Serum vitamin B 12 concentrations within reference values do not exclude functional vitamin B 12 deficiency in PKU patients of various ages

Abstract

Homocysteine (Hcy) and in particular methylmalonic acid (MMA) are considered reliable parameters for vitamin B 12 status in healthy individuals. Phenylketonuria (PKU) patients are at risk for functional vitamin B 12 deficiency based on their diet. Objective The aim of this study was to investigate the prevalence of functional vitamin B 12 deficiency in continuously treated PKU patients and the association of parameters of vitamin B 12 and metabolic control. Methods In 75 continuously treated PKU patients of 1–37 years of age, serum vitamin B 12 concentrations, plasma Hcy, MMA, and phenylalanine concentrations were studied. Results Eight patients had vitamin B 12 concentrations below normal. Out of these eight patients, two had elevated MMA and/or Hcy concentrations. Ten other patients with normal vitamin B 12 concentrations had elevated concentrations of MMA and/or Hcy. Conclusions A vitamin B 12 concentration within the reference range does not automatically imply a sufficient vitamin B 12 status. We recommend measuring serum MMA, or alternatively plasma Hcy, yearly in all PKU patients to diagnose functional vitamin B 12 deficiency.