EPOR and anemia: As calcitriol has a synergistic effect with endogenous erythropoietin, it can up-regulate the expression of erythropoietin receptors on erythrocyte progenitor cells [24–26]; however, there is no clear explanation for the relationship between higher levels of 25(OH)D and increased risk of anemia, which appears contradictory to the established effect of 25(OH)D.