Qualitative Aspects of Protein in Human Milk and Formula: Amino Acid Pattern

Since analytical data on the different composition of human milk and cow's milk became available at the beginning of this century, human milk has always been the gold standard for creation of infant formulas. When it became evident that cow's milk contains three times the protein concentration of human milk, the first steps of protein adaptation consisted in dilution of cow's milk at a 1:1 ratio with water and supplementation with sucrose and starch to compensate for dilution-related energy losses. As methods for the determination of amino acid concentrations became more reliable, the considerable differences in almost all amino acid patterns in human milk and cow's milk proteins were recognized. Because of the comparatively low concentration in cow's milk of cystine, which was thought to be an essential amino acid in infancy, it was then recommended that cow's milk should be diluted with water in a ratio of 2:1 for the preparation of infant formulas in order to meet the infant's cystine requirements. These formulas, which contained 2.3% protein, were generally in use up to the 1950s. They contained the majority of all amino acids in abundance and represented a large excretory load in the form of urea and ammonia. At that time, attention was focused on the amino acid composition of the various fractions of human milk and cow's milk protein. The whey protein fraction of cow's milk was found to be rich in the essential amino acids deficient in whole cow's milk protein and the idea of whey protein supplementation of cow's milk came into being. The admixture of whey protein to cow's milk at ratios of 60:40 or 50:50 rendered it possible to adapt the amino acid pattern of cow's milk more closely to the amino acid composition of human milk protein and to avoid an excessive supply of aromatic amino acids and methionine, as well as avoiding cystine and tryptophan deficiency. The surplus of threonine, due to the whey protein admixture, was considered to be unimportant.