Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements

Background &Aims: Irreversible amino acid losses at the human ileum are not taken into account when tracer-derived amino acid requirements are calculated because the data available are scarce. We have investigated amino acid losses at the ileal level in humans after ingestion of a protein meal. Methods: Thirteen volunteers ingested a single meal of N-15 milk or soy proteins. The appearance of N-15 and N-15 amino acids in the ileal effluents collected using an ileal tube was monitored for 8 hours. Results: In the soy group, higher losses of endogenous nitrogen, especially originating from amino acids, were observed, as well as a higher flow rate of dietary non-amino acid nitrogen. With soy protein, the digestibilities of valine, threonine, histidine, tyrosine, alanine, and proline were significantly lower than with milk. Ileal losses of leucine, valine, and isoleucine amounted to 12, 10, and 7 mg (.) kg(-1) (.) day(-1), respectively. Threonine ileal loss (9-12 mg (.) kg(-1) day(-1) (.) was particularly high compared with the current amino acid requirement. Conclusions: Amino acid losses at the human terminal ileum are substantial and depend on the type of dietary protein ingested. Although it remains unclear whether intact amino acids are absorbed in the colon, we suggest that ileal losses should be considered an important component of amino acid requirements.