Interaction between vitamin D supplementation and sunlight exposure in women living in opposite latitudes (the D-SOL study)

Abstract

Vitamin D deficiency is associated with detrimental effects on bone health and is currently a major global public health issue, with increasing prevalence in both low and high latitude locations. Vitamin D can be synthesised in the skin via sunlight exposure as well as ingested through diet. This study aimed to investigate the interaction and relative contribution of vitamin D supplementation and individual sunlight exposure in raising vitamin D levels above recognized vitamin D cut-off points for deficiency/sufficiency, throughout winter, in ethnically identical adult women living in opposite latitudes. Within two parallel randomized controlled trials (RCT), 135 Brazilian women, (England, n=56, 51˚N; Brazil, n=79, 16˚S), were randomized to receive daily 15 μg vitamin D3 supplements or placebo, for 12 weeks. Oral vitamin D supplementation of 15 μg daily was significantly effective compared to placebo at raising 25(OH)D concentrations over winter, regardless of latitude, and response was dependent on initial 25(OH)D concentrations. Individual UV radiation level was strongly correlated with 25(OH)D concentrations. In both latitudes, supplementation prevented the seasonal concomitant increase in plasma parathyroid hormone (PTH) levels. This research shows: 1) an optimal vitamin D status for bone health around 70-80 nmol/l; 2) the required UV radiation to achieve this status was 1.5 SED; 3) the vitamin D dietary intakes required to achieve these serum levels are 4.5 μg/d at a low and 37 μg/d at higher latitude respectively, with a lower intake of 12 μg/d sufficient to achieve 50 nmol/l in high latitudes. The strength of these results is the novel analysis that directly links human in vivo individual sunlight radiation, increased vitamin D intake and 25(OH)D concentrations, within two parallel RCTs in opposite latitudes. This study demonstrates that a daily supplement of 15 μg vitamin D3 is an effective strategy to significantly raise vitamin D concentrations throughout the winter months in adult females, with important implications for bone health through the concomitant lowering of PTH, regardless of latitude.