INS and metabolic syndrome: Fibroblast growth factor 21 (FGF21) emerges as an insulin-mimetic hormone that regulates systemic energy balance and has beneficial effects on body weight, insulin sensitivity, dyslipidemia, and pancreatic β-cell growth (Kharitonenkov et al. 2005; Wente et al. 2006; Kharitonenkov et al. 2007; Coskun et al. 2008; Gaich et al. 2013).