Choose the major fuel that is being adapted by the brain after 1 week of fasting.
**Core Concept**
The brain's primary fuel source changes in response to fasting, requiring an adaptation in energy metabolism.
During normal conditions, the brain primarily relies on glucose as its main energy source. However, in the setting of fasting, the brain undergoes a process of metabolic adaptation to utilize alternative energy sources.
**Why the Correct Answer is Right**
After 1 week of fasting, the brain adapts to use ketone bodies (β-hydroxybutyrate, acetoacetate, and acetone) as its primary fuel source, a process known as ketosis. This adaptation is mediated by the enzyme HMG-CoA synthase, which plays a crucial role in the production of ketone bodies. The brain's ability to utilize ketone bodies as an energy source is due to the presence of a high-affinity transport mechanism for β-hydroxybutyrate.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose - Although glucose is the brain's primary fuel source under normal conditions, it is not the major fuel source after 1 week of fasting.
* **Option B:** Amino acids - Amino acids can be used by the brain as an energy source, but they are not the primary fuel source after 1 week of fasting.
* **Option D:** Fatty acids - Fatty acids can be used by the brain as an energy source, but they are not the primary fuel source after 1 week of fasting.
**Clinical Pearl / High-Yield Fact**
The brain's ability to adapt to using ketone bodies as an energy source during prolonged fasting is a critical adaptation that allows it to function normally despite the lack of glucose.
**Correct Answer:** C. Ketone bodies