A few weeks ago I was asked a question by a client about low intensity cardio and the ‘fat-burning zone.’ This is something I always address with my students at U of L, but I have never written about it here, and it is a myth that needs to be exposed!!
The myth of the fat burning zone arose because of the physiological fact that fat is primarily burned as fuel at rest and during low intensity exercise. During these activities aerobic metabolism is the primary supplier of fuel for the cells. The aerobic process takes longer to complete, but since exercise intensity is low or minimal, the cells do not require an immediate source of fuel. Fat is the fuel source because it takes longer to break down, but also yields a greater amount of energy.
During high intensity exercise, fuel is needed quickly- there is not enough time for the aerobic processes to carry out. Therefore, anaerobic (which simply means ‘without oxygen’) metabolism takes over to provide fuel to active cells. The anaerobic process is much faster, so it requires something that breaks down quicker and easier, and this source is carbohydrate. Unfortunately for us the process is very inefficient and produces a byproducts which ultimately accumulate and limit or halt our ability to keep exercising at a high intensity.
So, to recap, fat is the primary fuel source of low intensity exercise, carbohydrate is the primary fuel source of high intensity exercise. Considering that, it is fairly logical to conclude that exercising at a low intensity will burn more fat than exercising at a high intensity, and this is where the famous ‘Fat Burning Zone’ designation that you see on various cardio machines comes from.
Now let me bust up this myth! Calories, while important, are only part of the picture, but for the sake of this example let us assume that while walking on a treadmill at 3 mph you are burning 70% fat and 30% carbohydrate. Conversely, running on a treadmill at 8 mph has the opposite fuel mix: 30% fat and 70% carbohydrate. Based on that breakdown, you may assume that, yes, the lower intensity activity is more conducive to fat burning.
But consider this: walking on a treadmill for one hour at 3 mph may burn roughly 100 calories. Based on our formula above, we would therefore be burning 70 fat calories. Not bad. While running at 8 mph, however, you may burn 250 calories in 30 minutes. Since the intensity is higher, we are only burning 30% fat, BUT since the total calories are higher, we are actually burning 75 fat calories during the running versus 70 with the walking.
In conclusion, yes you burn a higher percentage of fat at lower intensities, but it is a higher percentage of a smaller number. During high intensity exercise you burn more total calories in less time, and you place a physical stress on your body which will take hours to recover from, thus keeping your metabolism elevated long after you have stopped working out, and stored fat will be the primary supplier of fuel during this recovery process!
Low intensity physical activity is very important to your health, but to make changes in your body, high intensity activity is a must!