Nitrogenous components of human milk: non-protein nitrogen, true protein and free amino acids

This study reports the nitrogen distribution as total nitrogen, protein nitrogen, non-protein nitrogen and free amino acids in milk, from 195 healthy nursing mothers, of 1 month of lactation. The purpose was to provide data on mature breast milk with the aim of increasing the knowledge of the physiological and nutritional role of human milk. Human milk samples were collected on the 30th day of lactation from well-nourished mothers of term infants in seven pediatric centres in northern, central and southern Italy. The protein and total nitrogen levels were 12.6 g/l±2.0 and 2350 mg/l±390 respectively. The percentage of non protein nitrogen was 15% and the absolute value was 341 mg/l±76. The amount of nitrogen derived from each amino acid varied widely; the cumulative amino acid nitrogen presented a minimum of 13 mg/l and a maximum of 82 mg/l. Taurine, serine, glutamic acid and glutamine were the only free amino acids, accounting for a large proportion of the FAA in the non-protein nitrogen fraction. The sum of glutamic acid and glutamine (1430 μmol/l±489) in human milk samples represented 49% of total free amino acids.