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

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 x kg(-1) x 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 x kg(-1) x d(-1) with 35% and 65% of nonprotein energy as carbohydrate. Protein intake was targeted at 4 g x kg(-1) x 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.