FNDC5 and major depressive disorder: This research is designed to explore the circulating irisin levels in depressive patients.<h4>Methods</h4>From the earliest available records up to 18 July 2024, searches were conducted in databases of PubMed, Embase, Web of Science, the Cochrane Library, and Scopus, in order to identify the relevant articles assessing the correlation between irisin and depression in humans.