Study quality
The Cochrane Collaboration tool was used for assessing the risk of bias, since it has been accepted that trial inclusion with a high risk of bias may distort the results of a meta-analysis30,31. The quality of all included studies were evaluated by based on the following factors: randomization sequence generation; allocation concealment; blinding of participants, personnel, investigator, and assessor; attrition rates; and financial interest by companies. These were given a rating of low, unclear, or high risk of bias. A randomized controlled trial (RCT) was ranked as having low, medium, or high-risk overall based on the key areas of allocation concealment, reporting of attrition rates, and participants and assessor (high = all 4 factors rated high, medium = 2 or 3 factors rated high or unclear, and low = all key areas rated low).