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Loss of muscle mass during periods of disuse likely has negative health consequences for older adults. We have previously shown that β-hydroxy-β-methylbutyrate (HMB) supplementation during 10 days of strict bed rest (BR) attenuates the loss of lean mass in older adults. To elucidate potential molecular mechanisms of HMB effects on muscle during BR and resistance training rehabilitation (RT), we examined mediators of skeletal muscle mitochondrial dynamics, autophagy and atrophy, and intramyocellular lipids. Nineteen older adults (60–76 yr) completed 10 days BR followed by 8-wk RT rehabilitation. Subjects were randomized to either HMB (3 g/day HMB; n = 11) or control (CON; n = 8) groups. Skeletal muscle cross-sectional area (CSA) was determined by histology from percutaneous vastus lateralis biopsies. We measured protein markers of mitochondrial content [oxidative phosphorylation (OXPHOS)], fusion and fission (MFN2, OPA1, FIS1, and DRP1), autophagy (Beclin1, LC3B, and BNIP3), and atrophy [poly-ubiquinated proteins (poly-ub)] by Western blot. Fatty acid composition of several lipid classes in skeletal muscle was measured by infusion-MS analysis. Poly-ub proteins and OXPHOS complex I increased in both groups following BR ( P &lt; 0.05, main effect for time), and muscle triglyceride content tended to increase following BR in the HMB group ( P = 0.055). RT rehabilitation increased OXPHOS complex II protein ( P &lt; 0.05), and total OXPHOS content tended ( P = 0.0504) to be higher in HMB group. In addition, higher levels of DRP1 and MFN2 were maintained in the HMB group after RT ( P &lt; 0.05). BNIP3 and poly-ub proteins were significantly reduced following rehabilitation in both groups ( P &lt; 0.05). Collectively, these data suggest that HMB influences mitochondrial dynamics and lipid metabolism during disuse atrophy and rehabilitation.    NEW &amp; NOTEWORTHY Mitochondrial content and dynamics remained unchanged over 10 days of BR in older adults. HMB stimulated intramuscular lipid storage as triacylglycerol following 10 days of bed rest (BR) and maintained higher mitochondrial OXPHOS content and dynamics during the 8-wk resistance exercise rehabilitation program.

Effects of β-hydroxy-β-methylbutyrate on skeletal muscle mitochondrial content and dynamics, and lipids after 10 days of bed rest in older adults