title: Sunlight exposure is just one of the factors which influence vitamin D status.
abstract: Studies on the determinants of vitamin D status have tended to concentrate on input - exposure to ultraviolet B radiation and the limited sources in food. Yet, vitamin D status, determined by circulating concentrations of 25-hydroxyvitamin D (25(OH)D), can vary quite markedly in groups of people with apparently similar inputs of vitamin D. There are small effects of polymorphisms in the genes for key proteins involved in vitamin D production and metabolism, including 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol, the precursor of vitamin D, to cholesterol, CYP2R1, the main 25-hydroxylase of vitamin D, GC, coding for the vitamin D binding protein which transports 25(OH)D and other metabolites in blood and CYP24A1, which 24-hydroxylates both 25(OH)D and the hormone, 1,25-dihydroxyvitamin D. 25(OH)D has a highly variable half-life in blood. There is evidence that the half-life of 25(OH)D is affected by calcium intake and some therapeutic agents. Fat tissue seems to serve as a sink for the parent vitamin D, which is released mainly when there are reductions in adiposity. Some evidence is presented to support the proposal that skeletal muscle provides a substantial site of sequestration of 25(OH)D, protecting this metabolite from degradation by the liver, which may help to explain why exercise, not just outdoors, is usually associated with better vitamin D status.
authors.affiliation: Physiology, School of Medical Sciences, Sydney Medical School, Australia. rebecca.mason@sydney.edu.au and Bosch Institute for Medical Research, Australia and College of Sustainability Sciences and Humanities-Zayed University, Abu Dhabi, United Arab Emirates.
authors.forename: M
authors.initials: M
authors.lastname: Abboud
authors.affiliation: Physiology, School of Medical Sciences, Sydney Medical School, Australia. rebecca.mason@sydney.edu.au and Bosch Institute for Medical Research, Australia.
authors.forename: M S
authors.initials: MS
authors.lastname: Rybchyn
authors.affiliation: Department of Health Services Research, CAPHRI School of Public Health and Primary Care, Maastricht University, Maastricht, 6200 MD Maastricht, The Netherlands.
authors.forename: R
authors.identifier: http://orcid.org/0000-0002-8850-6502
authors.initials: R
authors.lastname: Rizk
authors.affiliation: Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: D R
authors.initials: DR
authors.lastname: Fraser
authors.affiliation: Physiology, School of Medical Sciences, Sydney Medical School, Australia. rebecca.mason@sydney.edu.au and Bosch Institute for Medical Research, Australia.
authors.forename: R S
authors.identifier: http://orcid.org/0000-0003-1534-9697
authors.initials: RS
authors.lastname: Mason
mesh_terms: D006801:Humans; D012307:Risk Factors; D013472:Sunlight; D014807:Vitamin D / Q000097:blood* / Q000378:metabolism
chemical_list: D014807:Vitamin D