When we exercise our body uses a combination of all three types of energy; carbohydrates, fat and protein. Different exercises will cause the energy system to shift towards using the most appropriate fuel, largely depending on the level of effort an individual produces during exercise. Anaerobic exercise causes the energy system to shift towards burning predominantly carbohydrates (carbs) simply because movements require more effort. Energy is needed quickly for fast and/or power movements and because sugars (carbs) burn much more easily they form the major source of energy for exercises with high effort such as weight training, squash or sprinting. As much as 90% carbs can be used during anaerobic exercises. Although bear in mind that high intensity exercise can only be performed for a limited time, depending on the fitness level of the individual, thus during periods of rest the energy system will shift back and a little more fat may be burned, although this is only a minor change. An example is weight training with frequent rest intervals between exercising; another is soccer, where a player may run around the pitch until out of breath (burning up to 90% carbs) then have slight periods of walking to aid recovery which reduces the percentage of carb usage.

Aerobic exercises which are generally performed at a slower pace and with less effort usually burn a greater degree of fat. Fat is a slow-burning fuel which requires oxygen and exercising at lower effort enables the cells to use this energy more efficiently, meaning a greater percentage of fat can be burned, up to 70% of total energy used when exercising aerobically. Obviously some people will have greater fitness levels and able to workout at a higher rate of effort and still be exercising aerobically, this will allow the fitter person to burn more calories. An example of this would be an experienced long distance runner. They are very lean because they burn lots of energy and still use mostly fat even though training is fairly intense. Individual fitness levels mean it is important to workout within your own Target Heart Rate when trying to lose fat weight. Or as a beginner a good tip to use when exercising is to make sure you can still manage a conversation during a workout. In other words you want breathing to be deeper than normal but not out of breathe so you cannot talk.

Protein is also burned during exercise but this is usually a small percentage. Protein usage may increase to as much as 20% during high intensity training. However again bear in mind that exercising at such high levels of effort are not maintained for long so the actual amount of protein used for energy will still be small in comparison to carbohydrates and fats.

At this stage you may be thinking that the best way to burn fat is to hardly move so the energy system shifts towards higher degree of fat burning. Yes good thinking, in fact resting will shift the energy system into fat burning mode but because there are so few calories burned at rest it doesnt contribute to effective weight loss. Remember calories do count so if youre consuming 2500 calories per day but only using up 2000 then you will still gain weight, even if most of those calories come from fat stores. All extra energy will simply be converted into fat for storage replacement. For this reason there needs to be a happy medium between using up excess fat stores when exercising and burning enough calories so you always remain in a state of negative energy balance (more calories used up than taken in).

There are a few experts that say as long as youre burning more calories it shouldnt matter what type of fuel is burned, thus high energy exercises would be great for a dieter to lose weight. I disagree with this point, I believe if anaerobic exercise makes up the majority of exercises in your regime then most energy burned will come from carbs. This will drive appetite and encourage sugar cravings in order to replace the glycogen stores used, forcing the dieter to consume more calories. I believe many dieters struggle with cravings and overeating so anything which may cause an upset cannot be good. With aerobic exercise it helps regulate the appetite because glycogen stores are not reduced rapidly. Another point is that for the less fit individual anaerobic exercise is just too intense forcing the body to work very hard thus requiring longer for recovery. This could mean fewer workouts per week resulting in less energy burned over a greater time span. Aerobic exercise often helps people feel more energetic and motivated to workout again within a day or two.

Anaerobic exercise can be great for results but I believe this mainly applies to those with average to good fitness levels. The only exception is doing a weights session a few times per week purely to help boost or maintain lean mass and a higher metabolism. Otherwise I think the major part of an exercise regime should be made up of aerobic exercises, especially for the first 3 months of starting a weight loss program.