FNDC5 and chronic obstructive pulmonary disease: In addition, an 8-week endurance training program increased circulating Irisin only in middle-aged and not in young healthy adults (Irisin concentrations ~140–168 ng/ml, p<0.05)74, while an 8-week chronic exercise program in COPD patients increased circulating Irisin (p<0.05)34.