Well here are two (one single study, and one meta-review of multiple studies):
http://www.ncbi.nlm.nih.gov/pubmed/19772843
http://www.ncbi.nlm.nih.gov/pubmed/17452559
http://www.ncbi.nlm.nih.gov/pubmed/19772843
http://www.ncbi.nlm.nih.gov/pubmed/17452559