Acute illness in maple syrup urine disease: dynamics of protein metabolism and implications for management.

Acute metabolic decompensation in maple syrup urine disease (MSUD) during otherwise minor illnesses has generally been presumed to result from massive release of leucine from protein catabolism. A stable isotope method based on the continuous infusion of (2H5)phenylalanine was used to measure protein metabolism in vivo in two children with MSUD during acute illness and when well. Net protein catabolism was greater in the unwell state (0.51 and 0.40 gm/kg per 24 hours in each child, respectively) than in the basal state (0.34 and 0.32). This rate of release of leucine from protein is compatible only with a slow (several days) rather than a dramatic rise in plasma leucine levels during acute illness in MSUD. Poor oral intake leading to a relative increase in time spent in the fasting state appears to be a more important determinant of increasing leucine levels than the catabolic effect of infection in itself. These factors suggested that branched-chain amino acid restriction should be commenced at the start of minor illness in children with MSUD, and that intake of other nutrients should be maintained or increased throughout the illness. A regimen based on these concepts was used during nine episodes of minor illness in two children with MSUD. Plasma branched-chain amino acid levels remained acceptable (less than 700 mumol/L) throughout each of these episodes. Dietary supplementation of this type may reduce the risk of metabolic decompensation during acute illnesses in children with MSUD.