Make vitamin D while the sun shines, take supplements when it doesn't: a longitudinal, observational study of older adults in Tasmania, Australia.

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

Relative proportions of participants with serum 25(OH)D concentrations (nmol/L) below different clinical thresholds.¹

1<p>Clinical thresholds were chosen to encompass the recommendations in the current literature <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059063#pone.0059063-Ross1" target="_blank">[2]</a><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059063#pone.0059063-PrezLpez1" target="_blank">[4]</a><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059063#pone.0059063-Nowson1" target="_blank">[7]</a><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059063#pone.0059063-BritishAssociationof1" target="_blank">[30]</a><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059063#pone.0059063-DawsonHughes1" target="_blank">[6]</a>. <25 nmol/L = deficient, 25–50 nmol/L = insufficient, 50–75 nmol/L = sufficient, >75 nmol/L = optimal.</p>2<p>Measurements for each time period were made over three weeks at the end of each season. “Winter 2011” represented the follow-up period, nine months after completion of the primary study, and after the participants were released from restricting vitamin D supplementation (study exclusion criterion>800 IU/day).</p>3<p>Number of subjects assessed at each time period are shown, and mean serum 25(OH)D concentration (±standard deviation) were estimated by mixed methods linear regression, adjusted for participant age and time from beginning of study. Large variation in participant numbers (Winter 2009, Spring 2010) is because a small group of participants commenced the study at the end of Winter (September) 2009 and completed at the end of Winter 2010; the majority commenced at the end of Spring (December) 2009 and completed at the end of Spring 2010.</p>4<p>The relative proportion of participants having serum 25(OH)D concentration below different clinical thresholds shown was compared to the relative proportion at the end of Winter (September) 2010 (chosen as the references because the end of Winter 2009 represented the pilot group only), estimated using repeated-measures negative binomial regression and expressed as an incidence rate ratio (IRR; 95% confidence intervals; p-values).</p

The most significant determinants of vitamin D status (serum 25(OH)D concentration, nmol/L).

1<p>Mean serum 25(OH)D concentration (nmol/L) at end of Winter, the mean difference (95% confidence intervals; p-values) at the end of other seasons, and the effect of taking Vitamin D supplements, wearing protective clothing, and percentage body fat mass, were estimated using repeated-measures mixed methods linear regression analysis adjusted for age and time of subject visit. Variables for inclusion in this model were selected using stepwise regression from: Age, gender, sun exposure, sun avoidance, use of sunscreen, wearing hat, wearing protective clothing, weight, vitamin D supplements, dietary vitamin D, dietary fat as percentage of energy, saturated fat as percentage of total fat.</p>2<p>Data was organised into four seasonal groupings, to enable investigation of the determinants of vitamin D status based on season rather than of specific time-points.</p>3<p>The effect of wearing protective clothing and percentage body fat mass (as standardised normal transformations) on serum 25(OH)D concentration was expressed as the slope of the association: one standard deviation rise in each measure was associated with change shown in the table (95% confidence intervals of the slope; p-values).</p

Relative proportions of participants ingesting vitamin D supplements during the study and at follow-up.

1<p>Seasonal groupings for winter and spring included data from two time points (i.e. September 2009 and September 2010 for winter and December 2009 and December 2010 for spring).</p>2<p>The relative proportion of participants taking Vitamin D supplements of different doses at the end of different seasons was compared to the relative proportion at the end of Winter (September), estimated using repeated-measures negative binomial regression and expressed as an incidence rate ratio (IRR; 95% confidence intervals; P-values).</p>3<p>Follow-up: at the end of Winter (September) 2011, nine months after the final study time point and after participant were released from restricting vitamin D supplementation (study exclusion criterion was>800 IU/day).</p

Relationship between mean daily solar exposure and serum 25(OH)D concentrations during the study.

<p>¹Diamonds represent mean daily solar exposure during the study measured as Mega joules/m². Data obtained from the Australian Bureau of Meteorology (<a href="http://www.bom.gov.au/tas/observations/index.shtml" target="_blank">http://www.bom.gov.au/tas/observations/index.shtml</a>). ²Square points and y-axis error bars represent mean and 95% confidence intervals of serum 25(OH)D concentrations (nmol/L) measured at each study time point estimated by repeated-measures mixed methods linear regression; and sign-wave lines were calculated by mathematical modelling using coefficients for sine-wave analysis estimated using repeated-measures nonlinear regression.</p

Amplitude of annual variation in serum 25(OH)D concentrations (nmol/L) in different supplementation groups.

1<p>Mean amplitude of annual variation, estimated by repeated measures non-linear regression using a sine wave model, adjusted for age and gender.</p>2<p>Mean difference of amplitude between 100–600 IU/day and 800 IU/day: 5.2 (95%CI-1.6 to 12.0; p = 0.13).</p>3<p>Comparison of the mean amplitude between non-supplement and supplement groups.</p

Effect of vitamin D supplementation on serum 25(OH)D concentrations (nmol/L) during the study and at follow-up.

1<p>Mean (±standard deviation) and comparison of serum 25(OH)D concentration (nmol/L) in patients with and without (low- and high-dose) supplements at the end of different seasons, estimated by repeated-measures mixed methods linear regression (mean difference; 95% confidence intervals; P-values corrected for multiple comparisons by the Holm method), adjusted for age and time of subject visit.</p>2<p>Large variation in participant numbers (Winter 2009, Spring 2010) is because a small group of participants commenced the study at the end of Winter (September) 2009 and completed at the end of Winter 2010; the majority commenced at the end of Spring (December) 2009 and completed at the end of Spring 2010.</p>3<p>Maximum allowable dose of vitamin D supplements increased between the end of the study and the follow-up appointment.</p