Serum 25-hydroxyvitamin D status of the US population: 1988–1994 compared with 2000–2004 - Looker,A.C.; Pfeiffer,C.M.; Lacher,D.A.; Schleicher,R.L.; Picciano,M.F.; Yetley,E.A. (Am J Clin Nutr, 2008)