LABELING WITH N-15 AS COMPARED WITH HOMOARGININE SUGGESTS A LOWER PRECECAL DIGESTIBILITY OF CASEIN IN PIGS

Calculations of prececal protein digestibility based on the stable iso tope N-15 and the chemical label homoarginine were compared, using cas ein doubly-labeled with both markers. After food was withheld overnigh t 24 miniature pigs were given a meal containing 15 g/100 g casein, in cluding 4 g/100 g doubly-labeled protein, and chromic oxide as an indi gestible 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, N-15 and homoarginine were determined in diet a nd chyme. Digestibility of casein in the distal third of the small int estine was 93.5 +/- 0.5% and 97.6 +/- 0.3% (P < 0.05) according to N-1 5 and homoarginine label, respectively. Potential causes for this syst ematic difference were assessed. The data suggest that incorporation o f N-15 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 N-15 as compared with the homoargini ne labeling technique. A preferential occurrence of homoarginine in mo re easily digestible sections of the protein, faster release during th e digestive process and absorption of homoarginine, or incorporation o f N-15 into proteins of intestinal bacteria are less likely to cause t his difference.