Review Question - QID 5752

During prolonged steady-state endurance events (such as a marathon), the predominant muscle energy source is in glycogen early in the event and free fatty acids (FFAs) after about 90 minutes.

The different energy substrates used during aerobic and anaerobic activities is based upon duration, intensity, primary muscle fiber type activated, and energy source. Aerobic activities are low to moderate intensity that utilize type I muscle fibers and glycogen as it’s primary energy source for about 90 minutes or until stores have been depleted (different for each athlete depending on diet, training regiment, and genetics). Prolonged steady-state exercise lasting several hours is characterized by a shift towards increased lipid oxidation and reduced carbohydrate oxidation rates. This shift in oxidation rates is accompanied by an increased contribution of plasma fatty acids towards energy expenditure and a decreased reliance on both muscle glycogen and intramuscular triglycerides.

Knuiman et al. state glycogen is an essential substrate during high-intensity exercise by providing a mechanism by which adenosine triphosphate (ATP) can be resynthesized from adenosine diphosphate (ADP) and phosphate. The relative use of energy sources during exercise is mainly determined by the intensity and the duration of the exercise bout, as well as the athlete’s training status. Although the amount of liver and skeletal muscle glycogen is relatively small compared to endogenously stored fat, glycogen is recognized as the major source for fuel during prolonged moderate to high-intensity endurance exercise

Tsintzas et al. studied the effects of carbohydrate supplementation during exercise. When carbohydrates are taken during exercise, the carbohydrate feeding during exercise augments exercise performance via multiple mechanisms, consisting of muscle glycogen sparing, liver glycogen sparing and maintenance of plasma glucose and carbohydrate oxidation rates.

Burgstrom et al. found glycogen availability is essential to power ATP resynthesis during high-intensity exercise which relies heavily on glycogenolysis. Furthermore, it has been well documented that the capability of skeletal muscle to exercise is impaired when the glycogen store is reduced to a certain level, even when there is sufficient amount of other fuels available.

Illustration A is a depiction of energy substrate metabolism based on duration of exercise.

Incorrect answers:
Answer 1: Creatine phosphate has a short energy supply and is utilized during short, high energy anaerobic metabolism.
Answer 2: Lactate metabolism occurs during intense exercise in low glucose conditions.
Answer 3: Muscle glycogen is a predominant energy substrate early (<90 minutes) in endurance exercise.
Answer 5: Liver glycogen is also mobilized early during endurance exercise and is a secondary source of glucose to the larger muscle glycogen stores.