4. Discussion

We have used the doubly labelled water technique to measure non-invasively total energy expenditure over a period of 7 days, to provide the first centiles for total energy expenditure for infants up to 6 months of age.

There are a number of potential sources of error in the doubly labelled water technique that need careful consideration. These include the choice of the respiratory quotient (RQ), and the proportion of water output fractionated, the possible sequestration of isotope into body tissue, and the effect of weaning, or major changes in diet during an isotope study period.

In adult studies, using the doubly labelled water technique, an RQ of 0.85 is usually applied. Although RQ may vary throughout a study, over a period of 10-14 days, the length of most adult studies, RQ will tend towards 0.85 and the use of this figure will not introduce a large error. However, an RQ value used in infants must take into account growth. BLACK et al. (1986) list unadjusted and adjusted food quotients (FQ) for infants, and conclude that the FQ and RQ can be used interchangeably in the doubly labelled water technique, due to the length of study period (7 days in our study). The value for RQ we have used at periods A and B (i.e., 0.87) is the mean of some 143 individual calculations of FQ, adjusted to allow for growth using the body composition data reported by FOMON (1974). At period C an adjusted FQ of 0.855 is used; this is based on 253 individual measurements of FQ between the ages of 4 and 12 months. Consequently, we feel that our choice of RQ is appropriate and justified.

Another source of potential error in the doubly labelled water technique is the use of an inappropriate value for the proportion of total water output that is subject to isotopic fractionation (fractionation factor). The magnitude of error produced in the final estimation of total energy expenditure due to an inappropriate choice of fractionation factor depends strongly on the kd:ko ratio. The closer this ratio is to 1 the greater the final error. In adults, the kd:ko ratio is usually about 0.75. In young infants, as water turnover is high in relation to carbon dioxide production rate, the ratio is higher (0.86, 0.85, and 0.82 at periods A, B and C, respectively, in this study). COWARD (1988) recently showed that with a kd:ko ratio of 0.90 as much as an 8% error in carbon dioxide production rate will be induced by an error of 0.1 in the value of fractionation factor used. The factor assumed in this present study (0.13) is based upon an unpublished water balance study (LUCAS, unpublished results), in a subset of infants in the present cohort.

In the validation study reported by JONES et al. (1987) a higher value (0.18) was used. This value was based upon the assumptions that breath is saturated with water and contains 3.5% carbon dioxide. Thus, breath water losses can be related to carbon dioxide production rate. Skin losses (non-sweat) were estimated using a value of 0.18 g/min/m2 insensible skin water loss, and assuming that 75% of the infants' skin was exposed to the air. While appropriate for the infants studied by JONES et al. (1987), their assumed value for the proportion of exposed skin was undoubtedly too high for the infants in our study. We estimate that a figure of about 30% would be more appropriate. Using this, and the approach of JONES et al. (1987), the mean proportion of water output fractionated in our study would have been 0.15; close to our value of 0.13.

The sequestration of isotope into tissues during the rapid growth experienced in early infancy could lead to error in the calculation of total energy expenditure. Labelled hydrogen exchanges with non-aqueous hydrogen in proteins and other tissues. In contrast, there is little non-aqueous oxygen in the body with which ¹⁸O could exchange, and in any case such an exchange does not occur readily at physiological temperature and pH, JONES et al. (1987) addressed the potential problem of label sequestration in rapidly growing infants. They hypothesised that if there was significant isotope sequestration, notably of labelled hydrogen, the difference in energy expenditure (calculated by doubly labelled water) and respiratory gas exchange would be correlated with the change in body weight during the study period. No such relationship was found and these workers concluded that sequestration rates were insignificant.

Finally, changes in the enrichment of ²H and ¹⁸O in dietary water during a study period can produce major error in the calculation of energy expenditure with doubly labelled water (ROBERTS et al., 1988a; JONES et al., 1988). During infancy the principal factor in the change in dietary water ²H and ¹⁸O content is weaning. At periods A and B. the infants studied here were exclusively being breast- or formula-fed, while at period C, care was taken that no major change in diet occurred during the study period.

We have attempted to define the most appropriate way that an infant's energy expenditure should be expressed in relation to body weight in order to minimize the correlation between body weight and energy expenditure. We have found that a statistically valid and numerically convenient adjustment of energy expenditure would be to express energy expenditure as kcal/kg^0.5 or kJ/kg^0.5, that is, per square root of body weight. We suggest that in studies on energy metabolism in early infancy this expression should be used.