BackgroundExercise is primarily sustained by energy derived from lipids (plasma free fatty acids and intramuscular triglycerides), and glucose (plasma glucose and muscle glycogen). Substrate utilization is the pattern by which these fuel sources are used during activity. There are many factors that influence substrate utilization. We aim to delineate the effect of exercise intensity and body composition on substrate utilization. ObjectiveThe objective of our study was to discern the differences in substrate utilization profiles during a maximal and submaximal graded exercise test, and to determine the extent to which body composition influences substrate utilization during the exercise tests. MethodsA total of 27 male athletes, 32.5 +/- 11 years of age, were recruited for this study. Body composition was analyzed using a bioelectrical impedance analyzer. Maximal and submaximal exercise tests were performed on a treadmill. A novel graded submaximal treadmill protocol was used for the submaximal test. ResultsAverage percent body fat (PBF) was 15.8 +/- 5%. Average maximal oxygen consumption (VO(2)max) was 47.6 +/- 9 mL/kg/min, while the average exercise intensity (percent VO(2)max) at which participants were shifting to glucose predominance for energy during the maximal and submaximal tests were 76 +/- 8.3% and 58.4 +/- 21.1%, respectively. A paired-samples t-test was conducted to compare percent VO(2)max at crossover point in maximal and submaximal graded exercise tests. There was a significant difference in percent VO(2)max at the crossover point for maximal (76 +/- 8.3%) and submaximal (58 +/- 21.1%) tests (t = 4.752, p = 0.001). A linear regression was performed to elucidate the interaction between exercise intensity at the crossover point and body composition during a maximal and submaximal graded exercise test. There was a significant effect of PBF on percent VO(2)max at crossover point during the maximal graded exercise test [F(1,24) = 9.10, P = 0.006] with an R-2 of 0.245. However, there was no significant effect of PBF on percent VO(2)max at crossover point during the submaximal graded exercise test (P > 0.05). ConclusionSubstrate utilization, represented by the crossover point, is dependent on the rate of increase in exercise intensity. At maximal efforts, the crossover to carbohydrates from fats as the predominant fuel source occurs at a significantly later stage of percent VO(2)max than at submaximal efforts. Furthermore, body composition represented by PBF is a significant predictor of substrate utilization during maximal efforts. Athletes with a relatively higher PBF are more likely to have increased lipid oxidation during high intensity exercises than those with a lower body fat percentage.