In a fasting state, what becomes the primary fuel source for the body?

In a fasting state, the primary fuel source for the body shifts toward fat oxidation. When carbohydrates are not readily available—such as during fasting—the body needs to find an alternative source of energy. During this process, stored fats are broken down into fatty acids through a process known as lipolysis, which then becomes the main energy substrate.

This shift is essential because the body aims to preserve glucose for the brain and other vital organs that rely heavily on glucose for energy. By utilizing fat oxidation, the body efficiently mobilizes fat stores, which provides a substantial amount of energy while sparing protein and preventing the breakdown of muscle tissue for energy. This metabolic adaptation is vital for maintaining energy balance and ensuring proper functioning of physiological processes during periods without food intake.