After carbohydrate reserves are depleted, what does the body burn next?

After carbohydrate reserves are depleted, the body turns to lipids, or fats, as the next source of energy. This transition occurs because the body requires a continuous supply of energy for its various functions, and once glycogen stores are used up, it needs to utilize alternative energy stores.

Lipids serve as an important energy source because they contain more than double the calories per gram compared to carbohydrates. The body breaks down fats through a process called lipolysis, converting triglycerides into fatty acids and glycerol. These fatty acids can then be oxidized in the mitochondria to produce ATP, which is the energy currency of cells.

Proteins, while they can eventually be used as an energy source, are usually not the body's first choice after carbohydrates and fats. The body prefers to use proteins for other essential functions such as tissue repair and synthesis of hormones, enzymes, and other vital molecules. Burning proteins for energy generally occurs only in prolonged fasting or malnutrition when fat stores are insufficient.

Sugars, specifically glucose, are the primary energy source when available and are derived from carbohydrates initially. Once carbohydrate stores are exhausted, the focus shifts away from sugars to fats.

Vitamins do not serve as an energy source; instead, they are essential micronutrients