Exercise requires a continuous supply of energy, met by burning a mix of fuels, mainly carbohydrates and fat. The term “sugar” in this context refers primarily to stored carbohydrates called glycogen. The exact ratio of this fuel mix constantly adjusts based on the activity’s intensity and duration, with carbohydrates becoming the predominant fuel source as effort increases.
The Body’s Primary Energy Reservoirs
The body maintains two major reservoirs for fuel during physical activity: stored carbohydrates and stored fats. Carbohydrates are stored in the form of glycogen. The majority of this glycogen is held within the skeletal muscles and the liver. Muscle glycogen is directly available to the working muscle fibers, providing a rapid source of fuel.
Liver glycogen is broken down to release glucose into the bloodstream, which helps maintain stable blood sugar levels for the brain and other tissues. The total amount of stored glycogen in a 70 kg person is relatively limited, equating to about 1,700 to 2,000 kilocalories of energy. Once these stores are significantly depleted, a person will experience fatigue, a feeling often described as “hitting the wall.”
Fat is stored primarily as triglycerides in adipose tissue. This reservoir is vastly larger than the carbohydrate stores. In addition to adipose tissue, fat is also stored in the muscle fibers themselves as intramuscular triglycerides. Fat is a highly energy-dense fuel, providing about 9 kilocalories per gram compared to the 4 kilocalories per gram from carbohydrates. However, breaking down fat for energy is slower and requires oxygen, making it better suited for lower-intensity, longer-duration activities.
Fuel Utilization Based on Exercise Intensity
The intensity of exercise is the single most important factor determining the mix of fuel the body burns. During low-intensity activities, the body relies heavily on fat for fuel. This is because the body has ample time to transport oxygen and process fatty acids through aerobic metabolism, which yields a large amount of energy efficiently.
As the exercise intensity increases, the body’s demand for energy production outpaces the rate at which fat can be mobilized and processed. The body must then shift its reliance to the faster-acting fuel source: carbohydrates. This transition point, where the body switches from using fat as the primary fuel to using carbohydrates, is known as the “crossover point”.
At high-intensity levels, carbohydrates become the near-exclusive fuel source. This is due to the rapid recruitment of fast-twitch muscle fibers, which are highly dependent on glucose, and the increased activity of anaerobic metabolism. During very high-intensity exercise, the energy demand is so great that the body relies on glycolysis, the breakdown of glucose. This rapid carbohydrate utilization quickly depletes muscle glycogen stores.
How Training Status and Diet Influence Fuel Selection
While exercise intensity sets the basic fuel mix, an individual’s training status and diet can significantly modulate this ratio. Endurance training causes adaptations in the muscle that make the body more efficient at utilizing fat. Regular training increases the number and size of mitochondria, the cellular structures responsible for aerobic energy production. This increased capacity enhances the muscle’s ability to oxidize fat, allowing the trained individual to burn a higher percentage of fat at a given submaximal exercise intensity.
This physiological adaptation effectively shifts the crossover point to a higher intensity level. By becoming better “fat burners,” trained athletes spare their limited glycogen stores, which delays fatigue during long-duration events.
Dietary composition also influences the fuel selection process. Consuming a high-carbohydrate diet increases the body’s glycogen stores, leading to a greater reliance on carbohydrates during subsequent exercise. Conversely, following a low-carbohydrate, high-fat diet can induce metabolic changes that promote a greater use of fat for fuel. While this adaptation helps conserve limited carbohydrate stores, it can impair performance during high-intensity efforts that depend on rapid carbohydrate metabolism.