D-amino acid utilization in infants measured with the 15N-tracer technique

The utilization rates of D-[15N] valine, D-[15N] leucine, D-[15N] phenylalanine and D-[15N] alanine were investigated in seven infants being fed parenterally. Retention of the (15)N in the protein pool varied from one amino acid to another and ranged from 23.2% for D-[15N] valine to 48.6% for D-[15N] alanine. In the case of valine it was shown that cumulative renal excretion amounted to 70% of the applied (15)N-dose, 44% being overflow of unchanged D-[15N] valine, 16.6% appearing as [15N] urea and 1.2% as [15N] ammonium. This indicates that doubling the concentrations of D-amino acid racemates compared with their L-isomer counterparts in parenteral feeding solutions must inevitably lead to imbalances. The use of D-amino acids for the purposes of parenteral nutrition remains however ineffective due to their poor and variable utilization compared to L-amino acids.