Recreational athletes were tested at multiple aerobic exercise intensities (55%, 65%, 75%, 85% and 95% of anaerobic threshold) to determine fat oxidation rates.
No difference was found between the intensity and energy from fat metabolism. The additional energy demands of higher intensity exercise must have been met by carbohydrates. Unfortunately, the post exercise period was not studied.
The researchers did find a significant increase in fat metabolism with increased exercise duration.