Assessment of net postprandial protein utilization of N-labelled milk nitrogen in human subjects

The nutritional quality of milk proteins, evaluated both in terms of digestibility and postprandial oxidation and retention in human subjects, was investigated in this study. Five healthy adult volunteers were given 480ml N-15-labelled milk (i.e. 190mmol N)N-15 was subsequently determined at the ileal level, using a naso-intestinal intubation technique, as well as at the faecal level. Plasma and urine were sampled for sh after meal ingestion. Dietary exogenous N recovered at the terminal ileum after 8 h reached 8.6 (SE 0.8) mmol while the amount collected in the faeces was 6.5 (SE 0.7) mmol after 5d. The true ileal and faecal digestibilities were 95.5 (SE 0.4) % and 96.6 (SE 0.4)% respectively. The appearance of [N-15]amino acids in the plasma was rapid and prolonged. The measurement of N-15 in the body urea pool and in the N excreted in the urine allowed us to calculate the deamination occurring after [N-15]milk protein absorption. The net postprandial protein utilization (i.e. NPPU = (N-absorbed - N-deaminated)/N-ingested), calculated as an index of protein quality 8h after milk ingestion, was 81.0(SE 1.9) %. Our data confirm that milk protein has a high oro-ileal digestibility in man and demonstrate that milk protein has a high NPPU, an index corresponding to a period in which the dietary protein retention is maximal.