CRD commentary
The review objective and inclusion criteria were clear. Relevant sources were searched for literature, but the search string was apparently limited to two Medical Subject Headings, which means that some studies might have been missed. It is unclear whether steps were taken to minimise the risk of error and bias in the review process, such as having more than one reviewer independently select studies and extract the data and there is no indication that study quality was systematically assessed. This makes it difficult to assess the reliability of the findings, especially as the authors noted that there were high losses to follow-up in the primary studies. The methods used to pool the data appear appropriate. The primary studies were heterogeneous, especially with respect to the intensity and duration of the interventions. This was appropriately addressed by regression analyses, although the authors noted that only a limited number of variables could be analysed because of the small number of studies. The authors' conclusions appear to be supported by the data presented, but the lack of information about review methods and study quality makes it hard to assess their reliability.