After a 48-hour fast, what becomes the brain's primary fuel source?

After a 48-hour fast, what becomes the brain's primary fuel source?

• A. Carbohydrates
• B. Amino acids
• C. Essential fatty acids
• D. Ketone bodies

Answer: (D)

When fasting lasts into the second day, the brain increasingly uses ketone bodies for energy instead of glucose. As insulin falls and fatty acids flood the bloodstream, the liver converts those fatty acids into ketone bodies (mainly beta-hydroxybutyrate and acetoacetate). These ketones cross the blood–brain barrier and are readily used by neurons to generate ATP, which reduces the brain’s reliance on glucose. This shift helps spare muscle protein that would otherwise be broken down for gluconeogenesis. So, after a 48-hour fast, ketone bodies become the brain’s primary fuel source. Carbohydrates aren’t supplying the main energy at this stage, amino acids are mainly reserved for maintaining blood glucose, and essential fatty acids aren’t used by the brain for energy in the same way ketones are.

When fasting lasts into the second day, the brain increasingly uses ketone bodies for energy instead of glucose. As insulin falls and fatty acids flood the bloodstream, the liver converts those fatty acids into ketone bodies (mainly beta-hydroxybutyrate and acetoacetate). These ketones cross the blood–brain barrier and are readily used by neurons to generate ATP, which reduces the brain’s reliance on glucose. This shift helps spare muscle protein that would otherwise be broken down for gluconeogenesis. So, after a 48-hour fast, ketone bodies become the brain’s primary fuel source. Carbohydrates aren’t supplying the main energy at this stage, amino acids are mainly reserved for maintaining blood glucose, and essential fatty acids aren’t used by the brain for energy in the same way ketones are.