Six normal men consumed a mixed test meal while adapted to high (1.5 g · kg¹ · day¹) and low (0.3 g · kg¹ · day¹) 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 of ¹⁵N 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 dietary ¹⁵N 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.