Energy cost of growth during infancy.

Adequate energy and a balance of essential nutrients are dietary requisites if optimal growth and development are to be achieved during infancy. Energy deposition will be determined by the relative balance between the oxidative and synthetic processes within the body. The energy cost of growth may be regarded as two components, the energy deposited in newly synthesized tissues and the energy expended to support the metabolic processes necessary to achieve tissue accretion. Theoretical and experimental derivations of the total cost of growth and its components during infancy are the subject of this paper. The total energy cost of growth will be referred to as E, and its parts as Em nents and Esynthesis. Estimation of Ecompnents depends on the accurate assessment of bo$ composition. Derivation of Evnthesis requires knowledge of the biochemical transformations necessary to maintain and support net tissue accretion. Deposition of dietary fat as fat tissue is the most efficient transformation with a loss of only 0.04 kJM deposited (Millward et al. 1976). The interconversions of carbohydrate into fat, and protein into protein, result in a loss of 0.63 kJM deposited. Synthesis of fat from protein is the least efficient with a loss of 1.30 kJIkJ deposited. Developmental aspects of body composition and whole body metabolism affect the energy cost of growth. Throughout the first year of life, changes occur in the composition of weight gain. Chemical maturation and the differential contribution of various organs to body-weight gain alter not only the maintenance requirement for energy, but also the energy cost of growth. Changes in relative organ size influence both energy and protein metabolism. Protein synthesis proceeds at a high rate in the neonate and declines throughout infancy. High protein turnover contributes to the relatively high energy requirement of the newborn (Reeds ef al. 1982). Protein synthesis accounts for approximately 23% of the daily energy expenditure in the human neonate (Young, 1981) and 7% in the older infant (Millward et al. 1976). Because of these developmental changes, the energy cost of growth is expected to vary throughout infancy.