Final answer:
Without carbohydrate consumption, muscle glycogen levels will decrease and perceived effort will increase during sustained endurance exercise due to higher reliance on fat metabolism.
Step-by-step explanation:
Without carbohydrate consumption, muscle glycogen levels will decrease during long-duration endurance exercise. As muscle glycogen is a primary source of energy, its depletion leads to an increased reliance on the aerobic metabolism of fats. However, because fats and oils provide more energy per mass than glycogen due to being in a more reduced form, the body will experience a higher perceived effort when it switches to utilizing fats as a primary energy source instead of glycogen.
This switch to fat metabolism happens because, with prolonged exercise and no intake of carbs to replenish glycogen levels, the body needs to access stored fat for energy. Since glycogen is hydrophilic and retains water, which makes it quicker to metabolize, using fat as an energy source can lead to a feeling of increased exertion. The body's energy systems are designed to use a combination of both glycogen and fat reserves depending on the intensity and duration of the exercise. With low glycogen levels, this balance shifts more toward fat utilization, making the exercise feel more challenging.