What is the metabolic reason for increased fat utilization during endurance exercise?

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BlackTom AI · Accepted Answer

When examining why endurance exercise shifts substrate use toward fats, several physiological mechanisms come into play.
Option 1: Greater glycogen availability would tend to promote greater carbohydrate oxidation rather than fat utilization, since ample glycogen supports glycolysis; thus this option does not explain increased fat use.
Option 2: Enhanced activity of Carnitine palmitoyl transferase (CPT) is a key rate-limiting step in importing long-chain fatty acids into mitochondria for beta-oxidation. During prolonged, steady-state endurance efforts, CPT activity increases, facilitating greater fatty acid entry into mitochondria and therefore greater fat oxidation to spare limited glycogen stores.
Option 3: More muscle mass by itself does not directly explain a shift toward fat utilization during endurance activity. While larger muscle can affect overall metabolism, the immediate substrate shift is driven more by enzyme regulation and substrate availability than by mass alone.
Option 4: Increased carbohydrate oxidation would imply more carbohydrate use, not fat oxidation. In endurance contexts, a shift away from carbohydrate toward fat occurs to preserve glycogen; therefore this option contradicts the observed metabolic adaptation that underpins fat utilization.
Overall, the most consistent mechanistic explanation among the choices is the upregulation of CPT activity, which directly facilitates fatty acid entry into mitochondria for oxidation during prolonged exercise.

KNHES 3415 SP2025 (14442) Exam 3

What is the metabolic reason for increased fat utilization during endurance exercise?

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