Which statement best describes the role of ketone bodies during fasting?

Which statement best describes the role of ketone bodies during fasting?

• A. They provide brain energy and minimize gluconeogenesis and protein breakdown
• B. They increase hepatic glycogen synthesis
• C. They stimulate glycolysis in brain
• D. They are produced in adipose tissue

Answer: (A)

During fasting the body shifts to using fats for fuel, and the liver converts fatty-acid–derived acetyl-CoA into ketone bodies (acetoacetate and beta-hydroxybutyrate, with acetone as a volatile byproduct) which are released into the blood and used by tissues, especially the brain. This provides an important alternative energy source for the brain, reducing its reliance on glucose. By supplying ketones, the brain’s need for glucose falls, which in turn lowers the demand for gluconeogenesis from amino acids and helps spare muscle protein from breakdown. Glycogen stores are depleted during fasting and insulin is low, so hepatic glycogen synthesis is not promoted. The brain’s use of ketones also means glycolysis in the brain is not stimulated; the brain preferentially uses ketones when they are available. Ketone bodies are produced in the liver, not adipose tissue.

During fasting the body shifts to using fats for fuel, and the liver converts fatty-acid–derived acetyl-CoA into ketone bodies (acetoacetate and beta-hydroxybutyrate, with acetone as a volatile byproduct) which are released into the blood and used by tissues, especially the brain. This provides an important alternative energy source for the brain, reducing its reliance on glucose. By supplying ketones, the brain’s need for glucose falls, which in turn lowers the demand for gluconeogenesis from amino acids and helps spare muscle protein from breakdown. Glycogen stores are depleted during fasting and insulin is low, so hepatic glycogen synthesis is not promoted. The brain’s use of ketones also means glycolysis in the brain is not stimulated; the brain preferentially uses ketones when they are available. Ketone bodies are produced in the liver, not adipose tissue.