title: Make vitamin D while the sun shines, take supplements when it doesn't: a longitudinal, observational study of older adults in Tasmania, Australia.
abstract: Low vitamin D status has been associated with a number of chronic conditions, particularly in older adults. The aim of this study was to identify how best to maintain optimum vitamin D status throughout the year in this high-risk population. The main objectives of the study were to assess seasonal vitamin D status; identify the main determinants of vitamin D status; determine if taking part in the study led to alterations in participant behaviour and vitamin D status. A longitudinal design across four consecutive seasons observed ninety-one 60-85 year old community-dwelling adults in Tasmania (41π S) over 13 consecutive months, with a follow-up assessment at next winter's end. Associations between solar UVB exposure, sun protection behaviours, dietary and supplemental vitamin D with serum 25(OH)D concentrations were assessed. Variation in serum 25(OH)D demonstrated an identical pattern to solar UVB, lagging 8-10 weeks. Serum 25(OH)D was positively associated with summer UVB (mean 15.9 nmol/L; 95%CI 11.8-19.9 nmol/L, p<0.001) and vitamin D supplementation (100-600 IU/day: 95%CI 10.2 nmol/L; 0.8-19.6 nmol/L; p = 0.03; 800 IU/day: 21.0 nmol/L; 95%CI 8.1-34.0 nmol/L; p = 0.001). Seasonal variation in serum 25(OH)D was greatly diminished in supplement users. The most common alteration in participant behaviour after the study was ingesting vitamin D supplements. Post-study vitamin D supplementation ℘800 IU/day was seven times more likely than during the study resulting in mean difference in serum 25(OH)D between supplement and non-supplement users of 30.1 nmol/L (95%CI 19.4-40.8 nmol/L; p<0.001). The main limitation was homogeneity of participant ethnicity. Solar exposure in summer and ingestion of vitamin D supplements in other seasons are the most effective ways of achieving and maintaining year-round vitamin D sufficiency in older adults in the Southern hemisphere. Vitamin D supplementation has greatest effect on vitamin D status if ingested during and after winter, i.e. between the autumn and spring equinoxes.
authors.affiliation: School of Human Life Sciences, University of Tasmania, Launceston, Tasmania, Australia. Jane.Pittaway@utas.edu.au
authors.forename: Jane K
authors.initials: JK
authors.lastname: Pittaway
authors.forename: Kiran D K
authors.initials: KD
authors.lastname: Ahuja
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Beckett
authors.forename: Marie-Louise
authors.initials: ML
authors.lastname: Bird
authors.forename: Iain K
authors.initials: IK
authors.lastname: Robertson
authors.forename: Madeleine J
authors.initials: MJ
authors.lastname: Ball
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002112:Calcifediol / Q000097:blood; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D012621:Seasons; D013472:Sunlight; D013648:Tasmania; D014807:Vitamin D / Q000097:blood*
chemical_list: D014807:Vitamin D; D002112:Calcifediol