How does the body primarily fuel low-intensity exercise?

During low-intensity exercise, the body primarily relies on fat metabolism for energy. This is largely due to the availability and efficiency of fat as a fuel source at lower intensities. As exercise intensity decreases, the body shifts towards oxidizing fat for energy, taking advantage of its abundant stores, as fat provides a greater amount of ATP per gram compared to carbohydrates or protein.

At rest and during low-intensity activities, the body uses a higher percentage of energy derived from fat, primarily in the form of free fatty acids released from adipose tissue. These fats are then oxidized in the mitochondria of muscle cells to produce ATP, which is essential for sustaining prolonged, lower-intensity activities such as walking or light jogging.

While carbohydrates can also contribute to energy production during exercise, they are utilized more significantly during higher intensity activities where quick energy is needed. Protein catabolism generally occurs when carbohydrates and fats are insufficient to meet energy demands, which is not a primary source during low-intensity exercise. Phosphocreatine is predominantly used for short bursts of high-intensity activities and does not play a major role in sustained low-intensity exercise normally. Therefore, fat metabolism is the predominant fuel source during low-intensity exercise.