After the body's carbohydrate reserves are depleted, which of the following does the body primarily burn for energy?

The body primarily turns to lipids for energy after it has depleted its carbohydrate reserves. Once the glycogen stores, which are the stored form of carbohydrates, are used up, the body shifts its metabolism to utilize fat as its main source of energy. This process involves breaking down triglycerides from fat cells into fatty acids and glycerol, which can then be used for energy production through a process called beta-oxidation. This mechanism is crucial during periods of fasting, prolonged exercise, or low carbohydrate intake, as it ensures that the body continues to receive the energy it needs to function effectively.

In contrast, while proteins can also be used for energy, they are typically utilized only when carbohydrates and fats are insufficient, as proteins have vital roles in the body beyond energy production, such as building and repairing tissues. Simple sugars are a quick source of energy, but once carbohydrate stores are exhausted, they are not a viable option for sustained energy needs. Alcohol can provide calories, but it is not a preferential source of energy for the body and may actually disrupt metabolic processes. This makes lipids the primary choice during times when carbohydrate stores are low.