Increased risk of vitamin B12 deficiency in patients with phenylketonuria on an unrestricted or relaxed diet

Abstract

Objective: To investigate whether dietary relaxation or cessation in patients with phenylketonuria (PKU) predisposes to vitamin B12 deficiency.Study design: Patients with PKU aged 11 to 38 years underwent a neurologic examination and dietetic assessment and were divided according to their diet into 1 of 3 groups: Strict — those on a strict low phenylalanine (phe) diet with amino acid, mineral, and vitamin supplements; Relaxed — those on a total protein intake of ∼1 g/kg/d with 50% of this from natural protein and 50% from amino acid, mineral, and vitamin supplements; Unrestricted — those on no formal protein restriction and not taking amino acid supplements. Assays of blood samples were taken for vitamin B12 and folate levels by standard assays. Results were analyzed with Student t test.Results: Vitamin B12 levels were significantly lower in the PKU groups on relaxed or unrestricted diets compared with the normal population (P < .0001 [unrestricted] and .0034 [relaxed]). Folate levels were significantly elevated in all PKU groups (<.0001).Conclusion: Patients with PKU who are no longer under strict dietary control may be at risk from vitamin B12 deficiency. We recommend that all patients should remain under medical and dietetic supervision and in particular have their vitamin B12 status monitored.