Final answer:
As exercise duration increases and glycogen stores deplete, the body increasingly relies on plasma free fatty acids for energy, especially during low-intensity activities where aerobic metabolism is most efficient.
Step-by-step explanation:
As exercise duration increases, the body transitions from primarily using muscle glycogen as a fuel source to relying more on plasma free fatty acids for energy production. Initially, during increased exertion, muscle glycogen is broken down into glucose, which undergoes glycolysis and enters the Krebs cycle to produce energy. However, as glycogen stores begin to dwindle, the liver releases glucose into the bloodstream, and fat metabolism ramps up to maintain energy production.
During exercise, as the intensity level shifts from high to low, there is a higher reliance on aerobic metabolism, which primarily uses fatty acids as an energy source. Unlike glycogen stores, which are limited and can be depleted, leading to fatigue, fat reserves offer a nearly unlimited energy supply over prolonged periods, making them an efficient fuel during extended low-intensity activities.
When glycogen reserves are exhausted, the body increases the use of fatty acids as the principal metabolic fuel for several days, eventually leading to the liver synthesizing ketone bodies from fatty acid breakdown. It's worth noting that these energy pathways are intricately tied to the intensity and duration of the physical activity.