Results Searches through databases and review articles located 7605 records (Fig. 1). After removing 2351 duplicated records or records before January 1, 1990, 3656 irrelevant titles, and 675 in eligible abstracts, 923 articles were retained for full text review. Of them,894 were eliminated for not fulfilling one of the criteria. Thus,29 studies were included for outcome-level analysis. Eight studies without an inactive control or minimum intervention were excluded; 21 studies were retained for study-level meta-analysis.