Aerobic vs. Anaerobic, Which Burns More Fat?

As a personal trainer, this is a question I have heard too many times to count and one in which there has been a lot of confusion. Rather than just looking at them as two types of exercise, we should first understand that these are two different energy systems in the body. Aerobic uses oxygen to burn stored fat. Anaerobic uses the body's sugar stores from muscles and the liver. Which system our bodies use depends on the intensity of the activity it is performing. Lower intensity exercise like walking or light jogging depends mainly on the aerobic energy system. Higher intensity exercise like fast running, an up hill climb or resistance training uses the anaerobic energy system.

From this description it would seem that aerobic exercise is the best activity for burning fat. It is important to note that the body will use stored sugar energy from the bloodstream, muscles and liver before really digging into fat stores. It could take 30 minutes of aerobic activity before fat becomes 90% or more of your energy source. However, with aerobic exercise, you are limited to burning only the calories you need to complete the exercise with some metabolic increase for perhaps a couple of hours after exercise.

While anaerobic exercise only relies on stored sugar or glycogen for energy, it has the ability to do something that aerobic exercise doesn't. It can cause the body to release growth hormone and other fat burning hormones. These hormones are released during the recovery and repair of muscle fibers from the stress caused by anaerobic exercise. This hormone release occurs about 12 to 48 hours after the bout of exercise and creates an increase in fat burning. We have the greatest potential for fat burning while we are asleep between the hours of 12:00 and 4:00 a.m. We can burn far more fat during this time than during an hour of aerobic exercise.

For this to be effective, there have to be a few other factors in place. You shouldn't eat any sugar an hour before, during or right after exercise and no sugar before bed. An increase in insulin levels will prevent the release of growth hormone. You also have to be getting enough sleep for the body to recover. Seven hours a night is recommended. A lack of sleep can add stress and too much stress will cause cortisol to be released which also inhibits fat burning. The exercise should be intense and difficult or growth hormone won't be released. The bout of exercise should be 20-45 minutes long. Much longer than this will also increase cortisol levels and counteract the desired effect. These factors make the difference between your exercise routine being effective for losing weight or just being another frustrating attempt.