Effect of protein restriction on N-15 transfer from dietary [N-15]alanine and [N-15]Spirulina platensis into urea

Six normal men consumed a mixed test meal while adapted to high (1.5 g.kg(- 1).day(-1)) and low (0.3 g.kg(-1).day(-1)) protein intakes. They completed this protocol twice: when the test meals included 3 mg/kg of [N-15] alanine ([N-15] Ala) and when they included 30 mg/kg of intrinsically labeled [N-1 5] Spirulina platensis ([N-15] SPI). Six subjects with insulin-dependent di abetes mellitus (IDDM) receiving conventional insulin therapy consumed the test meal with added [N-15] Ala while adapted to their customary high-prote in diet. Protein restriction increased serum alanine, glycine, glutamine, a nd methionine concentrations and reduced those of leucine. Whether the prev ious diet was high or low in protein, there was a similar increase in serum alanine, methionine, and branched-chain amino acid concentrations after th e test meal and a similar pattern of N-15 enrichment in serum amino acids f or a given tracer. When [N-15] Ala was included in the test meal, N-15 appe ared rapidly in serum alanine and glutamine, to a minor degree in leucine a nd isoleucine, and not at all in other circulating amino acids. With [N-15] SPI, there was a slow appearance of the label in all serum amino acids ana lyzed. Despite the different serum amino acid labeling, protein restriction reduced the postmeal transfer of dietary N-15 in [N-15] Ala or [N-15] SPI into [N-15] urea by similar amounts (38 and 43%, respectively, not signific ant). The response of the subjects with IDDM was similar to that of the nor mal subjects. Information about adaptive reductions in dietary amino acid c atabolism obtained by adding [N-15] Ala to a test meal appears to be equiva lent to that obtained using an intrinsically labeled protein tracer.