BDNF and chronic obstructive pulmonary disease: Although, similarly to our current findings in asthma, a significant linear relationship was present between the Impacts score and reciprocal of serum irisin levels among COPD patients (β: 419.97; CI: 204.31, 635.63; p < 0.001), the interaction between serum irisin and BDNF levels was found to be different.