Question 167

Question:

Which of the following is used as a metabolic energy source by erythrocytes after an overnight fast?

Correct Answer: B.  Glucose

Glucose is correct because erythrocytes do not have mitochondria. Accordingly, glucose is their only metabolic energy source, regardless of fasting status.

Beta-hydroxybutyrate can be used as a metabolic energy source by muscle or renal cortex tissue after an overnight fast, or by brain tissue in a prolonged fasting state. It is never an energy source for erythrocytes.

Free fatty acids can be metabolized by beta-oxidation in the mitochondria of cardiac and skeletal muscle. They are not an energy source for the brain or erythrocytes.

During fasting or exercise, pyruvate in the erythrocyte is converted into lactate then shuttled to the liver as part of the Cori Cycle. In other tissues, the mitochondrial enzyme pyruvate dehydrogenase can use pyruvate to generate acetyl-CoA for the citric acid cycle.

Triglycerides are the storage form of fatty acids, and are primarily found in the liver and adipose tissue.