Microbial amino acid synthesis and utilization in rats: the role of coprophagy

Four rats were housed in cages with mesh floors; another four rats were housed in tubular anticoprophagy cages, in which they could not turn round to reach their own faeces. Both groups were fed for 6 d on a low-protein diet containing fermentable carbohydrates and 15NH4Cl. At the end of the experiment the rats were killed and their carcasses were homogenized, lysine was isolated by ion-exchange chromatography and its 15N enrichment measured by isotope-ratio mass spectrometry. The 15N enrichment in the lysine of the microbial fraction of faeces and the total amount of lysine in the body were also determined in order to estimate the amount of microbial lysine absorbed. The 15N enrichment in body lysine of non-coprophagic rats was not different from that previously measured in rats given unlabelled NH4Cl, but in coprophagic rats it was significantly higher. The daily absorption of microbial lysine by the coprophagic rats accounted for 20.7 (SE 2.55) mg/kg body weight0-75, but was only 0.5 (SE 1.04) mg/kg body weight0-75 for the non-coprophagic rats. This value was not significantly different from zero. The utilization of microbial amino acids via coprophagy resulted in a higher weight gain (adjusted for intake) in the coprophagic group (15.5 g/6 d) than in the non-coprophagic rats (3.1 g/6 d). It was concluded that, in rats, the utilization of microbial lysine occurred exclusively via coprophagy.