Which is the primary fuel used by resting muscle and during low-intensity exercise?

The primary fuel used by resting muscles and during low-intensity exercise is fatty acids. This is primarily because, at rest and during low-intensity activities, the body relies on aerobic metabolism, which predominantly oxidizes fatty acids to provide energy.

Fatty acids are more efficiently utilized in these conditions due to the body's availability of oxygen, allowing for the aerobic breakdown of fats. This process generates more ATP (adenosine triphosphate) per gram compared to carbohydrates and does not require as quick a supply of energy as carbohydrates would provide during higher intensity activities. Additionally, the body's fat stores are substantial, thus allowing prolonged energy supply.

During higher intensity activities, the body would rely more on carbohydrates for quick energy due to their faster rate of metabolism. Proteins are generally not used as a primary energy source in these situations and are typically reserved for more critical functions, such as tissue repair and synthesis when carbohydrate and fat stores are low. Sugars are a type of carbohydrate, and while they provide quick energy, they are not the predominant fuel during low-intensity exercise compared to fatty acids.