TRUE EXOGENOUS AND ENDOGENOUS NITROGEN FRACTIONS IN THE HUMAN JEJUNUMAFTER INGESTION OF SMALL AMOUNTS OF N-15-LABELED CASEIN

The purpose of this work was to determine exogenous and endogenous con tributions to the overall nitrogen flux in the upper jejunum in humans . After the ingestion of water or 8 g of [N-15]casein, the behavior of gastro-jejunal nitrogen and electrolyte (Na+, K+, Cl-) movements were studied in seven volunteers using the ''slow marker'' perfusion techn ique. The jejunal flow-rate of the chyme peaked during 0-20 min and 20 -40 min periods following water and casein ingestion, respectively. Os molarity, Cl- and Na+ concentrations decreased significantly (P < 0.05 ) during the first 20 min following meal ingestion and returned to the basal levels in the 20-60 min period. The gastric half-emptying times (min) of the liquid phase differed significantly (P < 0.05) for water (7.9 +/- 0.4) and casein (21.4 +/- 9.1). The basal flow rate of endog enous nitrogen was 9 +/- 3.3 mmol/h in the jejunum. After casein inges tion, the total nitrogen content reached maximum values in the 20-40 m in period and then progressively returned to the initial level. The ga stro-jejunal casein absorption was 58% and the remaining amount of cas ein in the jejunum was degraded more than 80% to alcohol-soluble compo unds. After the ingestion of a low amount of casein no significant inc rease in the endogenous nitrogen fraction was observed. The N-15-dilut ion technique is an ap propriate method in humans for the direct measu rement of endogenous and exogenous contributions to the intestinal nit rogen fraction.