Gastrojejunal kinetics and the digestion of [N-15]beta-lactoglobulin and casein in humans: The influence of the nature and quantity of the protein

The evolution and luminal effects of different quantities of casein and beta-lactoglobulin were investigated in the upper jejunum of 35 volunteers who ingested 400 mL water with either beta-lactoglobulin or casein in either low or high doses (72.6 mmol N, L beta lg; 71.7 mmol N, LCas; 368.2 mmol N, H beta lg; 386.8 mmol N, HCas). The flow rate of the liquid effluents as well as the nitrogen movements were measured and the exogenous ([N-15]) and endogenous nitrogen fractions analyzed in the upper jejunum after milk protein ingestion. The basal jejunal liquid flow rate (mL/min) was 3.88 +/- 1.84 and peaked in the 0-20 min period for water (9.92 +/- 3.72) and L beta lg (7.27 +/- 3.08) and during the 20-40 min period for LCas (5.69 +/- 2.49), HCas (6.32 +/- 1.85), and H beta lg (6.11 +/- 2.31). One hour after water, LCas, L beta lg, H beta lg, and HCas ingestion, 100%, 95%, 85%, 71%, but only 38% of the liquid phase of the meal were passed through the jejunum, respectively. The flow rate of the endogenous nitrogen was 12.93 +/- 5.22 mmol N/h before meal ingestion; remained unchanged after water, LCas, or H beta lg ingestion; but increased significantly (P < 0.05) after L beta lg and HCas ingestion. The net disappearance of exogenous nitrogen in the upper jejunum 240 min after HCas, L beta lg, LCas, and H beta lg ingestion was 82.6 +/- 9.5%, 61.6 +/- 9.6%, 58.4 +/- 14.7%, and 44.7 +/- 24.4%, respectively. The exogenous fraction of protein nitrogen recovered in the upper intestinal lumen represented 43.3% of the ingested H beta lg nitrogen, but only 4.9% of the ingested HCas nitrogen. In conclusion, casein and beta-lactoglobulin present differences in both the intestinal kinetics of amino acid delivery and in the nature of the products in the intestinal lumen. These differences have to be taken into account from both nutritional and physiologic points of view for the utilization of these proteins in humans.