Soldiers in combat conduct sustained operations (SUSOPS) characterized by prolonged physical activity coupled with inadequate recovery and nutrition. This high operational tempo creates a catabolic condition in soldiers causing decrements in anabolic hormones (e.g. IGF-1) along with decreased lean body mass (LBM) and physical performance. It is imperative to find ways to attenuate these decrements in order to optimize military performance. Recent evidence suggests â-hydroxy-â-methylbutyrate (HMB) attenuates loss of skeletal muscle under catabolic conditions (e.g. cancer). However, the efficacy of HMB has not been investigated under catabolic conditions imposed by caloric restriction and prolonged endurance exercise. Therefore, the objective of this study is to investigate the efficacy and underlying mechanisms of HMB on body composition, physical performance, muscle mass, myofiber dimensions and myogenic capacity under catabolic conditions induced by caloric restriction and daily endurance exercise simulating a SUSOP in soldiers. Sixty-two young (10 wks old) C57BL/6 male mice were used in the study. Prior to the experimental protocol, mice were divided into baseline (B group) and baseline + HMB (BH group) and underwent a run-in phase for 4 wks to simulate basic combat training (1h/d for 3d/wk). After baseline measurements, 21 mice were sacrificed for baseline tissue isolation. The remaining mice (N=40) were weighed and randomized into four experimental groups (n=10/group): 1) ad libitum-fed (1h/d for 3d/wk wheel walking) [AL]; 2) AL + HMB [ALH]; 3) caloric restriction (CR, -30% of ad libitum groups) + exercise (EX, 2 km/d, 6d/wk wheel walking) [C]; and 4) CR + EX + HMB [CH]. Two groups (C + CH) were designed to simulate a 6 week SUSOP that simulated catabolic conditions soldiers endure. The other two groups (AL + ALH) were designed to simulate the normal training that soldiers undergo when in a garrison environment. Mice taking HMB were given 1% HMB (0.46 g/kg/d) for 4 wks prior to and throughout the experimental phase. Dual X-ray absorptiometry (DXA), magnetic resonance (MR), grip strength, sensorimotor function, and whole-body endurance were assessed pre- and post-experimental period. Also, muscle isolation (gastrocnemius, soleus, quadriceps, hamstrings, triceps) was performed pre- and post-experiment in order to analyze muscle wet weights and transcript factors. Transcript factors involved in muscle cell regeneration and growth: myogenin, myogenic differentiation factor (MyoD), insulin-like growth factor-I (IGF-1), protein kinase B (Akt), mammalian target of rapamycin (mTOR), atrogin-1, and muscle ring finger 1 (MuRF1) were analyzed by RT-PCR. There was a main time effect for total body mass (TBM) driven by the decrease in both the C (-23%) and CH (-17.6%) from baseline. LBM demonstrated a decrease in and the CH group (-23.5%) from baseline. There were main time effects for fat mass (FM) driven by the decrease in both the C (-56%) and CH (-38%) groups from baseline. FM increased in the AL (+25%) group only. Grip strength decreased in C (-10%). The CH group had greater grip strength (+11%) than the C group in their post-measurement. Gastrocnemius muscle mass in the CH group was greater (+11%) than the C group. Also, triceps mass was greater in the ALH group (+15%) compared to the AL group. Fractional anisotropies (FA) increased (+44%) in the AL group from baseline. FA in the C group was significantly less (-29%) than the AL group at post demonstrating more isotropic properties in the muscle. Apparent diffusion coefficient (ADC) increased in the AL group (+11%) and C group (+9%) from baseline. Also, the CH group was greater at baseline than the AL group (+16%) and C group (+12%) in ADC indicating that there were less restrictions to water. The CH group was greater than the AL group (+11%) and C group (+11%) at baseline in ë2 demonstrating a greater long cross sectional area of the muscle fiber. There was a decrease in the AL group (-29%) from baseline in ë3 and the C group was greater (+46%) than the AL group at post measurement demonstrating that the short cross sectional area of the muscle fiber was higher. The most interesting findings in transcript factors were significant group effects for the soleus (p=0.003) for mRNA expression of agtrogin-1 demonstrated by the high expression in the C group above both ad libitum groups. Post hoc revealed that atrogin-1 mRNA expression in the gastrocnemius was higher in the C group (+48%) than the BH group. There were significant group effects for the gastrocnemius (p=0.013), soleus (p=0.0005), and quadriceps (p=0.022) for MuRF1 mRNA expression demonstrating the catabolic groups were elevated above both ad libitum groups. Interestingly, post hoc revealed that MuRF1 expression in the soleus was higher in the C group (+129%) than the CH group. Based on our findings, HMB positively affects body composition (higher LBM and lower FM) and muscle mass (greater triceps mass) under normal conditions. HMB is effective in attenuating loss of strength and muscle under catabolic conditions by attenuating the ubiquitin-proteasome system. Further research needs to be conducted in humans under the same conditions to substantiate these results.