de Boer IH, Katz R, Chonchol M, Ix JH, Sarnak MJ, Shlipak MG, Siscovick DS, Kestenbaum B. Serum 25-hydroxyvitamin D and change in estimated glomerular filtration rate. Clin J Am Soc Nephrol. 2011  ;6(9):2141-9.