Why are fatty acids the predominant energy source for long-duration, low-intensity exercise?

• A. More ATP can be made from one molecule of fatty acids than one molecule of glucose
• B. They are oxidized more quickly
• C. They produce CO2 more rapidly
• D. They are easier to transport in blood

Answer: (A)

In long-duration, low-intensity exercise the body relies on aerobic metabolism, and lipids provide a huge energy reservoir with a very high energy yield. Fatty acids undergo beta-oxidation to generate many acetyl-CoA units, which feed the TCA cycle and oxidative phosphorylation, producing much more ATP per molecule of substrate than glucose. Add in the fact that fats store about twice the energy per gram of carbohydrate, so there’s a larger total energy supply available from fat reserves. This combination makes fats the most efficient long‑term fuel source. The other statements don’t fit: fats aren’t oxidized more quickly (they’re slower to oxidize), they don’t inherently produce CO2 more rapidly for the same energy output, and fats aren’t easier to transport in blood than glucose.