FNDC5 and obesity due to melanocortin 4 receptor deficiency: In general, irisin serum levels are also affected by obesity, diabetes, and lipid metabolism alteration, revealing a negative correlation between irisin and total cholesterol (TC), triglyceride (TG), HDL-C, low-density lipoprotein cholesterol (LDL-C), and serum homocysteine (Hcy) [46,71,72,73].