A small group of obese and non-obese adolescents were tested to determine maximum fat oxidation rates during exercise. Maximum fat oxidation occured at around 58% maximum heart rate. Males had slightly higher fat oxidation at that intensity. Interestingly, there was no difference between the obese and non-obese groups.
Fat oxidation rates decline at higher intensities because carbohydrate becomes the preferred fuel. Fat simply cannot provide energy fast enough at higher intensities.