After approximately 2 days of starvation, what metabolic adaptation occurs?

• A. The brain switches its fuel source from glucose to ketone bodies
• B. The liver increases gluconeogenesis
• C. Muscle relies more on amino acids
• D. The brain continues to rely on glucose

Answer: (A)

During prolonged fasting, the body shifts from using glucose as the main brain fuel to using ketone bodies produced by the liver from fatty acids. After about two days of starvation, levels of ketone bodies such as beta-hydroxybutyrate and acetoacetate rise sufficiently to supply a substantial portion of the brain’s energy needs. This adaptation reduces the brain’s glucose demand and helps spare muscle protein from being broken down for gluconeogenesis. While hepatic gluconeogenesis continues to maintain blood glucose, the defining change at this stage is the brain switching to ketone bodies as a major fuel source.