FNDC5 and obesity due to melanocortin 4 receptor deficiency: Longitudinally, the greatest irisin increase group (T3) over 3 years had higher odds of obesity [odds ratio (OR) 2.39, 95% confidence interval (CI) 1.24–4.71], abdominal obesity (OR 2.19, 95% CI 1.04–4.72), sarcopenia (OR 2.11, 95% CI 1.14–3.97), sarcopenic obesity (OR 3.40, 95% CI 1.43–8.61) and low ALST (OR 2.21, 95% CI 1.24–3.99) at the follow‐up than the T1 group.