Effects of β‐hydroxy‐β‐methylbutyrate (HMB) on Myofiber Dimension and Myogenic Response in Old Sprague‐Dawley Female Rats during 10‐Week Resistance Training

Abstract

Aging mammalian skeletal muscles exhibit sarcopenia. We investigated the effects of HMB intake during intense resistance training (RT) on lean body mass (LBM) and myofiber dimensions (MD) of aged rats. Sixteen 19‐month‐old Sprague‐Dawley female rats were divided randomly into 3 groups: CON (control), HMB (0.46 g/kg/d) and Non‐HMB. CON was sacrificed for baseline; HMB and Non‐HMB underwent intense resistance training (weighted ladder climbing) every 3 days for 10 weeks. Dual energy X‐ray absorptiometry assessed pre‐ and post‐RT LBM. In the soleus, diffusion tensor imaging determined water diffusion in myofibers by calculating the fractional anisotropy (FA) and eigenvalues while myogenin mRNA levels were assessed with PCR. Overall, RT improved LBM (+21%, p<0.05) in both HMB and Non‐HMB. The longitudinal eigenvalue λ1 exhibited no change while λ2 (+17%) and λ3 (+20%) increased (p<0.01) in both groups after RT, indicating no change in diffusion along the myofiber length but increased fiber cross‐sectional area (CSA). Decreased FA and increased apparent diffusion coefficient appear to indicate hypertrophic fibers with RT. Myogenin mRNA expression increased (+69%, p<0.05) in both groups. No between group differences were identified for all variables. Our findings suggest that high intensity RT in aged individuals increases LBM, myofiber CSA and myogenic response; however, HMB may not facilitate the changes.