glycogen depletion during athletic exercise

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Glycogen Depletion During Athletic Exerciseby Maria I. Martos

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 is 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%. The liver is responsible for

maintaining adequate levels of glucose in the body. As the bodys

glucose level drops, the liver converts some of the glycogen backinto 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 athletes 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

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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 approximately60% 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 larger

the stores of glycogen in the liver and muscles, 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.It is important for a Sports Massage Therapist to understand the

process of glycogen depletion during athletic exercise. While

working with athletes during training or competitive events, the

Therapist can provide information to them about the process.

Additionally, the Therapist can encourage the athlete to eat a

proper diet during training, as well as prior to and after an

endurance event in order to provide the body with necessary

nutrients for optimum performance. Additionally, and possibly more

importantly, the Therapist can detect signs of glycogen depletion

and be able to assist the athlete during an event.

BIBLIOGRAPHY

Asimov, Isaac. The Human BodyIts Structure and Operation.

Boston: Houghton Mifflin Co., 1963. pp. 238-239.

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Crouch, McClintic. Human Anatomy and Physiology. New York:

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Crowley, Leonard. Introductory Concepts in Anatomy &

Physiology, A Manual for Students in the Health Professions.

Chicago: Year Book Medical Publishers, Inc., 1976. pp. 279-281.

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Guyton, Arthur C. Textbook of Medical Physiology. Philadelphia:

W.B. Saunders Co., 1991. pp. 74, 726-28, 745-51, 790, 857-58,

862, 940-44.

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Hickson, Wolinsky. Nutrition in Exercise and Sport. Boca Raton,

Florida:

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CRC Press, Inc.. 1989. pp. 19, 38-43, 47, 57, 93, 95, 98-99, 115,

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McArdle, Katch, Katch. Essentials of Exercise Physiology.

Philadelphia: Lea & Febiger, 1994. pp. 35-56, 61-66, 146-148,

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Mason, Elliot. Human Physiology. Menlo Park, California: The

Benjamin/Cummings Publishing Co., Inc., 1983. pp.

439-445,448-451.

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Memmler, Wood. The Human Body in Health and Disease, 6th

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