| Summary: | Postprandial urea production in subjects with insulin dependent diabetes mellitus (IDDM) on conventional insulin therapy is normal when the previous diet is high in protein, but there is an incomplete adaptive reduction in urea production following protein restriction. To evaluate the nutritional implications of restricted protein intake in human diabetes mellitus, it is first necessary to establish a reliable method to measure changes in urea production and amino acid catabolism in response to changes in dietary protein intake. We therefore tested (1) the accuracy of the urea production rate (Ra) to depict changes in urea production, (2) whether sulfate production can be accurately depicted using tracer or nontracer approaches, after establishing the use of electrospray tandem mass spectrometry to measure sulfate concentrations and 34SO4 enrichments following administration of the stable isotope tracer sodium [34S]sulfate, (3) the reproducibility of urea and sulfate measurements following a test meal low in protein (0.25 g/kg) in subjects previously adapted to high (1.5 g/kg.d) and low (0.3 g/kg.d) protein intakes, and compared the metabolic fate of [ 15N]alanine added to the test meal with that of [15N] Spirulina platensis, a 15N-labeled intact protein, and (4) whether we could identify the differences in postprandial urea and sulfate productions between normal subjects and persons with IDDM receiving conventional insulin therapy previously adapted to high protein intake, when the test meal was limiting in protein. Under basal conditions, steady state urea Ra is an accurate measure of urea production. Following changes in urea production, both the tracer and nontracer methods seriously underestimated total urea Ra. The tracer method overestimated sulfate production by 20%, but the nontracer method provided an accurate measure of sulfate production and, hence, sulfur amino acid catabolism. Postprandial changes in urea and sulfate productions following normal ada
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