Running significantly increases the body’s metabolic rate, demanding substantial energy to sustain movement. This vigorous exercise forces the body to ramp up its energy-producing processes, leading to measurable changes in energy expenditure. Understanding the energy sources the body uses and how this process is calculated is important for anyone focused on fitness. The total energy required involves not just the immediate activity but also metabolic shifts that continue long after the run is finished.
Calculating Energy Burn
Energy expended during a run is quantified by calculating the number of calories burned. This calculation relies on Metabolic Equivalents (METs), which compare the energy cost of an activity to the energy used while sitting at rest. One MET is equivalent to consuming roughly 3.5 milliliters of oxygen per kilogram of body weight per minute. Running is a high-MET activity, ranging from 6.0 METs for slow jogging to 12.5 METs or more for faster paces. Total energy output is directly influenced by body weight, and distance covered is the most reliable predictor of total calories burned.
Fuel Sources Used During Running
Energy is generated primarily by drawing upon the body’s stored macronutrients: carbohydrates and fats. The relative contribution of each fuel source is determined by the intensity of the run, a concept known as the “Crossover Concept.” Carbohydrates are stored as glycogen in the muscles and liver, and their breakdown provides a fast, readily available source of energy. Fat, stored as triglycerides, is a vast energy reserve but requires more oxygen and time to process. During lower-intensity running, the aerobic system relies more heavily on fat oxidation, but as intensity increases, the body shifts toward utilizing carbohydrates as the predominant fuel source.
The Post-Run Afterburn Effect
Even after a run is finished, the body continues to burn calories at an elevated rate through Excess Post-Exercise Oxygen Consumption (EPOC), often called the “afterburn effect.” EPOC represents the body’s effort to return to its resting state and repair physiological disruptions caused by exercise. During this recovery period, the body uses extra oxygen and energy to restore depleted stores of adenosine triphosphate (ATP) and phosphocreatine. Energy is also expended to re-oxygenate tissues, balance hormones, and fuel cellular repair. The magnitude and duration of the EPOC effect are strongly correlated with the intensity of the run, meaning higher-intensity efforts lead to a more prolonged afterburn effect.
Running and Muscle Preservation
While carbohydrates and fats are the primary fuels, runners often worry about whether the activity “burns” muscle tissue, which is composed of protein. Running is a catabolic activity, meaning it involves breaking down molecules for energy or repair. Protein generally contributes a small fraction of the total energy cost of exercise, estimated to be up to 5.5% of total calories. However, prolonged running when glycogen stores are low or in a fasted state can lead to increased reliance on protein breakdown for fuel. To prevent excessive muscle catabolism, runners must consume adequate nutrition, particularly protein, post-exercise, and incorporate resistance training to stimulate muscle protein synthesis.