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Total energy expenditure of infants

The energy requirements of older children have been estimated from multiples of basal metabolic rates (BMR), reflecting various levels of physical activity (FAO/WHO/UNU, 1985). Even though information on the BMR of infants has been available, this approach was not applicable to infants because reasonable allowances for physical activity were undefined. Newly emerging data on total energy expenditure (TEE), however, may be used to derive energy requirements of infants. TEE encompasses BMR, thermoregulation, synthetic cost of growth, and physical activity.

The doubly labeled water method for the measurement of TEE has been used and validated in a number of studies in preterm infants and hospitalized term infants. Although these validation studies were not conducted under free-living conditions of term infants, the high rates of water turnover and high percentages of body water common to all infants were tested. Mean errors between the doubly labeled water method and respiration calorimetry were 0.3 ± 2.6% (Roberts et al, 1986), - 0.9 ± 6.2% (Jones et al, 1987), - 4.5 ± 6.0% (Westerterp et al, 1991), and Ä0.4 + 11.5% (Jensen et al, 1992). Although errors for individuals may be large the doubly labeled water method provides an accurate, unbiased measurement of total energy expenditure for groups and may be used for recommendations of energy intakes of infants. Available data on the TEE of infants are summarized in Table 4. The data published by Davies et al (1989, 1991) have been updated to include more infants (Davies, 1993 private communication). There are 268 data points available on presumably well nourished infants studied in Cambridge, UK and Houston, USA. The majority (90%) of the infants studied were £ 6 months of age (specific ages given in Table 4). TEE of infants living in The Gambia (n = 59) (Prentice et al, 1988; Vasquez-Velasquez, 1987, 1988), rural Mexico (n= 38) (Butte, 1993), and Peru (n= 19) (Fjeld et al, 1989) also have been studied.

Table 3 Energy requirements of infants estimated from dietary energy intake

* BF Breast-fed; FF Formula-fed infants.

First, we performed an analysis on the group mean values for TEE of presumably well-nourished infants (Table 4). Mean TEE was 449 ± 161 kcal/d for infants who were 4.0 ± 3.0 months old and weighed 6.1 ± 1.5 kg. Weighted for sample size, TEE was regressed on age (months), feeding mode (breast-fed, coded 0, and formula-fed, coded 1) and weight (kg) (BMDP1R: Dixon, 1990).

TEE (kcal/d) = 73.8 + 38.6 age + 40.4 mode + 35.4 weight
SEE = 25.7
r² = 0.98;
n = 14. (11)

TEE (kcal/d) was significantly affected by age (P = 0.005), feeding mode (P = 0.01), but not weight. Weight was highly correlated with age (r = 0.98). Interactions between age, mode and weight were not significant. Mean TEE for the breast-fed and formula-fed infants were 420 ± 151 and 495 ± 190 kcal/d, respectively. The high r² does not imply that the TEE of individual infants can be predicted with such a high degree of certainty. It should be remembered that the analysis was performed on group mean values. The SEE provides an indication of the error for predicting group mean values of TEE.

Table 4 Total energy expenditure of infants by doubly-labeled water method

^a Abbreviations: Fx = isotope fractionation; RQ = respiratory quotient; TEE = total energy expenditure; BF = breast-fed; FF = formula-fed; MF = mixed-fed; SMR = sleeping metabolic rate.
^b Mean (s.d.).
^c 1993 unpublished compilation of data used.

Standardized by body weight, TEE averaged 72.6 ± 8.1 kcal/kg/d overall, and 69.2 ± 7.8 and 76.6 ± 9.3 kcal/kg/d for the breast-fed and formula-fed infants, respectively. TEE (weighted by sample size, kcal/kg/d) was significantly affected by age (P = 0.001) and feeding mode (P = 0.01); the interaction between age and feeding mode was not significant. Within studies, the TEE of breast-fed infants has been shown to be lower than that of formula-fed infants (Butte, 1990a; Davies, 1992).

TEE (kcal/kg/d) = 60.1 + 2.6 age + 6.5 mode
SEE = 3.7
r² = 0.83; n = 14. (12)

We calculated BMR according to the Schofield equation for children under the age of 3 years (1985). Mean BMR was 54.6 ± 1.6 kcal/kg/d for the boys and 52.8 ± 1.7 kcal/kg/d for the girls. The physical activity level of the infants (TEE/BMR) increased from 1.3 at 1 month to 1.7 at 12 months of age. TEE rose steadily and gradually as activity increased through infancy.

Second, we examined the TEE data from infants living under harsh environmental conditions. We compiled 88 data points on Gambian and Mexican infants under 12 months of age (Vasquez-Velasquez, 1987; Butte, 1993). The mean TEE of these infants (5.7 ± 3.1 months) was 513 ± 101 kcal/d or 79.2 ± 4.0 kcal/kg/d. The TEE (kcal/kg/d) of infants living under harsh environmental conditions was significantly higher than that of the more sheltered infants (t = 2.6, P = 0.02), but the Gambian and Mexican infants were older. The regression of TEE (kcal/kg/d) on age did not differ between the sheltered and unsheltered infants. Prentice did not find any significant differences in TEE (kcal/kg/d) between Gambian and British infants, aged 0 to 36 months (Prentice, 1993). However, we found the TEE (kcal/kg/d) of the Mexican infants to be higher than that of predominantly breast-fed infants studied in Houston (Butte et al, 1993). More data from different geographic locations are needed to resolve putative differences in TEE of infants exposed to infection and other environmental stresses.

Currently available data on TEE of infants are limited in number, age range, and geographic distribution. Nevertheless, TEE data provide strong evidence for the need to revise current recommendations for energy intake of infants. Prudently, more data should be sought, particularly in the second 6 months of life.

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