The effect of excess L-phenylalanine on mothers and on their breast-fed infants

Four lactating mothers—two who were heterozygous for phenylketonuria and two nonheterozygous primiparas—were given oral doses of l -phenylalanine. The concentrations of phenylalanine and tyrosine in the mothers' and their infants' serum, as well as the concentrations of phenylalanine and its metabolites in the breast milk and in the mothers' and infants' urine, were measured. The heterozygous mothers exhibited higher concentrations of these substances than the nonheterozygous ones. Since the breast milk of untreated homozygous mothers would presumably contain more phenlyalanine and its metabolites than that of heterozygous ones, the question of whether or not they should nurse their infants deserves serious consideration.