Which macronutrient is primarily utilized by muscles during low-intensity physical activity?

During low-intensity physical activity, the body predominantly relies on lipids, or fats, as its primary source of energy. This is due to the fact that at lower exercise intensities, the body has sufficient oxygen available to metabolize fat effectively. Fat oxidation is a slower process compared to carbohydrates, making it more suitable for prolonged, steady-state activities where energy demand is lower.

In addition, as exercise intensity increases, the body starts to utilize carbohydrates more significantly because they can be broken down quickly for energy in anaerobic conditions. However, during lower intensities, the slower energy release provided by lipids meets the muscle's energy requirements efficiently without overwhelming the system.

Carbohydrates, while important during higher intensity activities, are not primarily utilized at low intensities. Proteins usually serve a different role as they are more involved in repair and recovery rather than being a primary energy source during exercise. Fiber, while an important dietary component for digestive health, does not serve as a direct energy source for muscles during physical activities. Thus, lipids are the main macronutrient tapped into during low-intensity exercise.