Doing cardio at a moderate to high intensity elicits a greater degree of lipolysis (fat breakdown). By stimulating lipolysis, your body has more fatty acids at its disposal and is therefore more inclined to use fat as a fuel during the subsequent strength training workout.

Research has shown that a greater fat loss benefit can be expected if periods of higher intensity cardio are performed during the early phase of the workout. A 2003 study in the European Journal of Applied Physiology showed that fat oxidation was higher in a workout that was initiated with a short bout of high intensity cardio.

The second benefit to doing high intensity interval training before strength training (for fat loss purposes) is that it produces greater than normal cardiorespiratory and metabolic responses, such as increased post-exercise oxygen consumption (EPOC).