title: Studies of biomarker responses to intervention with vitamin B-12: a systematic review of randomized controlled trials.
abstract: BACKGROUND
  Mild vitamin B-12 deficiency is common among older adults, but evidence for setting dietary recommendations is limited because most studies have administered vitamin B-12 via nonoral routes or at doses several hundred times higher than current recommendations. Furthermore, different biomarkers of vitamin B-12 status have not been systematically reviewed.
  
  
  OBJECTIVE
  The aim was to assess the effectiveness of biomarkers of vitamin B-12 status through a systematic review of published randomized controlled trials of oral vitamin B-12 supplementation.
  
  
  METHODS
  Methods included a structured search strategy on Ovid MEDLINE, EMBASE (Ovid), and Cochrane databases; formal inclusion and exclusion criteria; data extraction; validity assessment; and meta-analysis.
  
  
  RESULTS
  Eight randomized controlled trials were included, and all studies measured serum and plasma total vitamin B-12, 3 studies measured methylmalonic acid, and 6 studies measured total homocysteine response. All 3 biomarkers were found to be effective measures of altered vitamin B-12 intake in populations with low and borderline baseline vitamin B-12 status (P < 0.00001); however, in the case of total vitamin B-12, substantial heterogeneity that could not be fully explained by subgroup analysis was observed. Insufficient data were available to determine the effectiveness of plasma holotranscobalamin, which was measured in only one randomized controlled trial.
  
  
  CONCLUSIONS
  The available evidence suggests that plasma and serum concentrations of total vitamin B-12, methylmalonic acid, and total homocysteine are all effective biomarkers of a change in vitamin B-12 intake; however, because the available data were limited, it was not possible to examine fully the factors that could explain the substantial heterogeneity in total vitamin B-12. Future trials should include low-dose vitamin B-12 in adults across the entire age spectrum and measure the holotranscobalamin response to supplementation.
authors.affiliation: Northern Ireland Centre for Food and Health, School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland, United Kingdom. l.hoey@ulster.ac.uk
authors.forename: Leane
authors.initials: L
authors.lastname: Hoey
authors.forename: J J
authors.initials: JJ
authors.lastname: Strain
authors.forename: Helene
authors.initials: H
authors.lastname: McNulty
mesh_terms: D015415:Biomarkers / Q000097:blood*; D019587:Dietary Supplements; D006710:Homocysteine / Q000097:blood*; D006801:Humans; D008764:Methylmalonic Acid / Q000097:blood*; D015596:Nutrition Assessment; D009752:Nutritional Status; D016032:Randomized Controlled Trials as Topic; D014805:Vitamin B 12 / Q000008:administration & dosage / Q000097:blood*; D014806:Vitamin B 12 Deficiency / Q000097:blood / Q000188:drug therapy; D014803:Vitamin B Complex / Q000008:administration & dosage / Q000097:blood*
chemical_list: D015415:Biomarkers; D006710:Homocysteine; D014803:Vitamin B Complex; D008764:Methylmalonic Acid; D014805:Vitamin B 12