Glycogen is the body’s stored form of glucose. This stored carbohydrate serves two main purposes: providing a quick source of energy to power high-intensity physical activity and maintaining stable blood glucose levels for the brain and nervous system. The body’s reserves of this fuel are finite, and when they are exhausted, depletion marks a profound shift in how the body generates the energy needed to continue functioning.
Glycogen Storage and Usage
The body stores glycogen predominantly in two locations: the liver and the skeletal muscles. Muscle glycogen accounts for approximately three-quarters of the body’s total store, serving as a localized fuel source to power muscle contraction during exercise. This muscle glycogen cannot be released into the bloodstream and is available only for the specific muscles in which it is stored.
The remaining glycogen is stored in the liver, which acts as the body’s central blood glucose regulator. When blood sugar levels drop, the liver breaks down its reserve and releases glucose into the circulation. This liver glycogen is crucial for sustaining the energy needs of the central nervous system and other organs. These combined reserves can be depleted after approximately 90 to 120 minutes of intense, sustained activity or after a period of prolonged fasting.
Immediate Physiological Symptoms of Depletion
The moment the body’s primary carbohydrate reserves are exhausted is often characterized by a sudden, overwhelming physical experience commonly called “hitting the wall” or “bonking.” Performance collapses dramatically, signaling a fuel crisis. The muscles, deprived of their fast-burning fuel source, feel heavy and leaden, making even a slight increase in pace feel impossible.
Muscle fatigue quickly sets in, sometimes accompanied by cramping or fine motor coordination difficulties. The heart rate may also increase as the body attempts to maximize the delivery of oxygen and remaining fuel to the working muscles. These acute physical symptoms force a reduction in activity intensity to conserve resources.
The Metabolic Shift to Alternative Fuels
Once glycogen stores are depleted, the body pivots energy production to alternative, slower-burning fuels. This metabolic shift primarily involves increased reliance on fat oxidation, known as lipolysis. Stored triglycerides in adipose tissue are broken down into free fatty acids and glycerol.
Fat Oxidation
The resulting free fatty acids become the main energy source for most tissues, including skeletal muscle, which uses them in a process called beta-oxidation. Fat metabolism is a slower, more oxygen-intensive process than carbohydrate metabolism, which prevents the body from maintaining high intensity effort.
Gluconeogenesis
The glycerol released during lipolysis is transported to the liver, where it becomes a substrate for gluconeogenesis—the creation of new glucose from non-carbohydrate sources. The liver uses glycerol, lactate, and specific amino acids derived from muscle protein breakdown to synthesize new glucose molecules. This newly created glucose is released into the bloodstream to maintain a minimal level of blood sugar, which is important for organs that cannot use fat for fuel. This adaptation helps sustain basic life functions.
Impact on Cognitive Function
The brain relies almost exclusively on a steady supply of glucose for energy. Although the liver initiates gluconeogenesis, the supply is often insufficient to fully meet the brain’s high energy demands, leading to a state called neuroglycopenia. When blood glucose drops to low levels, cognitive impairment begins.
The lack of sufficient fuel causes immediate deficits in mental processing. Symptoms include mental fog, difficulty concentrating, and impaired decision-making. Individuals may also experience irritability, emotional instability, or disorientation. The brain’s reliance on glucose means that a drop in blood sugar directly impacts the speed and quality of thought.