During starvation, which of the following is depleted first?
**Core Concept**
The body's energy stores are primarily composed of glycogen, which is a complex carbohydrate stored in the liver and muscles. During starvation, the body initially breaks down glycogen to maintain blood glucose levels.
**Why the Correct Answer is Right**
The liver is the primary site for glycogen storage, and when glucose levels are low, the body depletes glycogen stores through the action of glycogen phosphorylase. This enzyme catalyzes the breakdown of glycogen into glucose-1-phosphate, which is then converted to glucose-6-phosphate and eventually released into the bloodstream. As glycogen stores are depleted, the body begins to break down fat for energy.
**Why Each Wrong Option is Incorrect**
* **Option A:** Muscle glycogen is depleted concurrently with liver glycogen, but the liver is the primary site for glycogen storage and breakdown.
* **Option B:** Protein stores are not depleted first during starvation, as the body has a more limited capacity to break down protein for energy.
* **Option D:** Fat stores are broken down after glycogen stores are depleted, as the body's primary source of energy during starvation is glucose.
**Clinical Pearl / High-Yield Fact**
The body can store approximately 100-200 grams of glycogen in the liver and muscles, which is equivalent to about 1-2 days of energy storage.
**Correct Answer:** A. Liver glycogen.