Levels of Non-Proteic Tryptophan in Milk and Soybean Formulas

Although human milk is the main source of nutrition for infants, for many years artificial formulas based on cow’s milk have been introduced into infant nutrition. It is known that human milk contains far less protein but many more free amino acids (Lonnerdal et al., 1976; Svanberg et al., 1977) than cow’s milk (Macy et al., 1953; Fomon, 1974; Lee and Lorenz, 1978; George and Lebenthal, 1981). Studies on the content of free amino acids in milk have excluded tryptophan for methodological reason, and consequently little is known of the nutritional factors that may influence its availabil-lity in early life (Tricklebank et al., 1979; De Antoni et al., 1980). Tryptophan is in effect the only amino acid bound between 80 and 90% to plasma albumin. Only the small free fraction is able to cross the haematoencephalic barrier to enter the brain to form serotonin (McMenamy and Oncley, 1958).