When people eat carbohydrates, the body’s metabolic system is designed to either use that energy immediately or store it for later demand. The idea of “burning off” carbohydrates with exercise is accurate, but the process is more complex than simply moving after a meal. Understanding how the body utilizes this fuel source during physical activity reveals that exercise intensity determines how much of these stored carbs are used.
How the Body Stores Carbohydrates
Dietary carbohydrates, such as starches and sugars, are broken down into glucose, the body’s primary fuel source. This glucose enters the bloodstream to be used by cells for energy. Any glucose not immediately needed is packaged and stored through a process called glycogenesis.
The stored form of glucose is called glycogen, a complex molecule made of many connected glucose units. Glycogen is stored mainly in the liver and the muscles. Liver glycogen is primarily used to maintain stable blood glucose levels, releasing sugar into the bloodstream when levels drop.
Muscle glycogen is stored for immediate energy during physical activity. Muscle cells use these stores exclusively to power their own contractions, unable to share this fuel with the rest of the body. The body has a limited capacity for glycogen storage, generally holding only enough for about a half-day’s supply.
Fuel Selection Based on Exercise Intensity
The body does not burn only one type of fuel at a time; it uses a mixture of fat and carbohydrates, with the ratio shifting based on the intensity of the effort. At rest and during very low-intensity activities, the body relies mostly on fat oxidation for energy. As exercise intensity begins to increase, the body’s reliance on carbohydrates for fuel also increases.
This shift is explained by the “crossover concept,” which describes the point where carbohydrate-derived fuel starts to contribute more than fat to the total energy expenditure. Fat is a slow-burning fuel that requires a steady supply of oxygen, making it ideal for lower-intensity aerobic activities. When energy demand increases, the body needs a faster rate of energy production than fat can provide.
Carbohydrates, specifically stored muscle glycogen, are broken down rapidly through glycolysis to produce adenosine triphosphate (ATP), the cell’s energy currency. This allows for quick bursts of power or sustained high-output effort, characteristic of anaerobic exercise. The crossover point often occurs around 60% of an individual’s maximum oxygen consumption (VO2max), at which point carbohydrates become the predominant fuel source.
Activities That Maximize Carbohydrate Use
To maximize the use of stored carbohydrates, exercise must be performed at an intensity that forces the body to cross the metabolic threshold.
High-intensity interval training (HIIT) is highly effective because the brief, all-out work periods rely almost exclusively on rapid glycogen breakdown. A short 20- to 30-minute HIIT session can deplete between 40% and 50% of muscle glycogen stores, despite the workout’s short duration.
Sustained high-volume endurance activities also rapidly burn through carbohydrate reserves, as the body must maintain a high power output for a prolonged time. Long-distance running, cycling, or swimming, especially performed at a fast pace, leads to significant glycogen depletion. For example, running at a moderate-to-high intensity uses glycogen at a rate that can exhaust stores in under two hours.
Heavy resistance training also falls into the category of glycogen-depleting activities, as the high force demands of lifting heavy weights activate fast-twitch muscle fibers that are highly dependent on anaerobic glycolysis. The goal in all these types of exercise is to create an energy demand that exceeds the speed at which fat can be broken down, compelling the body to tap into its finite carbohydrate reserves for immediate power.