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SummaryStudy aim: To compare the effects of caffeine on metabolic and cardiovascular responses to exercise in lean and obese subjects. Material and methods: In a double blind random design, 6 lean (BMI<20) and 6 obese (BMI>28) sedentary young men performed treadmill running for 30 min at similar exercise intensities (60% ‡O 2 max) one hour after caffeine (5 mg/kg body mass) or placebo ingestion. Gas exchange was measured by indirect calorimetry/open-circuit spirometry. Heart rate (HR) was measured throughout the exercise. Blood pressure (BP) measurements were taken at baseline, 1 h after caffeine/placebo ingestion and immediately after exercise. Repeated measures ANOVA was used in data analysis. Results: Caffeine significantly (p<0.05 -0.01) increased the exercise-induced oxygen uptake, energy expenditure, systolic blood pressure and heart rate in both groups, the respiratory exchange ratio remaining unchanged. The effects of caffeine were in both groups alike. Conclusions: Caffeine activates metabolism without inducing major changes in fat/carbohydrate oxidation. The differences in body fat content seem not to affect the caffeine-induced effects in a submaximal exercise.