Running burns carbohydrates, utilizing them as a primary energy source alongside fat to power muscle movement. Metabolism during a run is a complex, dynamic process that mixes fuel sources to produce Adenosine Triphosphate (ATP), the body’s energy currency. Immediate energy generation occurs through glycolysis, the metabolic pathway that breaks down glucose derived from stored carbohydrates. The degree of carbohydrate use depends largely on the speed and duration of the effort, which determines the body’s energy demands.
The Body’s Preferred Energy Sources
The body powers all activity by oxidizing a combination of fat and carbohydrates. These two macronutrients serve fundamentally different roles in energy provision. Fat is an abundant fuel source, stored as triglycerides in fat tissue, providing a virtually unlimited supply for endurance running. However, fat metabolism is a comparatively slow process that requires a continuous supply of oxygen to break down fatty acids.
Carbohydrates provide a fast and readily accessible form of energy, but their storage capacity is limited. Carbohydrates are stored as glycogen in the muscles and liver, typically amounting to only about 2,000 to 2,500 kilocalories of total energy. The body quickly converts this stored glycogen into glucose to meet immediate energy needs. This makes carbohydrates the preferred fuel for efforts requiring a rapid rate of energy production.
Intensity Dictates Fuel Use
The intensity of a run is the primary factor dictating the ratio of fat to carbohydrate utilization. As a runner’s pace increases, the demand for ATP rises sharply, and the slower process of fat oxidation cannot keep up with this accelerated energy requirement. This causes the body to shift its fuel mixture to a higher proportion of carbohydrates, as glucose is metabolized much faster than fat to generate ATP.
This metabolic shift is described by the “crossover point,” the exercise intensity where carbohydrate use surpasses fat use. For many moderately active individuals, the highest rate of fat oxidation occurs at approximately 60 to 65% of their maximal oxygen uptake (VO2 max). Beyond this point, the body relies increasingly on the anaerobic pathway, which exclusively uses carbohydrates because it does not require oxygen. This explains why high-effort runs, such as interval training, are heavily dependent on carbohydrate energy.
Glycogen Reserves and Hitting the Wall
The finite nature of carbohydrate reserves means their depletion places a hard limit on sustained high-intensity running. Glycogen is stored within the working muscles and in the liver. Muscle glycogen provides local fuel, while liver glycogen helps maintain stable blood sugar levels. The rate at which these stores are depleted relates directly to both the intensity and duration of the activity.
When these reserves are significantly reduced, the runner experiences profound fatigue commonly referred to as “hitting the wall.” This physiological event occurs because the body is forced to rely almost entirely on slower fat metabolism. Fat metabolism cannot sustain the previous pace, leading to a marked collapse in performance. For many marathon runners, this depletion typically happens around the 20-mile (32 km) mark.
How to Fuel for Optimal Running Performance
Pre-Run Fueling
Strategic carbohydrate intake is necessary to ensure the body’s energy systems are optimally supplied for running performance. Pre-run fueling focuses on maximizing glycogen stores before the effort begins. Consuming a complex carbohydrate-rich meal two to three hours before a long run allows for proper digestion and complete glycogen loading.
Intra-Run Fueling
For runs lasting longer than 90 minutes, intra-run fueling prevents premature glycogen depletion. Runners should consume between 30 and 60 grams of easily digestible carbohydrates, such as gels or chews, per hour. This maintains blood glucose and spares muscle glycogen.
Post-Run Nutrition
Following a run, post-run nutrition is important for recovery and restocking used reserves. Consuming a combination of carbohydrates and protein within 30 to 60 minutes of finishing helps quickly replenish muscle glycogen stores and aids in muscle repair.