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. This evidence concerns the gene EPO and anemia (phenotype).