The Current ¹⁵N-Leucine Infusion Technique Is Not Suitable for Quantitative Measurements of Ileal Endogenous Amino Acid Flows in Pigs

Manuscript received 17 March 1998. Initial reviews completed 19 May 1998. Revision accepted 6 July 1998.

Faculté Universitaire des Sciences Agronomiques, Unité de Zootechnie, 5030 Gembloux, Belgium and ^* INRA Station de Recherches Porcines, F-35590 Saint-Gilles, France

The current ¹⁵N-leucine infusion technique may overestimate the ileal endogenous nitrogen losses in pigs. To determine the reason, we infused four cannulated pigs intravenously, fed them a pea-based diet with ¹⁵N-leucine, and examined some methodological variables. Neither the blood sampling time nor the choice of precursor pool (total N or amino acid N of deproteinized plasma) or the method of estimation of the isotopic equilibrium level significantly affected the results. On the other hand, the ¹⁵N-enrichment of purified mucin, isolated from ileal digesta, was higher than that of the plasma amino acid pool (0.114 vs. 0.077 atom % excess). The endogenous proportion of the labeled amino acids (Ala, Gly, Ile, Leu and Val) in the ileal digesta ranged from 23 (Leu) to 74% (Ala), compared with 70% for total N. The low value of leucine was ascribed to the constant marker infusion condition. In pigs infused with ¹³C-leucine, a similar endogenous proportion was obtained for lumenal leucine with ¹³C-leucine and ¹⁵N-leucine infusion. However, the ¹³C-enrichment of the leucine bound to mucin was markedly lower than that of plasma leucine (38%). The endogenous amino acid flows were also estimated by combining the ileal N flow measured with ¹⁵N and the endogenous amino acid profile obtained by means of an N-free diet. They were different from those obtained with the ¹⁵N-amino acid dilution technique. We conclude that the precursor pool currently used (plasma total N or total -amino acid N pools) is a poor indicator of the enrichment of the secretions and that the infusion of one labeled amino acid is not sufficient to extend the method at the amino acid level.

The Journal of Nutrition Vol. 128 No. 11 November 1998, pp. 1961-1968
Copyright ©1998 by the American Society for Nutritional Sciences

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