Objectives and methods: To compare the progression of milk proteins in the upper part of the digestive tract, gastro-jejunal nitrogen movements were studied in 6 healthy human volunteers after beta-lactoglobulin and casein ingestion. 400 mL of water (control), purified beta-lactoglobulin (20 g/L) or casein (20 g/L), each adjusted to 25 microCi with 14C-polyethylene glycol, were given per os. Samples were collected in the stomach and 20 cm below the Treitz ligament every 20 min for 2 hours and measured for volume, osmolarity, ions and nitrogen content.
Results: The jejunal flow rate peaked in the 0-20 min period following water and beta-lactoglobulin ingestion, and in the 20-40 min period after casein ingestion. The gastric half-emptying time (T1/2 min) of the liquid phase was significantly different (P < 0.05) for water (12.1 +/- 0.8), beta-lactoglobulin (14.5 +/- 3.3) and casein (26.5 +/- 9.3). Before ingestion of the test meals, the basal rate of nitrogen was 9.14 +/- 4.09 mmol/h in the jejunum. The total nitrogen content in the jejunum peaked significantly in the 0-20 min period after beta-lactoglobulin ingestion and the 20-40 min period after casein ingestion. The apparent gastro-jejunal protein absorption values were 63% for casein and 66% for beta-lactoglobulin in the 120 min period.
Conclusions: These results show that beta-lactoglobulin and casein behave differently in the upper part of the digestive tract due to different gastric emptying rates.