Re: Efficacy of vitamin and antioxidant supplements in prevention of cardiovascular disease: systematic review and meta-analysis of randomised controlled trials
Myung SK et el have shown in their recently published meta-analysis of randomized controlled trials (RCT's) (1) that there is no evidence to support the use of antioxidants for primary or secondary prevention of cardiovascular diseases. However, we believe that the findings of this study are greatly limited by the quality assessment methods.
Myung SK et al have used the Jadad scale to assess the quality of RCT's. The use of Jadad scale is explicitly discouraged because of the inherent limitations of the scale. A very important limitation of the use of this scale is that it does not take allocation concealment into account. It has been shown that without the use of proper allocation concealment, even rigorously designed allocation sequences can be subverted (2). This can lead to exaggerated or underestimated treatment effects, thus reducing the validity of the study. Thus the findings from even "high quality" studies as per Jadad scale are limited by selection or confounding bias.
The Cochrane Collaboration's tool addresses this limitation and should be used to assess quality of studies in RCT's (3).
1. Myung SK, Ju W, Cho B, Oh SW, Park SM, Koo BK, Park BJ; for the Korean Meta-Analysis (KORMA) Study Group. Efficacy of vitamin and antioxidant supplements in prevention of cardiovascular disease: systematic review and meta-analysis of randomised controlled trials. BMJ. 2013 Jan 18;346:f10.
2. Schulz KF, Grimes DA. Lancet. Allocation concealment in randomised trials: defending against deciphering. 2002 Feb 16;359(9306):614-8.
3. Higgins JP, Altman DG, Gøtzsche PC, Jüni P, Moher D, Oxman AD, Savovic J, Schulz KF, Weeks L, Sterne JA; Cochrane Bias Methods Group; Cochrane Statistical Methods Group. The Cochrane Collaboration's tool for assessing risk of bias in randomised trials. BMJ. 2011 Oct 18;343:d5928.