Background: Critically ill patients often develop muscle weakness, which hampers recovery. In septic mice, supplementing parenteral nutrition (PN) with ketone body 3-hydroxybutyrate (3HB) attenuated muscle weakness, but also normalized sepsis-induced low cholesterol1. As 3HB can be metabolized into cholesterol, we hypothesized that improved muscle function with 3HB was related to altered cholesterol metabolism. Methods: In a catheterized, fluid-resuscitated, antibiotics-treated mouse model of prolonged sepsis (cecal ligation and puncture), septic mice received PN supplemented with either D,L-3HB sodium salt (PN+3HB; 150 mg/day) or isocaloric glucose (PN+gluc) and healthy pair-fed mice served as controls (n=15-17 mice/group). After 5 days, ex vivo muscle force and markers of cholesterol metabolism were assessed. In 600 weak and non-weak human critically ill patients (weakness assessed on day 8±1 in ICU by MRC sum score), serum total cholesterol concentration was measured on ICU day 3 or last day for shorter stayers. Results: In mice, PN+3HB counteracted the sepsis-induced lowering of plasma cholesterol (p=0.04), which correlated positively with absolute muscle force (R2=0.19, p=0.002). Plasma mevalonate concentration, a surrogate marker of cholesterol synthesis, was reduced by sepsis (p=0.03 vs. controls), but normalized by PN+3HB (p=0.001 vs. PN+gluc). Skeletal muscle expression of cholesterol synthesis genes Srebf2, Hmgcr and Hmgcs1 was higher in PN+3HB than in PN+gluc septic mice (p≤0.01). Expression of cholesterol uptake receptor Ldlr was also increased in PN+3HB septic mice (p=0.02 vs. PN+gluc), whereas PN+3HB did not affect cholesterol efflux transporters. In contrast, PN+3HB did not alter sepsis-induced alterations in markers of hepatic cholesterol metabolism. Plasma concentration of ubiquinone, a central co-factor of the mitochondrial respiratory chain derived from mevalonate, was increased by sepsis, irrespective of PN+3HB (p
