FGF21 and metabolic disease: The connection between PPAR-γ, PPAR-α, and serum levels of FGF-21 in the context of metabolic disorders such as type 2 diabetes (T2DM), non-alcoholic fatty liver disease (NAFLD), obesity, poly-cystic ovary syndrome (PCOS), gestational diabetes mellitus (GDM), cardiac disease, and dyslipidemia arises from their roles in regulating glucose and lipid metabolism, insulin sensitivity, inflammation, and oxidative stress.