What is the brain's primary fuel source?

• A. Fat
• B. Protein
• C. Glucose
• D. Ketone bodies

Answer: (C)

Glucose is the brain’s main fuel under normal conditions. Neurons rely on glucose because it can be rapidly converted into ATP through glycolysis and oxidative phosphorylation to meet the brain’s high energy demands. The brain consumes a sizable portion of resting energy—roughly a fifth of the body’s energy—despite its small size. When glucose is scarce, the brain can switch to using ketone bodies produced from fats during prolonged fasting or very low carbohydrate intake, but this is a backup adaptation rather than the default fuel. Fat cannot be used directly by the brain for quick energy, and amino acids from proteins aren’t the brain’s typical energy source.