However, low levels of irisin were found in several metabolic conditions (obesity, metabolic syndrome, metabolic fatty liver syndrome, peri-/post-menopausal osteoporosis) and cardiovascular diseases including heart failure, acute myocardial infarction, and stable coronary artery disease [33,34,35,36]. This evidence concerns the gene FNDC5 and obesity due to melanocortin 4 receptor deficiency.