The Difference Between Burning Fat and Burning Calories

Over the weekend, a really good friend was discussing how he "just can't seem to lose 2 kilos". He feels that it is fat weight and we discussed ways in which he can change his workout to optimize fat loss. This got me started thinking about how many different methods of fat loss and fads have come and gone over the many years that I have been a fitness instructor.

If there is one thing I have learned – exercising regularly over time will work to burn fat. The methods that work for each individual vary depending on the individual's weight, cardiovascular fitness level, activity level, metabolism, muscle development, size and even gender!

For all of us, though, if we eat more than our body requires – we either store it as fat or have to "burn it off".

The body at rest uses energy (calories) to power the organs, brain, cardiovascular system, immune system and skeletal muscle. In other words, all of the cells of the body require energy. The rate in which the body uses energy to function is known as the basal metabolic rate. There are two other areas of calorie requirements, one known as the thermic effect of eating (energy required to digest food) and the thermic effect of exercise.

So how does the body convert energy for use during exercise? There are 3 principle metabolic pathways, which are the ways in which the body produces Adenosine Triphosphate (ATP). I promise not to go too technical here and risk boring you…

1. The ATP-PCr System

This is used during the first few seconds of exercise. Creatine phosphate (PCr) in the blood is used to produce ATP which powers the cells.

2. The Glycolytic System

Glycolysis means the breakdown of glucose. The body uses this system for the first few minutes of exercise. If the exercise is very intense (say sprinting), glucose in the blood is used without oxygen and the by product of this will be lactic acid. After a while, the lactic acid accumulates (the burning sensation in the muscle) and leads to muscle failure. This is known as anaerobic or fast glycolysis. If the exercise is less intense (jogging), glucose in the blood is used with oxygen to produce ATP and the by product will be pyruvate which then is used by the oxidative system below. This is known as aerobic or slow glycolysis.

3. The Oxidative System

This system is has 4 parts which I definitely won't go into except to give you the names: slow glycolysis (discussed above), the Krebs Cycle, Electron Transport Chain and Beta Oxidation It is in this system, fats more than carbohydrates (glucose) are used to generate ATP. This process is called lipolysis. The fat produces more "energy" (ATP molecules), but also requires more oxygen.

What Does It All Mean?

In order to burn more fat, the intensity level must be kept at a level that is sustainable for long enough to move into the "fat burning zone" or Oxidative System.

Most people have about 2 hours of stored glycogen (glucose) in the muscle when exercising at moderate intensity. So it's safe to assume that unless you are running a marathon, you are using both stored glucose and fat when doing your aerobic training. This applies to whatever type of aerobic training you choose.Weight training, on the other hand, relies on stored available carbohydrate (glucose) because the short duration/high intensity of each lift doesn't call for the body to burn fat for fuel.

The argument that low intensity long duration exercise burns a greater percentage of calories from fat still stands. Example: a comparison of the calories expended by the same person walking for 1 hour and jogging for 1 hour reveals that the walking burns a much greater percentage of calories from fat although the overall calorie expenditure is lower.

In terms of just pure calorie consumption during exercise – the more oxygen you use the greater your calorie use. If we look at the Oxidative System which uses fat, but requires more oxygen to do, we can infer that working out at medium intensity for longer durations will burn more calories AND a higher percentage of fat.

The current thinking is that the more muscle groups engaged during the activity (example: aerobic dance with arms and legs), the harder the body works requiring more oxygen and more calories. Do it long enough, the body will burn a higher percentage of fat.

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There's a lot of discussion about the "afterburn" or the thermic effect of exercise recovery. Yes, the body does burn energy to recover. A more intense workout requires more energy to recover – hence more calories burned after the workout.

There are many fitness programs that advocate interval aerobics for fat burning. There are just as many that advocate long duration aerobics for fat burning. I advocate a combination of the 2. Both will burn fat. Each has benefits beyond fat burning.

Interval training increases your cardiovascular strength, output, heart rate recovery and produces a stronger "afterburn".

Long duration trains the cardiovascular system to sustain an increased load for a longer period of time and teaches the body to convert fat to energy more efficiently for use during exercise (endurance).

Whatever aerobic activity a person does, the key is to keep doing it. Why? The more a person exercises, the more efficient the body becomes at converting and using available carbohydrate and fat. So… regular aerobic exercise over a long period of time is more important for fat and calorie burning than which method used.