Why exercising in the “fat burning zone” doesn’t mean you will lose more weight.
If you have heard the term “fat burning zone” and have been told that low intensity aerobic training will burn more fat than high intensity aerobic training you may have been mislead.
The fat burning zone is form of low intensity cardio in which fats in the body are burned to provide fuel. Although at a lower intensity you will burn a higher percentage of body fat, this doesn’t mean you will lose more body fat in the process. Let me explain.
How we use fuel in the body.
Your body has 2 main fuel sources, carbohydrates and fats. These both provide energy to the body in order to carry out daily tasks and exercise. Our body will use the appropriate fuel source for the job however, often using both in various percentages. The body will naturally want to use the fuel source that is most abundant in the body at the time to perform every day tasks.
Activities like walking, and low intensity cycling will use fat as the dominant fuel source. As a percentage you may be burning around 70-80% fats and 20-30% as carbohydrates. This will depend on exercise intensity. Using fat as the dominant fuel sources allows for greater endurance to the activity. This fuel can keep us going for several hours even if you are relatively unfit. Because of this, we require the use of oxygen in order to break down fats and convert in to ATP which can then be used for fuel in the muscles. Hence the reason it is used in low intensity tasks where we can comfortably keep up with the demand with oxygen present.
Moderate exercise such as a slow jog or a fast-paced cycle will again use both fuel sources. Maybe closer to 50% of each. As we increase intensity the more carbohydrates will be used to fuel activity as more power and force is required and less oxygen is available to break down fat to be used as fuel.
So why does the “fat burning zone” not burn more body fat?
As I mentioned before, our body will use the fuel source most abundant at the time in order to provide energy. If you have used available fat in the body for fuel during your hour cycle, it is likely carbohydrates (glycogen) will be more abundant post exercise and therefore become the dominant fuel source for any other body functions or activity which energy is needed.
The energy balance equation still apply’s
If you were to burn 200kcal during 1 hour of low intensity bike work, 150 of those calories may come from fat and 50 may come from carbohydrates. This is fantastic. However, If you were to cycle for an hour at a moderate speed you may burn 400kcal in this time while burning 200kcal from both fats and carbs making the moderate cycle more effective for fat loss due to the increased calories burned. Calories in/out will still determine fat loss over time while somewhat ignoring which fuel source was used.
This simply means you can burn more calories during moderate to intense exercise compared with low intensity exercise which is more optimal for fat loss.
This does not mean Low intensity exercise is useless!
In fact “Zone 2” training is used by amateurs and athletes in order to build a large aerobic base of fitness, while also allowing increased blood flow in the body to help with recovery. It also allows for effective training while accumulating very little fatigue in the body which means you can do it for longer periods.
Zone 2 or low intensity exercise helps to increase your aerobic capacity and makes you more efficient at using fuel and over longer distances, especially using fat for fuel instead of carbohydrates.
To summarise…
So, if burning as many calories and losing as much body fat as possible is your aim (this should never be the aim of any workout) you are better off working at a higher intensity and heart rate in order to burn the most amount of fuel and calories.
If you want to enjoy exercising for the many, many benefits it brings and focusing on your diet to control body weight. You should include a variety of different intensities in your cardiovascular training in order to increase fitness, boost performance and work on keeping your heart and lungs in a good, healthy condition.
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