Milk protein quantity and quality in low-birth-weight infants

Well, appropriate-for-gestational age, low-birth-weight infants weighing 2,100 gm or less were divided intothree gestational age groups and assigned randomly within each age group to one of five feeding regimens: pooled human milk; formula 1 (F 1 )=1.5 gm/dl protein, 60 parts bovine whey proteins: 40 parts bovine caseins; F 2 =3.0 gm/dl, 60:40; F 3 =1.5 gm/dl, 18:82; F 4 =3.0 gm/dl, 18:82. Plasma and urine concentrations of methionine and of cystathionine were higher in the infants fed F 1 to F 4 than in the infants fed BM. The plasma cystine concentrations of the infants fed F 1 or F 3 were no different from those fed BM. Plasma cystine concentrations of infants fed F 2 (which had a cystine content at least twice that of any of the other formulas) were significantly higher than those of infants fed BM. Plasma taurine concentrations of infants fed F 3 or F 4 , which were virtually devoid of taurine, decreased steadily during the course of study becoming lower than those of infants fed BM. Urine taurine concentrations of infants fed F 1 , F 3 or F 4 (but not F 2 which had more taurine than F 1 , F 3 , or F 4 ) were lower than those of infants fed BM. These results provide further evidence for the limited capacity of the preterm human infant to convert methionine to cystine, owing to delayed maturation of cystathionase, and suggest a limited capacity to convert cystine to taurine. The latter suggestion is consistent with low human hepatic cysteinesulfinic acid decarboxylase activity 0.26 (fetal) and 0.32 (adult) nmoles/mg protein/hour vs 468 in rat liver.