Abstract: Changes in physiological markers and physical performance in relation to paratroopers' initial body composition were investigated during a 20-day winter military field training (MFT) and the subsequent 10-day recovery period. Body composition, serum hormone concentrations and enzymatic biomarkers, and physical performance of 58 soldiers were measured before, during, and after MFT. Comparisons were done according to soldiers' body fat percentage before MFT between low-fat (<12% body fat) and high-fat (>12% body fat) groups. Correlations between body fat percentage preceding MFT and changes in muscle mass, physical performance, and serum hormone concentrations and enzymatic biomarkers were investigated. It was hypothesized that soldiers with a higher fat percentage would have smaller decrements in muscle mass, physical performance, and serum testosterone concentration. The change in muscle and fat mass was different between groups (p < 0.001) as the low-fat group lost 0.8 kg of muscle mass and 2.0 kg of fat mass, while there was no change in muscle mass and a loss of 3.7 kg of fat mass in the high-fat group during MFT. Fat percentage before MFT correlated with the changes in muscle mass (R2 = 0.26, p < 0.001), serum testosterone concentration (R-2 = 0.22, p < 0.001), and evacuation test time (R-2 = 0.10, p < 0.05) during MFT. The change in muscle mass was correlated with the changes in evacuation test time (R-2 = 0.11, p < 0.05) and countermovement jump test results (R-2 = 0.13, p < 0.01) during MFT. Soldiers with a higher initial fat percentage lost less muscle mass, and had smaller decrements in some aspects of physical performance, as well as in serum testosterone concentration during MFT.