Regarding diabetic populations, most studies demonstrated lower circulating irisin concentrations in patients with T2DM or prediabetes than controls [45,55,56,57,58], whereas in an Italian study, irisin concentrations were higher in children and adolescents with type 1 diabetes mellitus (T1DM) than controls [34]. The gene discussed is FNDC5; the disease is type 2 diabetes mellitus.