FNDC5 and chronic obstructive pulmonary disease: (1) As a pilot study, it has a small sample size: this factor did not allow us to perform a normalization for the several available variables in all the comparisons; moreover, at the same time, despite this small number of subjects, the smoking statuses of the COPD group and the control group, that is significantly different (Table 1), have an evident effect: in fact, we found a significant effect of smoking on irisin/muscle mass ratio (Figure 2E);