de Boer IH, Sachs MC, Chonchol M, Himmelfarb J, Hoofnagle AN, Ix JH, Kremsdorf RA, Lin YS, Mehrotra R, Robinson-Cohen C, Siscovick DS, Steffes MW, Thummel KE, Tracy RP, Wang Z, Kestenbaum B. Estimated GFR and circulating 24,25-dihydroxyvitamin D3 concentration: a participant-level analysis of 5 cohort studies and clinical trials. Am J Kidney Dis. 2014 Aug; 64(2):187-97.