Effect of protein restriction on15N transfer from dietary [15N]alanine and [15N]Spirulina platensisinto urea

Abstract

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 [15N]alanine ([15N]Ala) and when they included 30 mg/kg of intrinsically labeled [15N] Spirulina platensis([15N]SPI). Six subjects with insulin-dependent diabetes mellitus (IDDM) receiving conventional insulin therapy consumed the test meal with added [15N]Ala while adapted to their customary high-protein diet. Protein restriction increased serum alanine, glycine, glutamine, and methionine concentrations and reduced those of leucine. Whether the previous diet was high or low in protein, there was a similar increase in serum alanine, methionine, and branched-chain amino acid concentrations after the test meal and a similar pattern of15N enrichment in serum amino acids for a given tracer. When [15N]Ala was included in the test meal,15N appeared rapidly in serum alanine and glutamine, to a minor degree in leucine and isoleucine, and not at all in other circulating amino acids. With [15N]SPI, there was a slow appearance of the label in all serum amino acids analyzed. Despite the different serum amino acid labeling, protein restriction reduced the postmeal transfer of dietary15N in [15N]Ala or [15N]SPI into [15N]urea by similar amounts (38 and 43%, respectively, not significant). The response of the subjects with IDDM was similar to that of the normal subjects. Information about adaptive reductions in dietary amino acid catabolism obtained by adding [15N]Ala to a test meal appears to be equivalent to that obtained using an intrinsically labeled protein tracer.