Make vitamin D while the sun shines, take supplements when it doesn't
Pittaway, JK and Ahuja, KDK and Beckett, JM and Bird, ML and Robertson, IK and Ball, MJ, Make vitamin D while the sun shines, take supplements when it doesn't, Proceedings of the Nutrition Society of Australia, 27-30 November 2012, Wollongong, Australia, pp. 654. ISSN 1836-1935 (2012) [Conference Extract]
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.
To assess the seasonality of serum 25(OH)D and the
contribution of dietary and supplemental vitamin D in
older, community-dwelling Tasmanian adults.
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
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).
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.