Question 226

Question:

A 4-year-old girl has been unable to eat for 2 days because of a gastrointestinal tract disorder. Which of the following is the major source of fuel being oxidized by her skeletal muscles?

Correct Answer: D.  Serum fatty acids

This question tests knowledge of the changes in fuel metabolism that occur with starvation.
Skeletal muscle oxidizes fuels to produce Acetyl CoA, which enters the citric acid cycle to yield ATP for energy during use. We need only to know which fuel provides the Acetyl CoA after 2 days of starvation. Let’s go step by step.
In the resting state, skeletal muscles degrade adipose tissue triglycerides (C) and ketone bodies from the liver. During heavy bursts of activity, muscle uses phosphocreatine (A) stores and anaerobic respiration to break down muscle glycogen (B) to lactic acid. Moderately active muscle uses blood glucose (E) in addition to adipose-fatty acids and ketone bodies.
After an overnight fast, glycogen stores in the liver and muscle become depleted. After 24 hours, the blood glucose concentration begins to fall and glucagon is released.
After prolonged starvation (>24 hours), skeletal muscle uses circulating fatty acids (D) from triacylglycerol molecules that were broken down in adipose tissue. This permits the efficient use of fuel for muscle while reserving glucose (and later ketones) for the brain.