ENERGY EXPENDITURE - THE DUAL ISOTOPICALLY LABELLED WATER METHOD
Although indirect calorimetry has considerable advantages over direct calorimetry, it still only permits measurement of energy expenditure over a period of a few hours. A more recent technique permits estimation of total energy expenditure over a period of 1—2 weeks. This method depends on the administration of dual isotopically labelled water, 2H218O. The rate at which the labelled water is lost from the body is determined by measuring the amounts of these two isotopes in urine or saliva.
The deuterium (2H) is lost from the body only as water. As shown in Figure 5.1, the labelled oxygen (18O) is lost more rapidly. It can be lost as either water or carbon dioxide, because of the rapid equilibrium between carbon dioxide and bicarbonate: H2O + CO2 ^ H+ + HCO3-. As all three oxygen atoms in the bicarbonate ion are equivalent, label from H218O can be lost in both water and carbon dioxide.
The difference between the rate of loss of the two isotopes from body water (plasma, saliva or urine) thus reflects the total amount of carbon dioxide that has been produced:
CO2 production = (0.5 X total body water) (kO - kH)
Figure 5.1 The estimation of energy expenditure using dual isotopically labelled water (2H218O).
where kO = rate constant for loss of label from 18O and kH = rate constant for loss of label from 2H.
Estimating the average respiratory quotient over the period from the proportions of fat, carbohydrate and protein in the diet and allowing for any changes in body fat permits calculation of the total amount of oxygen that has been consumed, and hence the total energy expenditure over a period of 2—3 weeks.
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