Because 1 g of urinary nitrogen arises from approximately 6.25 g protein, the protein oxidation rate p (in g/min) is given by the equation
Thus, indirect calorimetry allows calculation of net rates of nutrient oxidation. It is important to appreciate that indirect calorimetry measures the net appearance by oxidation of a substrate. Moreover, it is important to understand that there is a slight difference in the heat produced per liter of O 2 consumed when one compares carbohydrate, lipid, and protein oxidation. An examination of substrate oxidation has broadened our knowledge of the effects of environment (i.e., diet, exercise), disease, and nutrient requirements in humans.
The doubly labeled water technique offers promise as a method of determining energy requirements in free-living populations and in subjects in whom traditional measures of energy expenditure, using indirect calorimetry, have proven impractical and difficult (e.g., infants and critically ill patients). The basis of this technique is that after a bolus dose of two stable isotopes of water (2H2O and H218O), 2H2O is lost from the body in water alone, whereas H218O is lost not only in water but also as C1SO2 via the carbonic anhydrase system (9). The difference in the two turnover rates is therefore related to the CO 2 production rate, and with a knowledge of the fuel mixture oxidized (from the composition of the diet), energy expenditure can be calculated.
The main advantages of the doubly labeled water technique are (a) it measures total daily energy expenditure, which includes an integrated measure of RMR, TEF, and the energy expenditure of physical activity; (b) it permits an unbiased measurement of free-living energy expenditure; and (c) measurements are conducted over extended periods of time (1 to 3 weeks). Thus, energy values derived from the doubly labeled water method are representative of the typical daily energy expenditure and therefore the daily energy needs of free-living adults. Furthermore, this technique provides an accurate estimate of free-living physical activity. Daily free-living physical activity is calculated from the difference between the total daily energy expenditure and the combined energy expenditures of the RMR and TEF. Thus the doubly labeled water technique provides the most realistic estimate in free-living subjects of the average daily energy expenditure associated with physical activity.
Disadvantages of the doubly labeled water method are its expense and limited availability ( TabJe...5...2). Consequently, the technique does not lend itself to epidemiologic studies or studies of large groups of subjects. However, this technique is now being used to examine energy requirements of persons in a variety of healthy and diseased states. With use of the doubly labeled water method, measurement of daily energy expenditure becomes a proxy measure of daily energy requirements.
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Table 5.2 Advantages and Disadvantages of the Doubly Labeled Water Technique
The labeled bicarbonate (13C or 14C) method has recently won favor as a technique for measuring energy expenditure over shorter periods of time (several days) than those covered by the doubly labeled water method (15). When labeled bicarbonate is infused at a constant rate, it reaches a rapid equilibrium with the body's CO 2 pool. The extent of isotopic dilution depends on the rate of CO 2 production, provided there is not isotopic exchange or fixation. Thus variations in the dilution of isotope reflect variations in CO2 production and hence energy expenditure. Because the method assesses CO2 production rather than O2 consumption, it requires assumptions about the respiratory quotient similar to those required by the doubly labeled water method.
In the final analysis, cost and the specific research questions generated should direct the selection of methods of measuring energy expenditure. Questions of substrate oxidation and its impact on the regulation of energy balance, for example, are most applicable to the techniques of indirect calorimetry using room calorimeters and ventilated hood systems. On the other hand, more reliable information on the adaptations of free-living subjects to environmental perturbations (exercise, dietary interventions, etc.) over long periods of time is provided by the use of the doubly labeled water method combined with indirect calorimetry systems.
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