Effect of vitamin D(3) supplementation on iron status: a randomized, double-blind, placebo-controlled trial among ethnic minorities living in Norway

BACKGROUND: Both vitamin D and iron deficiencies are widespread globally, and a relationship between these deficiencies has been suggested. However, there is a paucity of randomised controlled trials assessing the effect of vitamin D supplementation on iron status. PURPOSE: We aimed to investigate whether 16 weeks of daily vitamin D(3) supplementation had an effect on serum ferritin, haemoglobin, serum iron and transferrin saturation. METHODS: Overall, 251 participants from South Asia, Middle East and Africa aged 18–50 years who were living in Norway were randomised to receive daily oral supplementation of 10 μg vitamin D(3), 25 μg vitamin D(3), or placebo for 16 weeks during the late winter. Blood samples from baseline and after 16 weeks were analysed for serum 25-hydroxyvitamin D (s-25(OH) D), serum ferritin, haemoglobin and serum iron. In total, 214 eligible participants completed the intervention (86 % of those randomised). Linear regression analysis were used to test the effect of vitamin D3 supplementation combined (10 or 25 μg) and separate doses 10 or 25 μg compared to placebo on change (T2-T1) in each outcome variable adjusted for baseline s-25(OH)D values. RESULTS: There was no difference in change in the levels of s-ferritin (1.9 μg/L, 95 % CI: -3.2, 7.0), haemoglobin (-0.02 g/dL, 95 % CI: -0.12, 0.09), s-iron (0.4 μg/L, 95 % CI: -0.5, 1.3) or transferrin saturation (0.7 %, 95 % CI: -0.6.1, 2.0) between those receiving vitamin D(3) or those receiving placebo. Serum 25-hydroxyvitamin D increased from 29 nmol/L at baseline to 49 nmol/L after the intervention, with little change in the placebo group. CONCLUSIONS: In this population of healthy ethnic minorities from South Asia, the Middle East and Africa who had low vitamin D status, 16 weeks of daily supplementation with 10 or 25 μg of vitamin D(3) did not significantly affect the haemoglobin levels or other markers of iron status. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12937-016-0192-7) contains supplementary material, which is available to authorized users.