For years, uneducated consumers, fitness enthusiasts and even fitness professionals have misunderstood the mechanism of fat burning, or, using fat as a fuel for work, exercise and life. This is not surprising, as the physiology behind using fat as fuel for our bodies is a very complex process, one that necessitates a deep understanding of energy metabolism. Attempts to simply explain this process have most likely confused us even more! For example, for years, fitness companies told us that “low intensity” exercise burns more fat than higher intensity exercise. Treadmills, ellipticals and other fitness machines list training zones as “fat burning”. In the 90’s, “fat burning” aerobic classes became popular. Myths about this form of exercise still abound today!

So, although I am not an expert in energy metabolism, I’m still going to try and attempt a simplified explanation of how your body uses fat as fuel.

As we all know, our body stores all excess food as fat, a genetically programmed storage method that helped prehistoric man survive fasts. While storing fat is not hard, effectively using fat as fuel is more complex. When your body is moving: breathing, digesting, sitting, thinking throughout the day, we are burning fat as fuel. Fat is only burned in the presence of oxygen, so any activity with sufficient oxygen will burn fat. However, at rest, the amount of fat we actually burn is obviously miniscule. As we exercise, our bodies burn fat at much higher rates than at rest, but also at greatly varying rates, depending on whether there is oxygen available during the exercise and depending on how well we are fueled for our exercise session.

Let’s address the exercise component first. It is true-the lower the intensity of exercise, the greater the percentage of fat is burned as fuel compared to carbohydrates. However, the total amount of calories used is significantly LESS than exercising at higher intensities. And, because all nutritionists agree that weight loss and fat loss is about creating a calorie deficit, this is important to understand.

Here is an oversimplified example of how our bodies might use fat and carbs for fuel:

Walking for ½ hour, or 2 miles, burns 200 calories. Of these 200 calories, 100 may be from fat, 100 from carbohydrates- ie 50% of the fuel is from fat

Jogging for ½ hour, or three miles, burns 300 calories, 100 from fat- the same exact amount of fat, and 200 from carbs- ie 33% of the fuel used is fat. The same calorie amount of fat is burned, but it is a lesser percentage of total fuel. Does this mean walking 2 miles is a better fat burning exercise and will result in better weight loss? NO! In fact, the TOTAL calorie expenditure is much higher during jogging. This results in a greater calorie deficit, and better success at weight loss, as carbohydrates not used for fuel get- YES- stored as fat later! While this example is a bit oversimplified, it illustrates the difference between percentage of fuel used during various activities and TOTAL energy expenditure- which is much more important a criteria.

So, what happens with even higher intensities of exercise such as competitive sports or sprinting or heavy weight lifting?
At times, these activities actually are so intense that our body cannot supply oxygen fast enough to use fat as fuel (anaerobic).Our bodies thus must convert to using carbohydrates stored in our bodies as fuel. Again, using these stored fuels prevents them from becoming stored as fat. But, more importantly, the significant raising of the metabolism during and after these activities produces an afterburn that-YES- burns fat even after the event is finished! The bottom line is:

the higher the intensity of the exercise/activity, the more energy is expended during and after the exercise session, and the greater the ultimate potential for fat loss and weight loss.

Unfortunately, all of this wonderful potential for fat burning and carbohydrate usage can be negated with poor nutrition. THIS IS VERY IMPORTANT TO UNDERSTAND. If you are under fueled- in other words, low on carbohydrates due to dieting or poor meal timing, your body may not burn fat, as it is a very slow process releasing fat from our fat stores, and, may in fact begin to use protein as a source of energy. Using protein as a source of energy is very undesirable. It causes muscle wasting as your body cannibalizes its own muscle tissue for energy. When muscle tissue diminishes, so does your metabolism, and, over time, people who exercise when under fueled may actually become fatter!

Some popular diets advocate sparing carbs to enhance fat burning. But, fat burning is just not that simple, and most of these diets cause weight loss NOT because carbs are eliminated, but because the diets are generally very low in calories (1200 calories). And, as above, when severely under fueled, your body will burn muscle tissue for fuel instead of just fat, and this is often the case with low calorie diets.

Finally, eating too much fat in the diet can also sabotage fat burning potential. If you are consuming large amounts of fat in your diet, your body never needs to tap into its fat stores for energy. Fat is a very rich energy source- it provides the body with nearly twice as much energy as carbohydrates or protein, but overdoing the fat intake will cause it to be stored, not released from storage.

So- what can YOU do to maximize fat burning?

• Include higher intensity exercise several times a week to create a high energy expenditure.
• Include vigorous weight training which has been proven to produce the greatest metabolic afterburn and maintains muscle tissue.
• Make sure nutrition is adequate prior to exercise with plenty of whole grain carbohydrates to avoid muscle wasting.
• Make sure your diet is low in fat, less than 30% is most desirable. Always avoid saturated fats, but do include healthy fats such as vegetable fats, nuts and fish oils.
• Know your caloric needs based on your body composition, exercise regimen and weight loss goals. Since all excess food is stored as fat, eating too many calories will not promote fat usage. On the other hand, going too low in calories will sabotage efficient fat burning, and will eventually cause muscle cannibalization.