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Interest in infant feeding has focused on energy, protein, essential fatty acids, and specific minerals. Biological advantages of feeding human milk versus proprietary formulas have been examined, with particular attention being accorded to the appropriateness of human milk for the premature infant and to the need to optimize the nutrient balance provided in semisynthetic formulations. The present paper will focus on the requirements for very long chain polyunsaturated essential fatty acids (Cz0 and CZ2), excluding considerations of fatty acid utilization for energy metabolism, to assess the basis of current knowledge concerning the infant's requirements for essential fatty acids.Metabolism of long chain polyunsaturated fatty acids derived from C18:zw6 and C I R :~~, )~ by chain elongation-desaturation ( Fig. 1) is essential for synthesis of complex structural lipids and prostaglandins. In homeothermic animals these essential fatty acids are required for normal function in developing tissues and appropriate maturation of a wide variety of physiological processes (reviewed by 1). During development, fetal accretion of long chain metabolites of 0 6 and 0 3 fatty acids may result from maternal or placental synthesis and transfer, or alternatively, the fetus may be capable of metabolizing C18:2w6 and CIS: 3,,,3 to longer chain homologues (2). After birth the infant must synthesize the very long chain polyunsaturated fatty acids (LCPE) derived from CI8, 2~6 and C18: 3w3 or be fed these fatty acids as they are normal constituents of most membranes.

Following a bolus of 6.9 wmol. kg-' of NaH13C03, a main-

Indirect calorimetry and primed constant infusion of [U-13C]glucose were combined in 16 appropriate-for-gestational age newborn, parenterally fed infants, in order to measure glucose utilization and glucose oxidation respectively. Glucose intake ranged between 10.0 and 24.1 g day-1 kg-1 and energy intake between 156.9 and 439.3 kJ day-1 kg-1. Glucose utilization (P less than 0.001), glucose oxidation (P less than 0.001) and metabolic rate (P less than 0.005) increased significantly with rising glucose intake. The significant difference between glucose utilization and oxidation (P less than 0.001) can be accounted for by an increasing storage as fat. As lipogenesis from glucose consumes 15-24% of the original glucose energy, the increasing metabolic rate accompanying rising glucose intake is probably due to increasing lipogenesis.

1. Indirect calorimetry and primed constant infusion of [U-13C]glucose were combined in 28 appropriate-for-gestational age newborn, parenterally fed infants, in order to measure glucose utilization and glucose oxidation and to estimate lipogenesis from glucose. 2. The infants were randomly allocated to either a group receiving glucose as the non-protein energy source or a group having one-quarter of the glucose energy replaced by intravenous fat. The energy intake (370 kJ day−1 kg−1) and protein intake (3.4 g day−1 kg−1) were similar in both groups. 3. Energy expenditure (P < 0.005), non-protein carbon dioxide production (P < 0.005) and non-protein oxygen consumption (P < 0.05) were lower in the lipid-supple-mented group. 4. The significant excess of glucose utilization over oxidation (P < 0.001) can be accounted for by lipid synthesis from glucose. 5. Fat synthesis from glucose was higher in the glucose/amino acid group (P < 0.02), but total fat storage was higher in the lipid-supplemented group (P < 0.02). Nitrogen balance was similar in both groups. 6. As lipogenesis from glucose is an energy- and oxygen-consuming and a carbon dioxide-producing process, the data suggest that the differences between the glucose-only group and the lipid-supplemented group are due to different rates of lipogenesis from glucose.

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