CYP24A1 and colon adenoma: Soy protein contains high levels of the isoflavone genistein,19 which acts as a CYP24A1 inhibitor.20, 21 Genistein was already shown to inhibit growth in colonic adenomas and carcinomas22, 23 and combination of 1,25‐D3 with genistein resulted in synergistic growth inhibition of primary prostate epithelial cells.24 Thus, we hypothesized that adding soy to a vitamin D rich diet could reduce xenograft growth by inhibiting CYP24A1 function and thereby prolonging the half‐life of vitamin D metabolites.