The amino acid composition of human milk corrected for amino acid digestibility

Human milk was collected from women in their 10th-14th weeks of lactation, and was analysed for amino acids. Corrections were made for losses of amino acids which were presumed to occur during acid hydrolysis, using a non-linear mathematical model that describes the simultaneous processes of amino acid yield and decay. The mean amino acid composition of the human milk was found to be similar to previously reported estimates, although the cysteine content of the human milk in the present study was 20% higher than the average literature estimate. True (corrected for endogenous amino acid excretions) ileal amino acid digestibility of human milk was determined using the 3-week-old piglet as a model animal for the human infant. The piglets were given either human milk (n 6) or a protein-free diet (n 6) for a 6 d experimental period. Cr2O3 was added as an indigestible marker, to both the human milk and protein-free diet. At the end of the experimental period the piglets were anaesthetized and samples of digesta removed from the terminal ileum of each piglet. After sampling the piglets were killed. Endogenous ileal excretions of amino acids were determined in piglets fed on the protein-free diet. The true digestibilities of total N and amino acid N were 88% and 95% respectively. The true ileal digestibility of the non-amino acid N fraction in human milk, when calculated by difference was only 50%. The true digestibility of the amino acids in human milk ranged from 81-101% with threonine (86%) being the least digestible essential amino acid. When the true ileal digestibility values were used to correct the amino acid composition of human milk, the pattern of digestible amino acids in human milk was different compared with the currently recommended pattern of amino acid requirements for the infant.