After prolonged fasting, what energy source does the brain predominantly use?

After prolonged fasting, what energy source does the brain predominantly use?

• A. Glucose
• B. Fatty acids
• C. Ketone bodies
• D. Amino acids

Answer: (C)

During extended fasting the body shifts to conserve glucose for the tissues that require it most, and the brain adapts by using ketone bodies produced by the liver from fatty acids. These ketone bodies, mainly beta-hydroxybutyrate and acetoacetate, cross into the brain and provide most of its energy needs, which gradually reduces the brain’s reliance on glucose. Fatty acids themselves cannot be used efficiently by the brain because they don’t cross the blood-brain barrier in usable amounts, and while amino acids participate in gluconeogenesis, they’re not the brain’s primary energy source during prolonged fasting. So, ketone bodies become the predominant cerebral fuel after several days of fasting, helping to spare muscle protein and glucose for other essential functions.

During extended fasting the body shifts to conserve glucose for the tissues that require it most, and the brain adapts by using ketone bodies produced by the liver from fatty acids. These ketone bodies, mainly beta-hydroxybutyrate and acetoacetate, cross into the brain and provide most of its energy needs, which gradually reduces the brain’s reliance on glucose. Fatty acids themselves cannot be used efficiently by the brain because they don’t cross the blood-brain barrier in usable amounts, and while amino acids participate in gluconeogenesis, they’re not the brain’s primary energy source during prolonged fasting. So, ketone bodies become the predominant cerebral fuel after several days of fasting, helping to spare muscle protein and glucose for other essential functions.