title: Vitamin D deficiency as a risk factor for dementia: a systematic review and meta-analysis.
abstract: Sunlight exposure and high vitamin D status have been hypothesised to reduce the risk of developing dementia. The objective of our research was to determine whether lack of sunlight and hypovitaminosis D over time are associated with dementia.
  
  
  
  We systematically searched MEDLINE (via PubMed), Cochrane Library, EMBASE, SCOPUS, Web of Science, ICONDA, and reference lists of pertinent review articles from 1990 to October 2015. We conducted random effects meta-analyses of published and unpublished data to evaluate the influence of sunlight exposure or vitamin D as a surrogate marker on dementia risk.
  
  
  
  We could not identify a single study investigating the association between sunlight exposure and dementia risk. Six cohort studies provided data on the effect of serum vitamin D concentration on dementia risk. A meta-analysis of five studies showed a higher risk for persons with serious vitamin D deficiency (<25 nmol/L or 7-28 nmol/L) compared to persons with sufficient vitamin D supply (≥50 nmol/L or 54-159 nmol/L) (point estimate 1.54; 95% CI 1.19-1.99, I2= 20%). The strength of evidence that serious vitamin D deficiency increases the risk of developing dementia, however, is very low due to the observational nature of included studies and their lack of adjustment for residual or important confounders (e.g. ApoE ε4 genotype), as well as the indirect relationship between Vitamin D concentrations as a surrogate for sunlight exposure and dementia risk.
  
  
  
  The results of this systematic review show that low vitamin D levels might contribute to the development of dementia. Further research examining the direct and indirect relationship between sunlight exposure and dementia risk is needed. Such research should involve large-scale cohort studies with homogeneous and repeated assessment of vitamin D concentrations or sunlight exposure and dementia outcomes.
authors.affiliation: Department for Evidence-based Medicine and Clinical Epidemiology, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500, Krems, Austria. isolde.sommer@donau-uni.ac.at.
authors.forename: Isolde
authors.identifier: 0000-0003-3592-1507
authors.initials: I
authors.lastname: Sommer
authors.affiliation: Department for Evidence-based Medicine and Clinical Epidemiology, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500, Krems, Austria.|Cochrane Austria, Danube University Krems, Krems, Austria.
authors.forename: Ursula
authors.initials: U
authors.lastname: Griebler
authors.affiliation: Department for Evidence-based Medicine and Clinical Epidemiology, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500, Krems, Austria.|Cochrane Austria, Danube University Krems, Krems, Austria.
authors.forename: Christina
authors.initials: C
authors.lastname: Kien
authors.affiliation: Department for Clinical Neurosciences and Preventive Medicine, Danube University, Krems, Austria.|MAS Alzheimerhilfe, Bad Ischl, Austria.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Auer
authors.affiliation: Department for Evidence-based Medicine and Clinical Epidemiology, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500, Krems, Austria.|Cochrane Austria, Danube University Krems, Krems, Austria.
authors.forename: Irma
authors.initials: I
authors.lastname: Klerings
authors.affiliation: Institute of Building Research & Innovation ZT-GmbH, Vienna, Austria.
authors.forename: Renate
authors.initials: R
authors.lastname: Hammer
authors.affiliation: Institute of Building Research & Innovation ZT-GmbH, Vienna, Austria.
authors.forename: Peter
authors.initials: P
authors.lastname: Holzer
authors.affiliation: Department for Evidence-based Medicine and Clinical Epidemiology, Danube University Krems, Dr.-Karl-Dorrek-Straße 30, 3500, Krems, Austria.|Cochrane Austria, Danube University Krems, Krems, Austria.|RTI-UNC Evidence-based Practice Center, Research Triangle Institute International, North Carolina, USA.
authors.forename: Gerald
authors.initials: G
authors.lastname: Gartlehner
mesh_terms: D015331:Cohort Studies; D003704:Dementia / Q000097:blood* / Q000453:epidemiology / Q000523:psychology*; D005260:Female; D006801:Humans; D012307:Risk Factors; D013472:Sunlight; D014807:Vitamin D / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood* / Q000453:epidemiology / Q000523:psychology*
chemical_list: D014807:Vitamin D
keywords: Dementia; Meta-analysis; Systematic review; Vitamin D