Which macronutrient is primarily utilized during prolonged, low-intensity exercise?

During prolonged, low-intensity exercise, fats are the primary macronutrient utilized for energy. The body predominantly relies on a combination of aerobic metabolism and fat oxidation under these conditions. This is because fat, being a high-energy macronutrient, becomes a more efficient source of energy as the intensity of exercise decreases and the duration increases.

At low intensities, there is sufficient oxygen available to allow for the breakdown of fatty acids into usable energy through a process known as beta-oxidation, which occurs within the mitochondria of muscle cells. While carbohydrates can also supply energy during exercise, they are primarily utilized during higher-intensity workouts, where quicker energy release is necessary.

In contrast, proteins are generally not used as a significant energy source during exercise unless absolutely necessary (for example, in prolonged or extreme circumstances) and sugars (simple carbohydrates) tend to be utilized for shorter bursts of energy rather than prolonged low-intensity efforts. Thus, fats are indeed the primary fuel source in this context, making it the correct choice.