During aerobic activity, when do fat stores become the primary fuel source?

• A. During the first minutes
• B. Only at rest
• C. After glucose and glycogen are used up during aerobic activity
• D. Never

Answer: (C)

During sustained aerobic activity, the body’s fuel mix shifts toward fat as the quick carbohydrate stores are used up. Carbohydrates provide rapid energy at the start, so glucose and muscle glycogen are utilized first. As those stores dwindle during ongoing exercise, fat becomes the primary energy source to keep the activity going. This switch helps spare glycogen for longer endurance, even though fat oxidation is slower and may not meet all energy demands at higher intensities. The idea isn’t that fat is never used or that it’s primary from the very beginning; rather, fat becomes the main fuel once glucose and glycogen have been depleted during continued aerobic work.