The body’s primary method for producing energy during sustained activity is the aerobic energy system. This system, also known as the oxidative system, takes place primarily within the mitochondria of cells. It fuels activities lasting longer than a few minutes, such as walking, jogging, or marathon running. Unlike anaerobic systems, which provide fast, short bursts of power, the aerobic system is slower but offers a continuous and virtually limitless supply of adenosine triphosphate (ATP), the body’s energy currency.
Carbohydrates as Primary Fuel
Carbohydrates are the body’s preferred source of fuel for the aerobic system, especially when energy demand is high. They are broken down into glucose, which is circulated in the blood or stored as glycogen in the liver and muscles. Muscle glycogen is an immediately accessible fuel source readily converted back to glucose for use by muscle cells.
The conversion of glucose to usable energy is a rapid metabolic pathway compared to fat, making it the dominant fuel during high-intensity aerobic exercise. Although carbohydrates yield a smaller amount of ATP per gram than fats, the speed at which they are processed is a major advantage when the body needs a quick supply of energy. Stored carbohydrate, however, is limited, and muscle glycogen stores can become depleted after approximately one to two hours of sustained, moderate-intensity effort.
Fats as Long-Duration Fuel
Fats serve as a massive energy reserve for the aerobic system, providing a high yield of ATP that sustains activity over long durations. They are stored primarily as triglycerides in adipose tissue, with a smaller amount stored directly within muscle fibers. The total energy stored as fat is vast, capable of fueling extensive activity.
Metabolizing fat involves a slower process called lipolysis, which breaks down triglycerides into glycerol and free fatty acids (FFAs). These FFAs then undergo beta-oxidation within the mitochondria to enter the aerobic pathway. This slower metabolic rate means fat is the dominant fuel source during rest and low-to-moderate intensity exercise, typically below 65% of maximal oxygen uptake. Fat’s high energy density, generating considerably more ATP per gram than carbohydrates, is why it is the fuel for ultra-endurance.
Proteins as Auxiliary Fuel Sources
Proteins, broken down into amino acids, are not a primary fuel source for the aerobic system. They are structurally and functionally important, serving roles in muscle repair, hormone production, and enzyme function. Their use as energy is typically minimal, contributing only about 5% to 10% of the total energy expenditure during exercise.
The body resorts to catabolizing protein for fuel only when carbohydrate and fat stores are significantly depleted. This typically occurs during periods of extreme, prolonged endurance exercise or during starvation. Amino acids must first be deaminated, a process that removes the nitrogen group before they can enter the metabolic pathways to produce ATP. This process is energetically costly and avoided when other macronutrients are available.
How the Body Selects Aerobic Fuel
The body constantly adjusts the ratio of carbohydrates to fats used for aerobic energy. This relationship is often described by the “crossover concept,” which illustrates the shift in fuel reliance. At rest or during very low-intensity activity, fat is the predominant fuel, supplying approximately 85% of the total energy.
Intensity
As exercise intensity increases, the body needs to produce ATP more quickly than fat metabolism can allow, causing a greater reliance on carbohydrates. At moderate intensity, roughly 40% to 65% of maximal oxygen uptake, the contribution from carbohydrates and fats may be nearly equal. However, once intensity rises above this point, carbohydrates rapidly become the predominant fuel, supplying almost all the necessary energy for high-intensity work.
Duration
Exercise duration also influences fuel selection. As carbohydrate stores, particularly muscle glycogen, become depleted over several hours of continuous exercise, the body is forced to increase its use of fat to maintain energy production. The availability of stored fuel, influenced by diet and training status, also plays a significant role in this selection process.