I took a quick look at pubmed and found two studies. Both support the existence of an effect. http://www.ncbi.nlm.nih.gov/pubmed/25002555 http://www.ncbi.nlm.n

I took a quick look at pubmed and found two studies. Both support the existence of an effect. http://www.ncbi.nlm.nih.gov/pubmed/25002555 http://www.ncbi.nlm.nih.gov/pubmed/25002555 http://www.ncbi.nlm.nih.gov/pubmed/25141012 http://www.ncbi.nlm.nih.gov/pubmed/25141012 And a systematic review that says that an effect exists but little is know about how it might affect results. http://www.ncbi.nlm.nih.gov/pubmed/24275499 http://www.ncbi.nlm.nih.gov/pubmed/24275499