FNDC5 and chronic obstructive pulmonary disease: Furthermore, only in the COPD group, but not in controls, haptoglobin normalized for fat mass (haptoglobin/fat mass ratio) was significantly and negatively correlated to irisin and to irisin/muscle mass ratio (p = 0.002 and p = 0.003, respectively) (Figure 5B,C), whereas total adiponectin was significantly and negatively correlated to irisin, irisin/muscle mass ratio, and to the Physical Functioning score of SF-36 questionnaire (p = 0.012, p = 0.02, and p = 0.025, respectively) (Figure 5D–F).