Lee CC, Young KA, Norris JM, Rotter JI, Liu Y, Lorenzo C, Wagenknecht LE, Cole DE, Haffner SM, Chen YI, Hanley AJ. Association of Directly Measured Plasma Free 25(OH)D With Insulin Sensitivity and Secretion: The IRAS Family Study. J Clin Endocrinol Metab. 2017 08 01; 102(8):2781-2788.