RELATIVE DILUTION SPACES OF H-2-LABELED AND O-18-LABELED WATER IN HUMANS

The doubly labeled water method for measuring energy expenditure can b e very sensitive to small differences in the ratio of the H-2 to O-18 isotope dilution spaces. Recently it has been suggested that the avera ge ratio is higher than the 1.03 we previously recommended. We therefo re combined the data from 99 recently studied subjects. Subjects (85 f emales and 14 males) were between the ages of 4 and 78 yr (mean = 34 y r) and between 10 and 52% (mean = 35%) fat. The average H-2-to-O-18 di lution space ratio was 1.034 +/- 0.014, which was very similar to the original assumption. As in recent reports, we did find that most of th e variance (60%) was due to random analytic error and that there was n o correlation between the dilution space ratio and age or body fat. Ho wever, in contrast to recent reports we found no evidence of a gender difference. Use of the constant dilution space ratio of 1.034 to recal culate CO2 product in published validation studies demonstrated improv ed accuracy, and thus the value of 1.034 is suggested for use in futur e studies.