In starvation, what nutrient does the brain primarily use as an energy source?

During starvation, the brain primarily utilizes ketone bodies as its main energy source. In a fed state, glucose is the primary fuel for the brain because carbohydrates are readily available. As starvation progresses and glycogen stores are depleted, the body begins to shift its energy metabolism. This involves increased lipolysis, where fat stores are broken down to provide fatty acids. However, fatty acids cannot efficiently cross the blood-brain barrier.

In response to prolonged fasting or starvation, the liver converts fatty acids into ketone bodies through a process known as ketogenesis. These ketone bodies, such as acetoacetate and β-hydroxybutyrate, can cross the blood-brain barrier and serve as an alternative fuel source. This adaptation is crucial for preserving muscle protein and maintaining energy supply to the central nervous system, allowing the brain to function effectively even in the absence of glucose.

Understanding this process highlights the body's remarkable ability to adapt to different nutritional states by utilizing alternative energy sources to support vital functions.