We retrieved 8548 references until February 2015, including 6658 references through searching electronic databases, 1890 references through checking other sources, including reference lists, relevant web sites, or personal contact with authors of the included studies. We excluded 8380 duplicates and clearly irrelevant references through reading titles and abstracts. Of the 168 references considered potentially eligible after screening, 137 studies were excluded because they were not original article (i.e., letter, commentary, review) or did not meet the inclusion criteria (Fig 1). Eventually, 31 studies included in the meta-analysis, including 9 cohort studies [23–31] and 10 case-control studies [32–41] and 12 cross-sectional studies [42–53]. All included studies were published in English. In cases of multiple publication, the last report was used.
