Beta oxidation, the process by which adipose tissue triglyceride fatty acids (stored fats) are metabolized for energy, is an ongoing process in the body. It’s contribution to the total energy expenditure becomes proportionally larger as other energy substrates, like Glycogen (sugars stored in muscles and liver) and Glucose (sugars floating in the blood stream) are depleted.
In terms of duration, your body starts to burn a greater proportion of fat for energy roughly after 20-30 minutes of continuous moderate-intensity where you can carry on a conversation but are still working up a sweat. Fat oxidation occurs most effectively between 60-70% of your maximum heart rate (MHR), referred to as the “fat-burning zone”. This is due to glucose and glycogen stores in your blood, muscles and liver beginning to deplete.
When you start to exercise, your body first turns to blood glucose and muscle glycogen for energy because it’s quicker and easier to convert these into usable energy. Glucose and Glycogen break down and produce 30-32 units of adenosine triphosphate (ATP) while the Beta-Oxidation cycle breaks down adipose tissue body fat (fatty acids) into 129 units of ATP.
Before Beta-Oxidation Ramps Up:
– Blood Glucose: 40%
– Muscle Glycogen: 50%
– Stored Fat (Fatty Acids): 10%
After Beta-Oxidation Ramps Up:
– Blood Glucose: 20%
– Muscle Glycogen: 30%
– Stored Fat (Fatty Acids): 50%
As you continue to exercise and these quick energy sources deplete, your body begins to break down more stored fat through beta-oxidation to provide the energy it needs. This is why the proportion of energy derived from stored fat increases with the duration of exercise.
Your brain functions off of simple sugars, so this is why if you fee faint or dizzy, you need to stop exercising. This is because your exercise has gotten ahead of your energy production and there is less and less sugars getting to your brain and it starts shutting down.
Summary: This difference in adenosine triphosphate (ATP) generation is one reason why fats are such an efficient form of energy storage. They provide more than twice the amount of energy (ATP) per unit of weight compared to carbohydrates. However, the process of oxidizing fats is slower and requires more oxygen, which is why the body prefers to use carbohydrates for quick, intense activities, and fats for longer, lower-intensity activities.
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