Materials and Methods: 24 male athletes underwent this study. They were divided into three groups. The first group received only one glass of tap water, second group received glucose as high glycemic sugar (1.5 g/kg BWT) and the third group received lentils as low glycemic suger index (2.6 g/kg BWT) half an hour and , 1 hour before the exercise, respectively. Blood samples were taken 15 minutes before exercise. All athletes performed increasing exercise work load on ergometer 57-71% Veo2 max up to 300 watts. After every three minutes pause, 25 watts was increased, and blood samples were again taken. ANOVA method was used for data analysis.Results: The results showed fatigue occurance in 19.3±1.98 min, 22.03±6.68 min and 20.5±4.2 min in first, second and third group, respectively. Vco2 and RQ (Respiratory Quotient) changes were statistically significant (p<0.05) for all groups during exercise, but blood glucose, oxygen uptake, Vo2 max, O2-Puls, Eqo2, Eqco2, Vo2-kg, ventilation and heart rate changes were not statistically significant in all the groups. With increasing exercise intensity, RQ arose. Statistically RQ changes were not significant between first and third group, however, these changes were significant between first and second group as well as between second and third group as (p<0.05). Vco2 depletion was significant for three groups during exercise (p<0.05). Rate of Vco2 depletion was significant between first and second group (p<0.05), but Vco2 depletion was not significant between first and third group and, second and third group. Conclusion: Intake of carbohydrate rich food with low or high glycemic index does have an impact on Vco2 and RQ changes, however, blood glucose, oxygen uptake and heart rate changes are not much affected. To increase athletic performance in short time exercise, glucose intake should be more before exercise and to increase athletic performance in prolonged exercise, intake of low glycemic index carbohydrate should be increased before exercise.