Energy requirement for growth

Although the energy requirement for growth relative to maintenance is small, except for the first months of life, satisfactory growth is a sensitive indicator of whether needs are being met. To determine the energy cost of growth, the energetics of growth must be understood and satisfactory growth velocities must be defined. The 1985 requirements were based on the growth reference published for international use by WHO (1983), which were derived from the United States National Center for Health Statistics growth curves (NCHS, 1977). What constitutes appropriate infant growth is a topic of controversy and is currently under debate at WHO. Because of policy implications, the findings of the WHO Expert Committee on 'Physical Status: The Use and Interpretation of Anthropometry During Infancy' should be considered if the FAO/WHO/UNU Energy and Protein Requirements are revised. Quantitatively, revision of infant growth curves will minimally impact estimated energy requirements. If growth curves were revised to reflect the growth velocities of breast-fed infants, energy requirements would decrease by 10, 16, 24 and 12 kcal/d for 0-3 months, 3-6 months, 6-9 months and 9-12 months, respectively.

In addition to the growth velocity, the energy cost of growth must be known. This cost consists of the energy content of the newly synthesized tissues and the energy expended in synthesis. In the 1985 report the energy cost of weight gain was reviewed in Annex 4 (FAO/ WHO/UNU, 1985) The value proposed for healthy term infants was 5.6 kcal/g gained. We measured the energy cost of growth in term infants and arrived at an estimate, 4.8 kcal/g (Butte et al, 1989). An additional report appeared on the energy cost of growth of infants recovering from malnutrition; the total energy cost of growth was 6-7 kcal/g (Fjeld et al, 1989). The estimated energy cost of growth is more accurate when the separate costs of protein and fat deposition are taken into account, since the components of weight gain change dramatically through the first year of life. However, the practicality of this point is significantly diminished by the fact that the energy cost of growth as a percentage of total energy requirement decreases from 35% at 1 month to 3% at 12 months.

The total energy cost of growth and its components is presented in Table 5 (Figure 4). For the present discussion, the rates of weight gain and components of weight gain, as described by Fomon et al (1982), have been used. For lack of specific information on the composition of weight gain of breast-fed and formula-fed infants, no distinction was made with respect to potential differences in the energy cost of growth between feeding groups. Median NCHS weights were used to standardize the data. The energetic efficiencies of synthesizing protein and fat were taken to be 42% (1 kcal deposited/2.38 kcal used) and 85% (1 kcal deposited/ 1.17 kcal used), respectively (Roberts & Young, 1988). Energy equivalents for fat and protein were 9.25 kcal/g and 5.65 kcal/g, respectively.

Table 5 Energy cost of growth through infancy

^a Monthly rates of weight gain (Fomon et al, 1982).
^b Monthly rates of &t and protein deposition (Fomon et al, 1982).
^c Energy equivalents for fat and protein deposition were taken as 9.25 kcal/g and 5.65 kcal/g, respectively.
^d Energetic efficiencies of synthesizing protein and fat were taken to be 42% (1 kcal deposited/2.38 kcal used) and 85% (1 kcal deposited/1.17 kcal used), respectively (Roberts & Young, 1988).

As calculated, the energy cost of growth displays an abrupt increase at 1-2 months, followed by a gradual decline through 12 months. The abrupt increase in fat deposition may be an artifact due to interpolation of data compiled from different studies by Fomon et al (1982). Unpublished data of Southgate were used to estimate body composition at birth. Body fat was assumed to be linearly related to subscapular and infra-iliac skinfolds between the ages of 3 months and 10 years. A smoothed curve was constructed relating the percentage body fat to age from 1 month to 10 years.