Glycogen Depletion During Athletic Exercise
Glucose, the primary source of fuel for all body cells, is derived primarily from carbohydrates, although, if needed, glucose can also be metabolized from protein. After a meal, some of the glucose not used immediately for fuel travels to the liver or skeletal muscles, where it is converted to a compound called Glycogen–through a process called glycogenesis–and stored for energy. Any excess glucose not needed for storage as glycogen is then stored in adipose tissue as fat. The liver has a greater capacity for glycogen storage than muscle: Liver cells can typically store up to 8% of their weight as glycogen, while muscle cells can typically store up to only 3%. However, since muscle mass or tissue is greater than liver mass (only about 1.6 kilograms of weight, in an adult), more glycogen is stored in the muscles (about 300 grams of glycogen) than in the liver (only about 90 – 100 grams). The liver is responsible for maintaining adequate levels of glucose in the body. As the body’s glucose level drops, the liver converts some of the glycogen back into glucose–through a process called glycogenolysis–and releases it back into the bloodstream. Muscle cells, on the other hand, are unable to reconvert glycogen to glucose. Instead, they convert glycogen directly to fuel through a process called glycolysis.
Glycolysis is a cellular anaerobic process which, through a complex series of steps, breaks down muscle glycogen into pyruvic acid during high-intensity exercise. This process rapidly produces a small amount of adenosine triphosphate (ATP), the necessary fuel for body cells. However, if too much pyruvic acid accumulates in the muscle during glycolysis, it can substantially slow down or even stop the process of ATP formation. Therefore, after one or two minutes of high-intensity exercise, a subsequent process of energy formation begins–oxidation
Oxidation, an oxygen-requiring process of energy formation, produces over 95% of the energy used by muscles during moderate and prolonged exercise. Oxidation immediately converts much of the pyruvic acid formed through glycolysis to ATP. However, during prolonged exercise, if an athlete is unable to breathe in oxygen quickly enough to oxidize pyruvic acid into ATP, some pyruvic acid is converted to lactic acid and diffused out of the cell. It then circulates throughout the body until it can be reconverted to pyruvic acid once oxygen again becomes available. If excess accumulation of lactic acid occurs, extreme fatigue can set in, which can greatly impair the athlete’s performance.
Glucose is needed by the central nervous system to keep the body functioning. Therefore, during periods of moderate exercise lasting longer than 20 minutes, the body works to conserve stored muscle and liver glycogen. It does so by reducing the percentage of fuel derived from glycogen to only 40% or 50%, with the remainder supplied by fat. During exercise periods lasting longer than 4 or five hours, as much as 60% to 85% of fuel produced by oxidation may be derived from fat.
Fats need carbohydrates in order to burn efficiently. The breakdown of carbohydrates generates oxaloacetic acid, which is needed for the breakdown of fats into fuel. If insufficient carbohydrate levels exist, the levels of oxaloacetic acid may also drop, making it difficult for the body to continue producing a high level of fuel from fat. Although the body can break down fats in the absence of carbohydrates, it does so at a much slower rate. When the glycogen stores in the muscles and liver are depleted, and the blood glucose level begins to fall, athletes begin to experience fatigue, lack of coordination, light-headedness and lack of concentration. This experience is commonly known as “hitting the wall” or “bonking”.
Following exhaustive exercise, the body needs to replenish the depleted glycogen reserves. Increasing the intake of carbohydrates promotes the storage of glycogen in the liver and muscles. Therefore, according to Hickson and Wolinsky in their book Nutrition in Exercise and Sport, a diet consisting of approximately 60% or more of complex (starch) carbohydrates is recommended after strenuous exercise in order to promote glycogen replenishment. With adequate consumption of complex carbohydrates, coupled with extra rest, most of the glycogen replenishment occurs within 24 hours. If a diet high in protein and fat is consumed, glycogen replenishment may take longer than one week.
While proper diet is important after an endurance event, it is probably of even greater importance prior to an event. The maximum amount of glycogen stores is ideal when entering into intense exercise. The larger the stores of glycogen in the liver and muscles – within genetic and physiologically normal limitations – the longer and more effectively an athlete can perform during prolonged strenuous exercise. Although many schools of thought exist regarding appropriate nutrition for athletes, most seem to agree that the most important nutrient for endurance athletes is carbohydrates. As much as 60% to 70% of the diet should consist of carbohydrates prior to intense exercise.