‘Sarcopenic obesity,’ a recent medical term, refers to a new trend in aged individuals who simultaneously demonstrate reductions and increases in lean mass and fat mass, respectively. As the elderly population increases, the economic burden of sarcopenia also tremendously increases. Recent evidence reveals that HMB directly increases fat oxidation and regulates myoblast differentiation and survival in cell culture, supporting a possible role in attenuating sarcopenic obesity. However, the efficacy of HMB on myogenic capacity and myofiber dimensions has not been investigated. Therefore, the overarching aim of the present study was to determine the effects of 16 wks of HMB administration in young and old rats on age-related changes in body composition, functionality and myofiber dimensions. We also sought to investigate the possible cellular and molecular mechanisms, which mediated these effects. Twelve young (44 wks), 6 middle aged (60 wks), 10 old (86 wks), and 5 very old (102 wks) male Fisher 344 rats were studied. Their body compositions (dual X-ray absorptiometry, DXA), grip strength and sensory motor function (incline plane) were assessed pre- and post-intervention. After DXA, 6 young, 6 middle aged, 5 old and very old rats were sacrificed for baseline muscles, and remaining young (44 wks) and old (86 wks) rats were given 1% HMB (0.46 g/kg/d) feed for 16 wks and then sacrificed for pre- and post-HMB molecular analysis and for diffusion tensor imaging (DTI) of the soleus and gastrocnemius muscles. Transcript factors involved in muscle cell regeneration and growth: myogenin, myogenic differentiation factor (MyoD), and insulin-like growth factor-IEa (IGF-IEa), mechano growth factor (MGF), myostatin, eukaryotic initiation factor 4E binding protein 1 (4EBP1), and atrogin-1 were also assessed with RT-PCR. There was a group effect for total body mass (TBM) increasing (+23%) in the control, but not in the HMB group from 44-60 wks, while it maintained in the control, and declined (-20%) in the HMB group from 86-102 wks. Fat mass (g) increased in the middle aged control (+49%) but not in the HMB group. Fat mass declined (-56%) in the old HMB group but not in the control group. There were overall declines of 25% in absolute grip strength from 44 to 102 wks of age. However, when normalized to TBM, there was a significant group x time effect (p<0.05) in which strength declined in the control group, but was maintained in the HMB group from 44-60 wks. Moreover, HMB improved strength normalized to TBM (+23%) (p<0.05) from 86-102 wks, with no change in the control group. There was a main time effect for sensory motor function, declining 14% (p<0.05) from 44 to 102 wks of age. However, the HMB group demonstrated an 11% increase (p<0.01) from 44-60 wks. There were declines in gastrocnemius, plantaris, and soleus weights normalized to TBM from 60-102 wks. While HMB had no effects on the gastrocnemius, there were decreases in control planaris (-21%) and soleus (-15%) muscles, which maintained and increased (+12%), respectively, in the HMB group from 44-60 wks. There was a significant decline in DTI-determined myofiber dimensions in the gastrocnemius and soleus from 44 to 102 wks as indicated by declines in eigenvalues 2 and 3 (representative of the cross sectional area) in the 102-wk control, but not 102-wk HMB condition. There were significant group effects for both the soleus (p<0.05) and gastrocnemius muscles (p<0.05) for expression of atrogin-1 mRNA which was greater in the 102-wk control group than all other groups in both the soleus and gastrocnemius. However, the rise was blunted in the soleus in the 102-wk HMB condition. There was a main group effect in the soleus for myogenin (p<0.05) which while approaching significance in the 102-wk control group (p=0.056) only significantly increased in the 102-wk HMB group relative to the 44-wk group (p<0.05). While no other main effects were found in any genes analyzed, post hoc analysis revealed that myostatin was lower in the gastrocnemius, and IGF-IEa was greater in the soleus in the middle aged HMB group than the control group. Our findings suggest HMB may produce a leaner phenotype in middle aged and older individuals, and may blunt the age-related loss of myofiber dimensions in lower limb muscles. These effects on myofiber dimensions may be mediated by prevention of protein breakdown (atrogin-1) and improvement of myogenic capacity; however, the mechanisms underlying HMB’s effects on fat metabolism remain to be elucidated.