Protein recycling in growing rabbits: contribution of microbial lysine to amino acid metabolism
2005
To study the absorption of microbial lysine in growing rabbits, a labelled diet (supplemented with 15NH4Cl) was administered to six animals (group
ISOT); a control group (CTRL, four rabbits) received a similar, but unlabelled, diet. Diets were administered for 30 d. An additional group of six
animals were fed the unlabelled diet for 20 d and then the labelled diet for 10 d while wearing a neck collar to avoid caecotrophy (group COLL),
in order to discriminate it from direct intestinal absorption. At day 30 animals were slaughtered and caecal bacteria and liver samples taken. The
15N enrichment in amino acids of caecal bacteria and liver were determined by GC–combustion/isotope ratio MS. Lysine showed a higher enrichment
in caecal microflora (0·925 atom% excess, APE) than liver (0·215 APE) in group ISOT animals, confirming the double origin of body lysine: microbial
and dietary. The COLL group showed a much lower enrichment in tissue lysine (0·007 (SE 0·0029) APE for liver). Any enrichment in the latter animals
was due to direct absorption of microbial lysine along the digestive tract, since recycling of microbial protein (caecotrophy) was avoided. In such
conditions liver enrichment was low, indicating a small direct intestinal absorption. From the ratio of [15N]lysine enrichment between liver and bacteria
the contribution of microbes to body lysine was estimated at 23 %, with 97% of this arising through caecotrophy. Absorption of microbial lysine
through caecotrophy was 119 (SE 4·0) mg/d, compared with 406 (SE 1·8) mg/d available from the diet. This study confirms the importance of caecotrophy
in rabbit nutrition (15% of total protein intake).
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