore, except in a few basic studies of heat physiology, it is the method of choice. There are virtually no direct human calorimeters still in use.


INDIRECT CALORIMETRY

Indirect calorimetry calculates energy expenditure from measurements of a person’s oxygen consumption and carbon dioxide production, which are proportional to the rate at which energy-yielding substrates are burned. The technique requires some means of collecting expired breath and channeling it to accurate gas meters to measure the rate of air

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