- This study tested the hypothesis that of 23 days of β -hydroxy- β -methylbutyrate (HMB) supplementation can maintain muscle mass and attenuate the immune and inflammatory response in combat soldiers during highly intense military training. Soldiers were randomly assigned to either a HMB (n = 6) or placebo (PL; n=7) group and provided with 3 g · day −1 of either HMB or PL. During the final week of supplementation soldiers participated in extreme physical training, which included night navigation of 6–8 hours across difficult terrain carrying heavy loads combined with sleep deprivation (3.8 ± 3.0 h per night). Blood draws were performed prior to and following the supplementation period. Magnetic resonance imaging, which included diffusion tensor imaging sequence, was used for muscle fiber tracking analysis. Data was analyzed using a two-way mixed factorial analysis of variance. Magnitude-based inferences were used to provide inferences on the true effects that HMB may have had on the dependent variables compared to PL, calculated from 90% confidence intervals. Changes in tumor necrosis factor– α for HMB (−3.9 ± 8.2 pg · mL −1 ) were significantly lower ( P = .043) compared to the change in PL (+4.0 ± 3.7 pg · mL −1 ). HMB ingestion was also very likely (92%-95% Likelihood) to lower granulocyte colony-stimulating factor and interleukin 10 compared to PL. In addition, HMB supplementation was likely (78%-87% likelihood) to reduce interferon- γ , interleukin 8, CX3CL1, and increase muscle volume for the adductor magnus (77% likelihood) compared to PL. In summary, the results of this study provides evidence that HMB supplementation may attenuate the inflammatory response to high intense military training, and maintain muscle quality.