Vitamin B12 and vitamin B6 supplementation is needed among adults with phenylketonuria (PKU)

Summary Phenylketonuria (PKU) is caused by an autosomal recessive deficiency of the enzyme phelnylalanine hydroxylase leading to a failure to convert phenylalanine to tyrosine. To avoid irreversible neurological damage because of increased phenylalanine, treatment is instituted rapidly after birth. We examined 31 adult PKU patients living on a less protein‐restricted diet. Theoretically, these PKU patients had an increased risk of developing vitamin B12 and B6 deficiency because of a limited intake of animal products. Besides laboratory tests (n = 31) we obtained clinical information (n = 30) and detailed information on food consumption (n = 28). Three‐quarters of the patients had early biochemical signs of vitamin B12 deficiency. In spite of a normal folate status, 9 (29%) had a plasma homocysteine above 12 μmol/L. In accord with these findings, the food questionnaires indicated that 11 (39%) patients received less than the recommended daily vitamin B12, and 20 (71%) received less vitamin B6 than recommended. A significant association was found between reduced vitamin B12 intake and both reduced serum cobalamins (p = 0.04) and reduced serum transcobalamin saturation (p = 0.03). Eleven patients took a vitamin pill daily, and these patients had a significantly lower plasma homocysteine compared to the rest. The present study suggests that adult PKU patients were at increased risk of developing vitamin B12 deficiency, and their intake of vitamin B6 was below the recommended daily intake. In conclusion PKU patients need continuing dietary guidance throughout adult life, and considering the risks, costs and potential benefits, daily vitamin supplementation seems justified in these patients.