The body burns both carbohydrates and fat for energy during exercise, and the ratio constantly shifts. Metabolism converts these fuel sources into usable energy (Adenosine Triphosphate or ATP). During physical activity, the body pulls energy from stored carbohydrates and fats to power muscle contractions. The relative contribution of each fuel source is highly variable, depending on the intensity, duration, and the individual’s physiological state.
Understanding Carbohydrate and Fat Storage
The body maintains separate reserves for its two primary exercise fuels: carbohydrates and fats. Carbohydrates are stored as glycogen, primarily in the muscles and the liver. Muscle glycogen is an immediate, localized fuel source, while liver glycogen releases glucose into the bloodstream to maintain stable blood sugar levels.
Glycogen storage capacity is limited, providing approximately 1,800 to 2,000 calories. This is enough energy to sustain continuous, vigorous activity for about 90 to 120 minutes. Fat is stored mainly as triglycerides in adipose tissue and within muscle fibers.
Fat is the body’s most concentrated energy source, providing nine calories per gram compared to four calories per gram from carbohydrates. The storage capacity for fat is virtually limitless, even in lean individuals. However, fat metabolism is a slower and more complex process than carbohydrate metabolism, making it less accessible for immediate, high-speed energy production. This difference dictates which fuel the body favors during different types of exercise.
How Exercise Intensity Dictates Fuel Source
The intensity of the activity is the most significant factor determining the fuel mix during exercise. At rest, fat is the dominant fuel source, contributing approximately 85% of total energy production. When exercise begins, the body initiates a metabolic shift.
During low-intensity activity, such as a brisk walk, the body supplies sufficient oxygen for aerobic metabolism. The low energy demand allows the body to rely predominantly on the slow but efficient process of fat oxidation. At very low intensities (below 40% of maximal oxygen uptake), fat supplies the majority of the energy.
As intensity increases to a moderate level, energy needs rise rapidly, increasing the reliance on carbohydrates. Carbohydrate metabolism is faster than fat metabolism, making it the preferred source for quick energy production. The “crossover point” is where carbohydrate and fat contributions are roughly equal, typically occurring around 50% to 65% of maximum aerobic capacity.
When exercise becomes high-intensity, such as sprinting or heavy resistance training, the immediate energy demand is too fast for the body to sustain fat-based aerobic metabolism. Muscles shift overwhelmingly to anaerobic metabolism, which uses only glucose (from muscle glycogen or blood sugar). At very high intensities (above 85% of maximal oxygen uptake), carbohydrates become the nearly exclusive fuel source.
Training Status and Diet Influence on Fuel Use
Long-term training status and diet significantly influence the fuel mixture beyond immediate intensity. Endurance training causes physiological adaptations that make the body more efficient at using fat. This includes increasing the number and size of mitochondria, where fat oxidation occurs.
This improved efficiency allows trained individuals to rely more on fat at a given submaximal intensity than untrained people, effectively “sparing” limited glycogen stores. This glycogen-sparing effect allows athletes to sustain a higher workload for longer before fatigue. The adaptation also shortens the initial delay required for fat metabolism to fully engage at the start of exercise.
Dietary choices also force metabolic adaptation, shifting the body’s preferred fuel source. A high-carbohydrate diet leads to greater reliance on carbohydrate oxidation during exercise, ensuring glycogen stores are readily available. Conversely, a low-carbohydrate, high-fat diet can train the body to increase its capacity to oxidize fat, even at higher intensities, altering the crossover point toward a greater fat contribution.