MSTN and Insulin resistance: Although another limitation is that circulating levels of myostatin do not necessarily reflect tissue myostatin expression, previous studies have shown excellent correlation between myostatin tissue expression and secretion (Brandt et al., 2012; Hittel et al., 2009), and studies have shown that greater skeletal muscle myostatin mRNA and protein expression are associated with insulin resistance (Hittel et al., 2009; Park et al., 2006; Ryan et al., 2013).