Decreased essential amino acid requirements without catabolism in phenylketonuria and maple syrup urine disease.

The normal infant requirement for essential amino acids includes requirements for growth, obligatory catabolism, and other minor losses. The fraction required for obligatory catabolism was estimated by the difference between the normal requirement and the dietary tolerance of infants lacking the enzymes needed for catabolism of phenylalanine and leucine. The average dietary tolerance for phenylalanine in infants with classical phenylketonuria is approximately 42, 31, and 23 mg/100 kcal at 0 to 4, 4 to 12, and 12 to 24 months, respectively. This is equivalent to 68, 58, and 50% of the estimated normal requirement for phenylalanine at these ages. The average leucine tolerance of two infants with classical maple syrup urine disease was approximately 80, 50, and 38 mg/100 kcal at the same ages, corresponding to 53, 38, and 33% of the estimated normal requirement for leucine. The decrease in relative requirements for these amino acids with increasing age can be accounted for by decreased body protein accretion. The difference from the normal requirement is proportional to previous estimates of obligatory protein catabolism. Four infants with variant milder forms of phenylketonuria and one with maple syrup urine disease were found to tolerate amino acid intakes which were substantially greater than the tolerance of infants with the classical disorders, in some cases exceeding the normal requirements. These differences in tolerance, which may reflect partial enzyme activity, were not predictable in individual cases.