What is the sequence in which organic compounds are used by the body during prolonged fasting

The correct answer is: Carbohydrates first, proteins and then lipids.

During prolonged fasting or starvation, the human body goes through distinct metabolic phases to maintain energy homeostasis and preserve vital organs. The utilization of organic compounds occurs in a sequential manner based on their availability, storage, and physiological role:
1. Carbohydrates: The body first utilizes its stored carbohydrates (glycogen in the liver and muscles). Liver glycogen is broken down into glucose (glycogenolysis) to maintain blood glucose levels. This source is relatively small and is typically exhausted within the first 12 to 24 hours of fasting.
2. Proteins: Once glycogen stores are depleted, the body starts breaking down proteins (mainly from skeletal muscle) into amino acids. These amino acids are transported to the liver, where they are converted into glucose via gluconeogenesis to supply glucose-dependent tissues like the brain.
3. Lipids: Following the initial mobilization of proteins, the body shifts towards utilizing its lipid stores (triacylglycerols in adipose tissue) as the primary energy source. Lipids undergo lipolysis to yield free fatty acids and glycerol, which are oxidized to produce energy and form ketone bodies, ensuring long-term survival.