title: Vitamin D supplementation does not improve human skeletal muscle contractile properties in insufficient young males.
abstract: OBJECTIVE
  Vitamin D may be a regulator of skeletal muscle function, although human trials investigating this hypothesis are limited to predominantly elderly populations. We aimed to assess the effect of oral vitamin D3 in healthy young males upon skeletal muscle function.
  
  
  METHODS
  Participants (n = 29) received an oral dose of 10,000 IU day(-1) vitamin D3 (VITD) or a visually identical placebo (PLB) for 3 months. Serum 25[OH]D and intact parathyroid hormone (iPTH) were measured at baseline and at week 4, 8 and 12. Muscle function was assessed in n = 22 participants by isokinetic dynamometry and percutaneous isometric electromyostimulation at baseline and at week 6 and 12.
  
  
  RESULTS
  Baseline mean total serum 25[OH]D was 40 ± 17 and 41 ± 20 nmol L(-1) for PLB and VITD, respectively. VITD showed a significant improvement in total 25[OH]D at week 4 (150 ± 31 nmol L(-1)) that remained elevated throughout the trial (P < 0.005). Contrastingly, PLB showed a significant decrease in 25[OH]D at week 12 (25 ± 15 nmol L(-1)) compared with baseline. Despite marked increases in total serum 25[OH]D in VITD and a decrease in PLB, there were no significant changes in any of the muscle function outcome measures at week 6 or 12 for either group (P > 0.05).
  
  
  CONCLUSIONS
  Elevating total serum 25[OH]D to concentrations > 120 nmol L(-1) has no effect on skeletal muscle function. We postulate that skeletal muscle function is only perturbed in conditions of severe deficiency (<12.5 nmol L(-1)).
authors.affiliation: Research Institute for Sport and Exercise Science, Liverpool John Moores University, Tom Reilly Building, Byrom Street, Liverpool, L3 3AF, UK.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Owens
authors.forename: Daniel
authors.initials: D
authors.lastname: Webber
authors.forename: Samuel G
authors.initials: SG
authors.lastname: Impey
authors.forename: Jonathan
authors.initials: J
authors.lastname: Tang
authors.forename: Timothy F
authors.initials: TF
authors.lastname: Donovan
authors.forename: William D
authors.initials: WD
authors.lastname: Fraser
authors.forename: James P
authors.initials: JP
authors.lastname: Morton
authors.forename: Graeme L
authors.initials: GL
authors.lastname: Close
mesh_terms: D000328:Adult; D019587:Dietary Supplements; D004311:Double-Blind Method; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D010281:Parathyroid Hormone / Q000097:blood; D014807:Vitamin D / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D014815:Vitamins / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*
chemical_list: D010281:Parathyroid Hormone; D014815:Vitamins; D014807:Vitamin D