Just a few for the hell of it (after a couple minutes of searching): Oh yeah - read the citation tree (what it cited and what cited it) of just these four if you want to see TONS more studies.
http://www.ncbi.nlm.nih.gov/pubmed/21247521
http://www.ncbi.nlm.nih.gov/pubmed/7440282
http://circ.ahajournals.org/cgi/reprint/89/1/198
or