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True Exogenous and Endogenous Nitrogen Fractions in the Human Jejunum after Ingestion of Small Amounts of ¹⁵N-Labeled Casein¹

Sylvain Mahé², Nils Roos^*, Robert Benamouzig, Hinrich Sick^*, Agnès Baglieri, Jean-François Huneau and Daniel Tomé

Unité INRA de Nutrition Humaine et de Physiologie Intestinale, Faculté des Sciences Pharmaceutiques et Biologiques, 75006 Paris, France ^* Institut für Physiologie und Biochemie der Ernährung, Bundesanstalt für Milchforschung, D-2300 Kiel 14, Germany Service de Gastroentérologie, Hôpital Avicenne, 93009 Bobigny Cédex France

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.

KEY WORDS: • casein • digestion • humans • milk proteins • nitrogen-15

¹ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

² To whom correspondence and reprint requests should be addressed.

Manuscript received 14 September 1993. Revision accepted 17 November 1993.

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