Serum vitamin B12 concentrations within reference values do not exclude functional vitamin B12 deficiency in PKU patients of various ages

Homocysteine (Hcy) and in particular methylmalonic acid (MMA) are considered reliable parameters for vitamin B12 status in healthy individuals. Phenylketonuria (PKU) patients are at risk for functional vitamin B12 deficiency based on their diet.ObjectiveThe aim of this study was to investigate the prevalence of functional vitamin B12 deficiency in continuously treated PKU patients and the association of parameters of vitamin B12 and metabolic control.MethodsIn 75 continuously treated PKU patients of 1–37 years of age, serum vitamin B12 concentrations, plasma Hcy, MMA, and phenylalanine concentrations were studied.ResultsEight patients had vitamin B12 concentrations below normal. Out of these eight patients, two had elevated MMA and/or Hcy concentrations. Ten other patients with normal vitamin B12 concentrations had elevated concentrations of MMA and/or Hcy.ConclusionsA vitamin B12 concentration within the reference range does not automatically imply a sufficient vitamin B12 status. We recommend measuring serum MMA, or alternatively plasma Hcy, yearly in all PKU patients to diagnose functional vitamin B12 deficiency.