You hear it all the time: "Train in your aerobic zone to maximize fat burning." Training in your aerobic zone means exerting effort so your heart rate is maintained between 60% to 80% of maximum (depending on who you read) for an extended period of time (i.e., 45 minutes of steady state of exercise on a treadmill, elliptical, road-run, etc.). The question is this: is this the best exercise prescription if you're attempting to shed body fat?

Let's take a look at the facts.

On one end of the energy-demand continuum you have sleeping/lying down. This is pure aerobic (with oxygen) and fueled by body fat only. On the other end you have maximal exertion such as sprinting/intense strength training. This is anaerobic (without oxygen) and fueled primarily by immediate stores of ATP, then stored muscle glycogen and circulating blood glucose (blood sugar).
Doing absolutely nothing (i.e., sleeping - the essence of "aerobic") burns very little fat since the energy demands of sleeping is minimal and your body has virtually an unlimited supply of fat. As you increase intensity of effort, the substrate for energy demand shifts to immediate stores (ATP) and eventually muscle glycogen and blood glucose because the muscles do not have time to "aerobically" metabolize fat.

This is where confusion arises: training "aerobically" does burn a greater proportion of body fat, but due to the aforementioned facts, not a whole lot. Stepping up the intensity taps more of the "anaerobic" sources (immediate ATP, muscle glycogen and circulating blood glucose), so it is assumed that working at a higher intensity is not appropriate for targeting body fat stores. Hmmm, makes sense, right?

Not exactly!

The key to fat loss is creating a huge energy-output surplus so stored adipose fats (i.e., mid-section, triceps (women) and buttocks) are tapped to supply energy IN THE RECOVERY PROCESS. That is, working at a high level of intensity uses immediate ATP stores, muscle glycogen, blood glucose and intra-muscular fat stores (yes, fat is available within muscle) - but due to the high-demand exercise the adipose sites are visited during the post-workout recovery period in order to facilitate complete recovery and energy replenishment. E.P.O.C. = Exercise Post-Oxygen Consumption. Look it up.

A number of studies have been performed relative to the above. Check this one out: http://www.cbass.com/FATBURN.HTM

Time-efficient examples (much better than 45+ minutes workout at a steady-state):

:30 work/:15 rest x 20 minutes.
:20 work/:10 rest x 15 minutes.
1:00 work/:30 res x 25 minutes.
:45 work/:20 rest x 10 minutes.

The bottom line: Exercising at low intensity and steady state and for an extended period (i.e., treadmill walking for 45 minutes) does burn some calories, but performing high-intensity interval training (i.e., bust your butt for :30, rest :15, repeat x 20 minutes) actually uses more energy - and thus ultimately more stored body fat (all other factors being equal) -- in less the time as compared to conventional steady state exercise options.

Take home message: to shed body fat, train as hard as you can in 20 minutes rather than easing up and training for 45+ minutes.