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Labeling with ¹⁵N as Compared with Homoarginine Suggests a Lower Prececal Digestibility of Casein in Pigs^1,2,3,

Institut für Physiologie and Biochemie der Ernährung, Bundesanstalt für Milchforschung, D-24121 Kiel, Germany ^* Forschungsinstitut für die Biologie landwirschaflicher Nutztiere, FB Ernährungsphysiologie "Oskar Kellner," D-18059 Rostock, Germany

Calculations of prececal protein digestibility based on the stable isotope ¹⁵N and the chemical label homoarginine were compared, using casein doubly-labeled with both markers. After food was withheld over-night 24 miniature pigs were given a meal containing 15 g/100 g casein, including 4 g/100 g doubly-labeled protein, and chromic oxide as an indigestible marker. The intestine of eight animals each was removed 3, 6 or 12 h later, divided into 3 sections of equal length, and chyme was collected. Kjeldahl-N, ¹⁵N and homoarginine were determined in diet and chyme. Digestibility of casein in the distal third of the small intestine was 93.5 ± 0.5% and 97.6 ± 0.3% (P < 0.05) according to ¹⁵N and homoarginine label, respectively. Potential causes for this systematic difference were assessed. The data suggest that incorporation of ¹⁵N into endogenous proteins and re-entry into the intestinal lumen via secreta and desquamations is the major cause for the 4.2 ± 0.4% lower digestibility based on the ¹⁵N as compared with the homoarginine labeling technique. A preferential occurrence of homoarginine in more easily digestible sections of the protein, faster release during the digestive process and absorption of homoarginine, or incorporation of ¹⁵N into proteins of intestinal bacteria are less likely to cause this difference.

KEY WORDS: • ¹⁵N • protein digestibility • homoarginine • miniature pigs

¹ Presented in part at the Summer Symposium of the Nutrition Society, July 1989, Oxford, U.K. [Roos, N., Hagemeister, H. & Scholtissek, J. (1990) Protein digestibility measured by ¹⁵N and homoarginine. Proc. Nutr. Soc. 49: 48A (abs.)] and at the Sixth International Symposium on Protein Metabolism and Nutrition, June 1991, Herning, Denmark [Hagemeister, H. & Roos, N. (1991) Comparison of protein digestibility measured by ¹⁵N and homoarginine. Proceedings of the Sixth International Symposium on Protein Metabolism and Nutrition, EAAP publication No. 59, pp. 36–38].

² Supported by the Deutsche Forschungsgesellschaft (Grant HA 456/2-2).

³ 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.

⁴ To whom correspondence should be addressed.

Manuscript received 28 December 1993. Revision accepted 1 June 1994.

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