When do fats become the major source of energy during exercise?

Fats become the major source of energy during exercise when carbohydrate stores are depleted. This occurs especially during prolonged aerobic activities, where the body exhausts its glycogen reserves, leading to a greater reliance on fat metabolism for energy. As the duration of exercise increases and glycogen levels drop, the body shifts to utilizing fatty acids for fuel because they provide a significant amount of ATP for energy, especially in a resting or low to moderate intensity exercise state.

In the initial stages of exercise, the body primarily uses carbohydrates since they can be metabolized more quickly and provide fast energy. During high-intensity workouts, such as intense weight lifting, the body continues to rely on carbohydrates due to the demand for quick energy. A low protein intake may not necessarily influence whether fats become the major energy source, as proteins are not primarily used for energy during most forms of exercise; their role is more about muscle repair and growth rather than fueling exercise directly. Thus, the depletion of carbohydrate stores is the key factor that leads to the shift towards fats as the main energy source.