Growth velocity and plasma amino acids in the newborn

Plasma amino acid levels in newborn babies are often measured as part of a screening for inherited metabolic disease, or less commonly as part of a nutritional assessment. We have examined certain factors which affect the levels as measured in low birth weight babies during the first 3 wk of life. As in other studies, plasma amino acid concentrations were shown to fall with increasing gestational and postnatal age, and to reflect to some extent the amino acid composition of the dietary protein. In addition, however, plasma branched-chain amino acids and cystine were found to be inversely related to growth velocity and nitrogen retention. These results could be interpreted to suggest that some low birth weight babies were malnourished. However accompanying anthropometric and clinical data showed that these particular babies were healthy and growing rapidly; they were unlikely to be truly malnourished. Rather we suggest they were experiencing "protein economy" resulting from their high growth rate, and would, therefore, have benefited from increased dietary protein. It seems that growth velocity, as well as maturity and diet, must be considered when interpreting plasma amino acid values in an individual baby. There are two implications for the nutrition of the low birth weight baby. The higher plasma threonine concentrations found in the babies receiving the demineralized whey formula does prompt a word of caution when further increase in the whey:casein ratio is contemplated. At energy intakes around 130 kcal/kg/day, protein may become a limiting nutrient both in terms of quality and quantity.