when I see the word study, and I think oh good. Someone has done a clinical trial. Enrolled patients and compared, in a double blind manner, interventions against placebo. No. It is a meta-analysis. Calling a meta-analysis a study is not unlike a library declaring they acquired several dozen new books and magazines when their copy of Readers Digest arrives. Someone massaging preexisting data; hardly a study. Nice for an overview of a topic, but worthless for drawing conclusions definitive conclusions.
When applied to real treatments, those based on reality and known physiology, the results of meta-analysis are often not predictive of well done clinical trials:
The outcomes of the 12 large randomized, controlled trials that we studied were not predicted accurately 35 percent of the time by the meta-analyses published previously on the same topics