Adaptation to protein restriction is impaired in insulin-dependent diabetes mellitus

Although mild abnormalities of amino acid metabolism frequently exist in conventionally treated insulin-dependent diabetes mellitus (IDDM), their physiologic and nutritional importance is uncertain. We tested whether a tendency toward body N loss can be either masked or revealed in insulin-treated IDDM by changing the level of protein in the diet. After adaptation to a protein-free diet adequate in all other nutrients, obligatory urinary N excretion of intensively treated IDDM subjects was significantly greater than normal, indicating an impaired ability to recycle endogenous amino acids during protein restriction. When the preceding diet was high in protein, urea N production after consumption of a mixed test meal matched the amount of N consumed for both normal and diabetic subjects. However, when the test meal was preceded by 5 d of protein restriction, conventionally treated IDDM subjects failed to adaptively reduce postprandial urea production as effectively as normal or intensively treated IDDM subjects. Thus, even during insulin treatment, the ability to maximally recycle endogenous amino acids is impaired in IDDM, as is the ability to adaptively increase dietary amino acid retention in response to protein restriction.