They didn't conduct a study on subjects, it's a literature review.

The author's repeatedly demonstrated inability to discern valid information from not makes them fundamentally incapable of conducting such a review. Whether that's out of malice or incompetence, the result is the same.

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They've repeatedly promoted debunked information and fraudulent studies on their Twitter, and a quick skim of the sources for this paper indicates that they've cited such "studies" in this paper.

For example, the "Dr. Mercola" previously exposed as a fraud is...cited here, citation #84. And what a shock, his "study" was also published in this same journal and remains unretracted by it.