The term "hitting the wall" is commonly used when athletes become so fatigued that they can no longer perform due to a depletion in blood glucose levels. This often happens around the 19th mile of a marathon, and it may happen much sooner if proper nutrition practices are not followed prior to exercise.
Breakdown of Glucose
Glucose is ingested through the consumption of carbohydrates. It enters the bloodstream and may go through the process of glycogenesis to form glycogen, which is then stored in muscles and liver. Liver glycogen stores are used to replenish blood glucose when it is low. When needed, muscle glycogen is broken down through a process called glycogenolysis to create glucose 6-phosphate. Glycolysis then breaks down the glucose 6-phosphate into lactate, also producing adenosine triphosphate, or ATP--the energy currency used in metabolism.
Glucose as a Fuel
Carbohydrate-derived glucose is the major source of fuel for high-intensity exercise. As exercise is prolonged, the fuel source gradually switches to fat. During extremely prolonged exercise--three to five hours duration--protein may supply up to 15 percent of the fuel in the final minutes, as opposed to less than two percent that protein typically provides. However, because glycogen primarily powers the energy-producing cycle called the Krebs Cycle, and this cycle is the only method of metabolizing fat, depletion of glycogen will slow the rate of fat metabolism. Therefore, glucose is necessary for efficient metabolism of fat.
Depletion of Glucose and Glycogen
When blood glucose and muscle glycogen are depleted, muscular fatigue occurs. Without carbohydrates to undergo glycolysis, pyruvic acid in the muscle is reduced, thus reducing the amount of ATP produced aerobically in the Krebs-cycle. Ideally fat metabolism would occur at this time. However, since carbohydrates are needed for fats to burn, the metabolism of fat also slows. Proteins may kick in by degrading to amino acids, which can sometimes be metabolized in skeletal muscle, providing a short source of energy. Lactate that was previously produced through glycolysis can undergo gluconeogenesis to help synthesize liver glycogen and provide fuel for skeletal muscle and the heart. (See Reference 2)
Gluconeogenesis
Gluconeogenesis is a last-resort process of creating fuel. During this process, noncarbohydrate precursors--such as lactate, pyruvate, glycerol and some amino acids--form glucose. Glycerol, a primary precursor, is released from fat cells through the process of lipolysis, the breakdown of triglycerides. Two of these molecules can make one glucose molecule. Skeletal muscle is broken down to make glucose if the availability from food energy is too shy. If all of these methods are exhausted, complete fatigue will set in to the muscles as energy is no longer able to be produced.
Prevention of Glucose Depletion
Eating the proper amounts of carbohydrates each day will help you maintain optimal blood glucose and muscle glycogen levels. The brain and central nervous system use about 125 g of glucose each day, and other glucose reliant tissues need 30 to 40 g. Any exercise or work will add to the daily need. Fasting, starving and diets low in carbohydrates may decrease glucose availability to unsafe levels. During prolonged exercise, consuming carbohydrate electrolyte beverages will help you maintain the glucose levels you need for desirable energy production.
References
- "Biochemistry Primer for Exercise Science: Third Edition"; Michael E. Houston; 2006
- "Exercise Physiology: Theory and Application to Fitness and Performance: Fifth Edition"; Scott K. Powers & Edward T. Howley; 2004
