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Caffeine had been shown to have an ergogenic effect on trained individuals; however, studies investigating the physiological effects of caffeine on the sedentary population are limited. The aim of this study was to examine the effect of caffeine on oxygen consumption and rating of perceived exertion during moderate intensity exercise among sedentary young adult females. Subjects comprised 16 female undergraduates aged between 22 to 24 years studying at Universiti Kebangsaan Malaysia. Eligibility criteria were based on low physical activity level and daily caffeine intake of less than 50 mg a day, which was screened using the International Physical Activity Questionnaire (short version) and caffeine consumption questionnaire, respectively. The design of this study is single-blind, crossover, placebo-controlled with all subjects serving as their own controls. Subjects were required to report to the physical activity laboratory for two experimental sessions after either ingesting placebo or caffeine capsule with an interval of 3 days between these two experimental sessions. Sixty minutes after ingesting placebo capsule (Glucolin, glucose) or 100 mg caffeine (Pro-plus, United Kingdom), subjects were required to run on a treadmill for 30 minutes at a standardized power output equivalent to 60% of maximal heart rate. Oxygen consumption, heart rate, and rating of perceived exertion were recorded at 20th, 25th and 30th minutes, while blood pressure was recorded immediately after subjects completed their 30 minutes run. Mean body fat percentage was 28.4 ± 5.4. Differences were recorded after every subject completed both the placebo and caffeine experiments. Paired t-tests showed no significant difference between placebo vs caffeine trials for oxygen consumption (13.99 ± 2.47 vs 14.49 ± 1.73, p = 0.440), rating of perceived exertion (12.3 ± 2.5 vs 12.3 ± 2.1, p = 1.000), systolic blood pressure (113 ± 10 vs 117 ± 11, p = 0.129), diastolic blood pressure (67 ± 8 vs 69 ± 10, p = 0.408) and heart rate (127.3 ± 11.0 vs 127.1 ± 11.6, p = 0.912). There was strong significant negative correlation between body fat percentage and oxygen consumption (r = --0.568, p < 0.05) and strong significant positive correlation between body fat percentage and rating of perceived exertion (r = 0.515, p < 0.05). The non-significance in the results obtained could be due to the small effect size of the study (d = 0.24). Hence, future studies with a larger number of participants should be carried out to examine the effects of caffeine during exercise in a sedentary population. [ABSTRACT FROM AUTHOR]