Effect of supplementation with vitamin D on biochemical markers of iron status and erythropoiesis in older people: BEST-D trial.

Previous observational studies suggested that vitamin D may control absorption of iron by inhibition of hepcidin, but the causal relevance of these associations is uncertain. Using placebo-controlled randomization, we assessed the effects of supplementation with vitamin D on biochemical markers of iron status and erythropoiesis in community-dwelling older people living in the United Kingdom (UK). The BEST-D trial, designed to establish the optimum dose of vitamin D3 for future trials, had 305 participants, aged 65 years or older, randomly allocated to 4000 IU vitamin D3 (n=102), 2000 IU vitamin D3 (n=102), or matching placebo (n=101). We estimated the effect of vitamin D allocation on plasma levels of hepcidin, soluble transferrin receptor (sTfR), ferritin, iron, transferrin, saturated transferrin (TSAT%), and the sTfR-ferritin index. Despite increases in 25-hydroxy-vitamin D, neither dose had significant effects on biochemical markers of iron status or erythropoiesis. Geometric mean concentrations were similar in vitamin D3 arms vs placebo for hepcidin (20.7 [SE 0.90] vs 20.5 [1.21] ng/mL), sTfR (0.69 [0.010] vs 0.70 [0.015] µg/mL), ferritin (97.1 [2.81] vs 97.8 [4.10] µg/L) and sTfR-ferritin ratio (0.36 [0.006] vs 0.36 [0.009]), respectively, while arithmetic mean levels were similar for iron (16.7 [0.38] vs 17.3 [0.54] µmol/L), transferrin (2.56 [0.014] vs 2.60 [0.021] g/dL), and TSAT% (26.5 [0.60] vs 27.5 [0.85]). The proportions of participants with ferritin <15 µg/L and TSAT<16% were unaltered by vitamin D3 suggesting that 12 months of daily supplementation with moderately high doses of vitamin D3 are unlikely to alter the iron status of older adults.