Make vitamin D while the sun shines, take supplements when it doesn't

Background Research suggests ageing populations, especially those with limited sunlight exposure, have a greater dependency on dietary sources to maintain optimal vitamin D status, be it through increased consumption of vitamin D-rich foods, food fortification or vitamin D supplementation. Objective To assess the seasonality of serum 25(OH)D and the contribution of dietary and supplemental vitamin D in older, community-dwelling Tasmanian adults. Design This was a longitudinal observational study. Ninety-one adult volunteers aged 60–85 years were assessed on five occasions over 13 months. At each time point dietary intake was estimated using semi-quantitative food frequency questionnaires and participants also provided information on supplement use and sun protection behaviours. Fasting blood samples were collected for determination of serum 25(OH)D. Step-wise regression analysis was used to predict the determinants of serum vitamin D. Outcomes Mathematical modelling of serum 25(OH)D concentration applying a sine wave model, demonstrated an identical pattern to daily solar exposure (representing solar UVB exposure) with an 8-10 week time lag. The greatest magnitude effects on serum 25(OH)D concentration were summer solar UVB exposure (mean 15.9 nmol/L; 95% CI 11.8 to 19.9 nmol/L, p<0.001) and use of vitamin D supplements (100-600 IU/day: 10.2 nmol/L, 95% CI 0.8 to 19.6 nmol/L, p=0.03; 800 IU/day: 21 nmol/L, 95% CI 8.1 to 34.0 nmol/L, p=0.001). Seasonal variation in serum 25(OH)D concentration was significantly reduced in participants taking 800 IU/day (10.5 nmol/L; 95%CI: 5.6 to 15.4 nmol/L; p<0.001). Dietary vitamin D had a nonsignificant effect on serum vitamin D concentration. Body fat mass and use of protective clothing showed significant negative association with serum 25(OH)D concentration (- 4.2 nmol/L, 95%CI -8.8 to -0.8, p=0.02 and -5.4 nmol/L, 95%CI -10.3 to -0.5, p=0.03, respectively). Conclusion Results of this study suggest little or no contribution of dietary vitamin D towards serum vitamin D concentration. Major contributors are UVB exposure, especially in summer; and vitamin D supplements in winter. Source of funding Funded by UTAS and the Clifford Craig Medical Research Trust, Launceston, Tasmania.