The paper ( http://www.biomedcentral.com/1471-2458/12/1059 http://www.biomedcentral.com/1471-2458/12/1059 ) is a meta-analysis of papers addressing this correla

The paper ( http://www.biomedcentral.com/1471-2458/12/1059 http://www.biomedcentral.com/1471-2458/12/1059 ) is a meta-analysis of papers addressing this correlation. In the section "Limitations of the evidence base," the author notes the deficiency you pointed out in most papers (only two had a control group), among other deficiencies. The conclusion is a rallying cry for larger and more rigorous studies on the matter since the current (ragtag) evidence shows a noticeable correlation.