MSTN and obesity due to melanocortin 4 receptor deficiency: The common pathophysiological basis of sarcopenia and obesity may play a synergistic role in the progression of SO, including increased inflammatory factors (e.g. interleukin 6 [IL-6], C-reactive protein [CRP] and tumour necrosis factor-α [TNF-α]), IR (e.g. serum insulin), endocrine dysfunction (e.g. insulin-like growth factor 1 [IGF-1], leptin [LEP] and adiponectin [ADP]), changes in myostatin (MSTN) and decreased physical activity (PA) [10–12].