EVALUATION OF A MATHEMATICAL-MODEL FOR PREDICTING THE RELATIONSHIP BETWEEN PROTEIN AND ENERGY INTAKES OF LOW-BIRTH-WEIGHT INFANTS AND THE RATE AND COMPOSITION OF WEIGHT-GAIN

A model for predicting the relationship between protein and energy int akes of low-birth-weight (LBW) infants and the rate and composition of weight gain is described. It is based on linear multiple regression e quations summarizing the rates of weight gain, nitrogen retention, and energy retention of 101 previously studied LBW infants fed protein in takes ranging from 2.25 to 3.9 g.kg(-1).d(-1) and concomitant energy i ntakes ranging from 115 to 147 kcal.kg(-1).d(-1) plus current theory c oncerning nutrient retention and body composition. To test the validit y of the model, three combinations of protein and energy intake predic ted by the model to result in specific rates and compositions of weigh t gain were fed to 44 LBW infants, and the observed rates of weight ga in, protein accretion, and fat accretion were compared with the rates predicted by the model. Differences in these and other outcome variabl es between two of the groups, the intakes of which differed only in en ergy, also were compared to provide additional insight into the effect of concomitant energy intake on protein utilization. Across groups, a ctual outcomes correlated closely with predicted outcomes, supporting the validity of the model for the total population. However, outcomes of individual infants deviated as much as 30% from predicted outcomes; the magnitude of the deviation was independent of birth weight, gesta tional age, or size for gestational age. In addition, the mean rate of protein accretion of the group fed the highest protein/energy ratio w as significantly less than predicted. The higher mean urinary nitrogen excretion as well as blood urea nitrogen and plasma amino acid concen trations of this group versus the group that received a similar protei n intake with a higher energy intake suggest that the higher energy in take improved nitrogen utilization. In toto, the data support the conc ept that the rate and composition of weight gain of LBW infants can be manipulated by intake; however, for individual infants, the extent of manipulation seems to be dependent on as-yet-unidentified inherent bi ologic variables.