BACKGROUND: Fatty acids (FA) are the predominate fuel source at exercise intensities < 65% of maximal oxygen consumption (VO2max). Aerobically trained individuals tend to exhibit a greater ability to oxidize FA at higher relative intensities than untrained individuals. Eumenorrheic females have a greater ability to oxidize FA than men, partially due to hormonal differences. Interestingly, a recent review concluded that unlike males, body composition does not affect fat oxidation (Foxi) rates in women. However, no studies have yet matched subjects first for aerobic fitness status prior to testing Foxi rates. Therefore, the purpose of this study is to compare Foxi rates between lean and overweight females when matched prior for aerobic fitness status. METHODS: A between-subjects design will be used to compare Foxi between lean and overweight females with matched fitness statuses. Thirty healthy and eumenorrheic female participants between the ages of 18-39 will be recruited to complete two trials. Trial 1 will consist of descriptive data collection and a peak aerobic test (VO2peak) consisting of a walk phase (1.0-3.5 mph) and a run phase (4-8 mph) using a motorized treadmill set at a 3% grade. Participants will be matched for aerobic status (VO2peak between 35 ± 5 mL·kg-1·min-1) and stratified into 2 groups based on body fat percentage (BF%),(seca mBCA 515, Hamburg, Germany). A graded exercise test (GXT) for trial 2 will be completed during the menses phase of the participants’ menstrual cycle and will consist of 6 (25%, 35%, 45%, 55%, 65%, 75% of VO2peak), 3-minute stages at a 3% incline on a motorized treadmill. Throughout the GXT, metabolic data and heart rate will be collected via Parvo Metabolic cart and a sternal heart rate monitor, respectively. At each stage heart rate and cardiorespiratory measures will be recorded and later used to calculate Foxi rates across each stage. Data will be analyzed with a 2-way repeated measures analysis of variance to identify differences, if any, between lower-fat and higher-fat groups. ANTICIPATED RESULTS: It is hypothesized that, when matched for aerobic fitness, overweight females will demonstrate higher rates of Foxi compared to their lean counterparts at lower intensities.

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