Your body requires a constant supply of blood glucose to function properly. To maintain an adequate level of blood sugar, your body stores glucose as glycogen in your liver and skeletal muscles. During strenuous exercise, your body uses these stores to provide critical energy, resulting in a decrease in glycogen during exercise.
Glycogen is a polysaccharide and the primary storage form of glucose in your body. Your diet supplies your body with an irregular supply of glucose from carbohydrates. Your body achieves a balance, overcoming the dietary peaks and valleys, by using stores of glycogen. When your glucose levels are high, your body converts glucose to glycogen through glycogenesis and stores it. When your glucose levels are low, your body converts glycogen back to glucose through glycolysis, where it is used as an energy source.
During anaerobic exercise, your muscles use glycogen stored within their tissue to produce a small amount of adenosine triphosphate (ATP) and pyruvic acid. However, the buildup of pyruvic acid slows down the production of ATP energy. This is the reason your muscles become exhausted after one to two minutes of high-intensity anaerobic exercise. Furthermore, in the absence of adequate levels of oxygen, the pyruvic acid is converted to lactate acid, which produces the burn you feel during short, high-intensity muscle exercises.
During aerobic exercise, your body uses glycogen stored within your liver to produce energy. However, because aerobic exercise involves high levels of oxygen in your blood stream, lactate acid levels do not build up. Instead, the pyruvic acid is converted to energy. Exhaustion during aerobic exercise occurs when your glycogen reserve in your liver is consumed. This causes your blood sugar level to drop, resulting in fatigue, a decrease in coordination, light-headedness and reduced concentration. This effect is commonly referred to as “hitting the wall.”
Because glycogen levels are depleted during both anaerobic and aerobic exercise, your body requires a recovery period to return to normal levels. Glycogen synthesis is greatest during a two-hour window immediately after your workout. During this period, consuming simple carbohydrates can dramatically raise your glycogen levels. After two hours, your body replenishes glycogen at a rate of roughly 2 percent an hour. However, if you continuously consume carbohydrates, you can raise this rate to around 5 percent per hour. This means that your body needs 20 hours or more to reach pre-exercise levels of glycogen.
During prolonged moderate-intensity aerobic exercise, meaning longer than 20 minutes, your body attempts to conserve glycogen in your liver by limiting glycolysis to 40 to 50 percent of total energy production. The remainder of the energy your body uses is supplied by converting fat to energy. The longer you work out aerobically, the higher the percentage of energy you burn from fat. Therefore, if you are interested in using exercise to lose weight, focus on prolonged moderate-intensity aerobic exercise.
- Stanford University School of Medicine: Glycogen
- Biochemistry; Glycogen Metabolism; J.M. Berg, et al.; 2002
- Exercise Prescription: Glycogen
- Sagewood Wellness Center; Glycogen Depletion During Athletic Exercise; Maria I. Martos
- University of New Mexico; Glycogen and Resistance Training; Todd Atorino, et al.
- The Sport Journal; Glycogen Replenishment After Exhaustive Exercise; Gregory Tardie, Ph.D.
- Digital Vision/Digital Vision/Getty Images
This article reflects the views of the writer and does not necessarily reflect the views of Jillian Michaels or JillianMichaels.com.