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 ene rgy-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 die t containing 65% of nonprotein energy as carbohydrate oxidize relatively mo re carbohydrate and relatively less protein than do infants fed an isoenerg etic, isonitrogenous diet containing 35% of nonprotein energy as carbohydra te. Design: Sixty-two low-birth-weight infants weighing from 750 to 1600 a at b irth were assigned randomly and blindly to receive 1 of 5 formulas that dif fered only in the quantity and quality of nonprotein energy. Formula contai ning 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 inta ke 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 excre tion. Results: Carbohydrate oxidation was positively (r=0.71, P<0.0001) and fat o xidation was negatively (r = -0.46, P<0.001) correlated with carbohydrate i ntake. Protein oxidation was negatively correlated with carbohydrate oxidat ion (r = -0.42, P<0.001). Fat oxidation was not correlated with protein oxi dation. Protein oxidation was less in infants receiving 65% of nonprotein e nergy as carbohydrate than in groups receiving 35% nonprotein energy as car bohydrate. Conclusion: These data support the hypothesis that energy supplied as carbo hydrate is more effective than energy supplied as fat in sparing protein ox idation in enterally fed low-birth-weight infants.