True Exogenous and Endogenous Nitrogen Fractions in the Human Jejunum after Ingestion of Small Amounts of 15N-Labeled Casein

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Abstract
The purpose of this work was to determine exogenous and endogenous contributions to the overall nitrogen flux in the upper jejunum in humans. After the ingestion of water or 8 g of [15N]casein, the behavior of gastro-jejunal nitrogen and electrolyte (Na+, K+, Cl-) movements were studied in seven volunteers using the “slow marker” perfusion technique. The jejunal flow-rate of the chyme peaked during 0–20 min and 20–40 min periods following water and casein ingestion, respectively. Osmolarity, 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 endogenous nitrogen was 9 ± 3.3 mmol/h in the jejunum. After casein ingestion, the total nitrogen content reached maximum values in the 20–40 min period and then progressively returned to the initial level. The gastro-jejunal casein absorption was 58% and the remaining amount of casein in the jejunum was degraded more than 80% to alcohol-soluble compounds. After the ingestion of a low amount of casein no significant increase in the endogenous nitrogen fraction was observed. The 15N-dilution technique is an appropriate method in humans for the direct measurement of endogenous and exogenous contributions to the intestinal nitrogen fraction.

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