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Effects of quality of energy on substrate oxidation in enterally fed, low-birth-weight infants^1,2

¹ From the Division of Neonatal-Perinatal Medicine, Department of Pediatrics, Babies and Children's Hospital, College of Physicians and Surgeons, Columbia University, New York.

Background: Carbohydrate and fat may differ in their ability to support energy-requiring physiologic processes, such as protein synthesis and growth. If so, varying the constituents of infant formula might be therapeutically advantageous.

Objective: We tested the hypothesis that low-birth-weight infants fed a diet containing 65% of nonprotein energy as carbohydrate oxidize relatively more carbohydrate and relatively less protein than do infants fed an isoenergetic, isonitrogenous diet containing 35% of nonprotein energy as carbohydrate.

Design: Sixty-two low-birth-weight infants weighing from 750 to 1600 g at birth were assigned randomly and blindly to receive 1 of 5 formulas that differed only in the quantity and quality of nonprotein energy. Formula containing 544 kJ•kg^-1•d^-1 with either 50%, 35%, or 65% of nonprotein energy as carbohydrate was administered to control subjects, group 1, and group 2, respectively. Groups 3 and 4 received gross energy intakes of 648 kJ•kg^-1•d^-1 with 35% and 65% of nonprotein energy as carbohydrate. Protein intake was targeted at 4 g•kg^-1•d^-1. Substrate oxidation was estimated from biweekly, 6-h measurements of gas exchange and 24-h urinary nitrogen excretion.

Results: Carbohydrate oxidation was positively (r = 0.71, P < 0.0001) and fat oxidation was negatively (r = –0.46, P < 0.001) correlated with carbohydrate intake. Protein oxidation was negatively correlated with carbohydrate oxidation (r = –0.42, P < 0.001). Fat oxidation was not correlated with protein oxidation. Protein oxidation was less in infants receiving 65% of nonprotein energy as carbohydrate than in groups receiving 35% nonprotein energy as carbohydrate.

Conclusion: These data support the hypothesis that energy supplied as carbohydrate is more effective than energy supplied as fat in sparing protein oxidation in enterally fed low-birth-weight infants.

Key Words: Low-birth-weight infants • substrate oxidation • oxygen consumption • carbon dioxide production • respiratory quotient • energy expenditure • nonprotein energy