Make Vitamin D While the Sun Shines, Take Supplements When It Doesn′t: A Longitudinal, Observational Study of Older Adults in Tasmania, Australia

PLoS ONE

Mar 2013

DOI: 10.1371/journal.pone.0059063

Citations:48

Influential Citations:1

Observational Studies (Human)

Enhanced Details

Methods

Longitudinal, observational; community-dwelling adults aged 60-85 years from Launceston, Tasmania; 91 participants analyzed (65 female, 26 male); mean age 69.4 ± 6.5 years; recruitment via local media; exclusions: acute or uncontrolled medical conditions, kidney or liver disease, medications interfering with vitamin D activity, or taking vitamin D supplements >800 IU/day; data collected over 13 months with five assessments aligned to seasonal fluctuations; follow-up visit nine months after final time point.

Intervention

Oral vitamin D supplements; 100-600 IU/day or 800 IU/day; self-administered; duration: 13 months with a follow-up 9 months after; maximum dose during the main period was 800 IU/day; higher doses were permitted after restrictions were lifted.

Results

Serum 25(OH)D concentration varied seasonally with an 8-10 week lag behind seasonal solar UVB exposure. Vitamin D supplementation increased serum 25(OH)D; 100-600 IU/day raised levels by about 10-20 nmol/L (season dependent); 800 IU/day raised levels by about 22-23 nmol/L in winter and 16-17 nmol/L in spring/autumn. Supplement users showed greatly diminished seasonal variation; amplitude decreased from ~10.7 nmol/L in non-supplement users to ~0.26 nmol/L with 800 IU/day. In winter, 800 IU/day users were about 22.8 nmol/L higher than non-users; at follow-up, the difference was about 30.1 nmol/L. After restrictions were lifted, supplement use increased (any supplement IRR 2.48; ≥800 IU/day IRR 7.43). Conclusions: Solar exposure in summer plus vitamin D supplementation in other seasons are the most effective way to achieve and maintain year-round vitamin D sufficiency in older adults in the Southern Hemisphere; supplementation has the greatest effect when ingested during and after winter (autumn-spring equinox).

Limitations

Ethnic homogeneity: all participants were white of western European descent, limiting generalizability to other ethnic groups.

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. show less