FNDC5 and Sepsis: Circulating irisin was significantly lower in patients who did not survive sepsis during the 28 days of follow-up after enrollment (N = 30) compared to survivors (N = 72), both at enrollment (17.9 ± 6.3 μg/L vs. 24.1 ± 6.2 μg/L, p < 0.001) and one week after (21.8 ± 7.8 μg/L vs. 28.6 ± 9.5 μg/L, p < 0.001) (Figure 5).