In the newborn, tryptophan (Trp) and its metabolites are essential to brain maturation and to the development of neurobehavioral regulations of food intake, satiation and sleep-wake-rhythm. Due to the high Trp concentration in human milk in relation to the total of neutral amino acids, the blood-brain transfer of tryptophan as a precursor of its metabolites serotonin and melatonin is optimal. In contrast, commercial infant formulas are lower in Trp and higher in neutral amino acid levels resulting in comparatively lower Trp serum concentrations. alpha-lactalbumin enriched, protein-reduced formulas adapted to 2.2% Trp were shown to be capable of producing Trp serum values that did not differ from those in breast-fed infants.