FNDC5 and Sepsis: Unadjusted Cox regression analyses demonstrated that circulating irisin at enrollment (HR: 0.41, 95% C.I. 0.24–0.69, p < 0.001) and one week after (HR: 0.32, 95% C.I. 0.20–0.51, p < 0.001) were significantly associated with 28-day mortality of sepsis.