The contribution of the gastrointestinal microflora to the amino acid requirements of non-ruminants
The aim of this work was to quantify the utilisation by rats and pigs of microbial amino acids synthesised in their gastrointestinal tracts. The method which was developed is based on the oral administration of isotopes in non-amino acid form (15NH4Cl or 14C-polyglucose) and the measurement of the incorporation of these isotopes into the amino acids of the body protein of the animal. Since animals cannot incorporate 15N or 14C into certain of the essential amino acids, labelling of such amino acids is only possible via microbial amino acid synthesis and absorption. To calculate the amount of amino acid absorbed the total amount of labelled amino acid (or total radioactivity in the amino acid) in the body was divided by the enrichment (or specific radioactivity, SRA) of that amino acid in microbial protein. It was shown that germ-free rats fed 15NH4Cl were not able to incorporate inorganic 15N into lysine. Conventional rats, however, showed 15N-incorporation in body lysins. When rats fed 15NH4Cl were prevented from coprophagy they did not incorporate 15N in their body lysine. It was concluded that the utilisation of microbial amino acids in rats occurs exclusively via coprophagy. The estimated absorption of microbial lysine by the conventional rats was 21 mg/kg.75 per day. The direct (non-coprophagic) uptake of microbial amino acids in pigs was also measured. Pigs (prevented from coprophagy) were offered a low-protein diet containing 15NH4Cl and 14C-polyglucose. At the end of the experiment the 15N-enrichment of body lysine and the 14C SRA of lysine, histidine, tyrosine, phenylalanine, valine, isoleucine and acids of the microbial fractions of ileal and caecal digesta were also measured.