When glycogen stores are depleted, which macronutrient is primarily used for energy during distance running?

When glycogen stores are depleted during endurance activities such as distance running, the body begins to utilize fats as the primary source of energy. This metabolic shift occurs because fats provide a more sustainable energy supply when carbohydrate reserves are low.

During prolonged exercise, especially after about 60-90 minutes of moderate to intense activity, the body first relies on glycogen (a stored form of carbohydrates) for energy. As these stores diminish, the body transitions to mobilizing fatty acids from adipose tissue for energy production. This process allows for a continued supply of energy without the need for carbohydrates, which are in short supply when glycogen stores are low.

While proteins can be utilized for energy under extreme conditions or prolonged calorie deficit, they are not the primary source during distance running. Similarly, sugars, while a type of carbohydrate, do not represent the broader category of fats used in this context. Overall, the body's capacity to oxidize fats efficiently allows athletes to maintain performance even in the absence of sufficient glycogen.