Background. Load carriage is a heavy task often performed in soldiers, other professions and sports, and is known to be strenuous on the body. Aerobic capacity which includes oxygen uptake and maximal oxygen uptake is the body’s ability to create energy using different substrates and is for assessing performance in load carriage. Soldiers are known to have a high aerobic capacity but it is not known to what degree load carriage has an effect on aerobic capacity, maximal performance or substrate utilization. Nor is it know how substrate utilization is affected during load carriage in participants with different body compositions. Aim. The aim of the current study was to investigate differences in oxygen consumption and substrate utilization with and without military gear and to study the relationship between body composition and substrate utilization in Swedish ground combat soldiers. Methods. Seven male and one female (24.5±3.5 years) participated in the current study. VO2, VO2peak, Respiratory Exchange Ratio (RER) and performance in the form of levels completed in the VO2max test were measured with an Oxycon Pro during a biological calibration and a modified Bruce protocol for VO2max. Body composition was measured with a bioelectrical impedance analysis. Shapiro-Wilks test was used to test for normality, Two-way repeated measures ANOVA was used for analysis of the main effects with gear, main effects of the levels and interaction effects between the tests. Paired samples t-test was used to analyze the differences in VO2peak, VO2 and RER. Spearman’s correlation was used for correlations between fat percentage and levels completed in the VO2max test. Results. The results of the current study showed a significantly lower absolute (p=0.02) and relative (p=0.01) VO2peak with gear compared to without gear. Significantly higher submaximal VO2 values with gear were also found in all active levels of the biological calibration and VO2max test as well as significantly higher RER values in all levels of the VO2max test with gear compared to without. A significant decrease in levels completed in the VO2max test (49%) with gear was also found (p<0.001). No correlations between substrate utilization, body composition and levels completed were found. Conclusion. The main findings of the current study were that VO2peak decreased and submaximal VO2 and RER levels were increased when wearing military gear. No correlations between muscle mass, fat mass or fat percentage and RER and levels completed were found. The findings can help better the understanding of the physiological demands during load carriage and can act as a guide for future load carriage workers, trainers and athletes.