FNDC5 and metabolic syndrome: Indeed, treatment of L6 myotubes with retinoic acid (the carboxylic acid form of retinol, Rac) enhanced insulin-stimulated glucose uptake and GLUT4 expression, reduced body weight and adiposity in lean and obese mice, increased fatty acid oxidation and irisin expression in vivo and presented a protective effect against obesity and metabolic syndrome [57,68,69,70,71].