Gahche J, Bailey R, Burt V, et al. Dietary Supplement Use Among U.S. Adults Has Increased Since NHANES III (1988–1994). National Center for Health Statistics: NCHS Data Brief No. 61, April 2011.
http://www.cdc.gov/nchs/data/databriefs/db61.pdf
key info
"Dietary supplement use has increased in adults over age 20 since 1994, and we have over one-half of Americans taking one or more supplements a day"