In endurance training adaptations, which item contributes to decreased carbohydrate metabolism?

• A. Increase in glycolytic enzymes
• B. Activation of phosphofructokinase
• C. Increase in carbohydrate oxidation
• D. Inhibition of rate-limiting enzyme in glycolysis

Answer: (D)

Endurance training shifts fuel use toward fat by increasing mitochondrial capacity and fatty-acid oxidation, which reduces reliance on glycolysis for energy during submaximal work. The rate-limiting step in glycolysis is governed by phosphofructokinase; when its activity is inhibited, the glycolytic flux drops, meaning less carbohydrate is metabolized. Training tends to raise factors like ATP and citrate that inhibit this enzyme, thereby sparing carbohydrate and promoting fat oxidation. So inhibiting the rate-limiting enzyme in glycolysis directly lowers carbohydrate metabolism. Increasing glycolytic enzymes or activating phosphofructokinase would raise glycolysis, and increasing carbohydrate oxidation would increase carbohydrate use.