How Do You Know If Your Glycogen is Depleted?

December 23, 2025 •

4 min read

According to research, the body can store an average of 400 grams of glycogen, primarily in the muscles and liver, but this reserve can be quickly depleted during intense or prolonged exercise. Understanding how do you know if your glycogen is depleted is crucial for anyone looking to maintain peak athletic performance and energy levels.

Quick Summary

This guide explains the signs and symptoms of low glycogen, such as abnormal fatigue, loss of strength, and reduced exercise performance. It details the difference between muscle and liver glycogen and offers practical tips for identifying and preventing the effects of running on empty.

Key Points

Sudden, severe fatigue and weakness: A rapid and extreme drop in energy, often accompanied by shakiness and dizziness, is a prime indicator.
Noticeable performance decline: Your strength, speed, and endurance will all take a hit as your muscles lack their primary fuel source.
Increased rate of perceived exertion: Normal workouts will feel much harder than they should, a clear sign of impaired muscle function.
Mental fog and irritability: Since the brain relies on liver glycogen, its depletion can lead to confusion, poor concentration, and mood changes.
Muscle 'flatness' and heavy legs: Glycogen is stored with water, so depletion causes water loss and can make muscles feel less full or responsive.
Know the difference: Glycogen depletion leads to a sudden 'bonk,' while general exercise fatigue builds more gradually.

What Happens When Your Glycogen Stores Run Low?

When your body needs energy, it first taps into the glycogen stored in your muscles and liver. Muscle glycogen fuels intense, local activity, while liver glycogen helps maintain stable blood sugar levels for the brain and other organs. When these reserves are exhausted, your body switches to slower fuel sources, like fat, which can lead to a significant drop in performance and a host of noticeable symptoms. This state is often referred to as 'hitting the wall' by endurance athletes.

The Physiological Signs of Depletion

Recognizing the physical and mental symptoms of depleted glycogen is the key to preventing a performance crash. Here are the primary indicators:

Sudden, overwhelming fatigue: This is not your typical post-workout tiredness. It's an all-consuming exhaustion that makes continuing an activity feel impossible. You may feel dizzy, shaky, and weak.
Decreased strength and power: You'll notice an inability to produce the same force or explosive power you normally can. This occurs because muscles lack the readily available ATP needed for powerful contractions.
Increased rate of perceived exertion (RPE): A workout that would normally feel manageable suddenly feels incredibly difficult and taxing. You feel like you're putting in maximum effort for minimal results.
Heavy, unresponsive muscles: Your muscles may feel 'flat' or 'heavy' and less responsive than usual. This is partly due to the water loss that accompanies glycogen depletion, as each gram of glycogen is stored with several grams of water.
Mental fatigue and impaired concentration: The brain relies heavily on glucose from liver glycogen. When those stores are low, you may experience 'brain fog,' mental dullness, irritability, or confusion.
Water weight fluctuation: A noticeable overnight drop in body weight can suggest significant water loss from depleted glycogen stores.
The 'bonk' or 'hitting the wall': This is the ultimate sign of full glycogen depletion, characterized by a sudden and rapid onset of severe fatigue, weakness, and dizziness, especially common in long-distance events.

Monitoring Your Fuel Gauge

For most people, listening to your body's signals is the best way to gauge your glycogen status. However, certain training scenarios or medical conditions might require a more scientific approach. Invasive muscle biopsies or advanced, expensive methods like magnetic resonance spectroscopy (MRS) can measure glycogen levels directly but are not practical for daily monitoring. Therefore, paying close attention to your body and understanding your training and diet are the most reliable indicators.

Practical Glycogen Management Strategies

To avoid hitting empty, consider these simple nutritional and training practices:

Carbohydrate-rich diet: Ensure your diet contains a sufficient amount of carbohydrates to keep your stores topped off, especially during periods of heavy training. Aim for consistent, carbohydrate-rich meals.
Consume carbs strategically: For activities lasting over an hour, consuming carbohydrates (e.g., sports drinks, gels) during exercise can help maintain energy levels. Post-workout carbohydrate intake is also critical for rapid muscle glycogen replenishment.
Prioritize recovery: Allow for adequate rest between intense sessions. Continuous, hard training without recovery and proper refueling will lead to chronically low glycogen levels.
Increase your fitness level: Higher aerobic fitness can improve your body's efficiency at using fat for fuel at lower intensities, sparing your glycogen stores for when you need them most.

Glycogen Depletion vs. General Exercise Fatigue


Feature	Glycogen Depletion	General Exercise Fatigue
Onset	Sudden, rapid crash. Often feels like an immediate loss of power.	Gradual, progressive decline in performance over time.
Sensation	Extreme weakness, dizziness, and mental fogginess. May feel shaky and lightheaded.	Feeling tired, heavy muscles, and breathlessness.
Primary Cause	Exhaustion of stored carbohydrates (muscle and liver glycogen).	Accumulation of metabolic byproducts and general wear-and-tear on muscles.
Recovery Time	Requires carbohydrate intake to replenish stores, typically 24-48 hours with proper nutrition.	A few hours to a day of rest is often sufficient.
Effect on Mind	Significant mental impairment, confusion, and irritability due to low blood sugar affecting the brain.	Can cause general tiredness but typically doesn't impair cognitive function as severely.

Conclusion

Understanding how do you know if your glycogen is depleted is essential for maximizing athletic performance and avoiding the dreaded 'bonk'. The key is to recognize the combination of symptoms—severe fatigue, decreased strength, and mental fogginess—and to differentiate them from normal exercise-related tiredness. By strategically managing your carbohydrate intake before, during, and after intense training and prioritizing recovery, you can maintain your energy reserves and push your performance to new limits. Always listen to your body, as its signals are the most accurate non-invasive indicators of your fuel status.

This article is for informational purposes only and does not constitute medical advice. For personalized nutritional or fitness guidance, consult with a healthcare professional or a registered dietitian.

Frequently Asked Questions

Glycogen is the stored form of glucose, or sugar, that your body uses for energy. It's stored mainly in your muscles and liver and is the primary fuel source for high-intensity exercise.

For prolonged, high-intensity exercise, glycogen stores can be depleted in as little as 90 to 120 minutes. For sedentary activities, it can take 12-24 hours of fasting. However, this varies based on your fitness level, diet, and exercise intensity.

There is no simple, everyday test to measure your glycogen levels. The most accurate method is an invasive muscle biopsy, which is impractical for routine use. For most athletes, tracking performance, energy levels, and food intake provides the most practical insight.

These terms refer to the state of total exhaustion that occurs when an endurance athlete's glycogen stores are fully depleted. It results in a sudden, rapid decline in performance, often accompanied by weakness and dizziness.

Consuming carbohydrates is the most effective way to replenish glycogen. For faster recovery, consuming carbohydrates within a few hours after exercise is particularly effective. A high-carbohydrate diet and adequate rest are also key.

A diet consistently rich in carbohydrates is crucial for maintaining adequate glycogen stores. If you follow a low-carb diet, your body will have fewer glycogen reserves available for high-intensity performance.

Yes, once glycogen stores are exhausted, the body shifts to burning fat for fuel in a process called gluconeogenesis. However, this process is slower and less efficient for high-intensity efforts, leading to a significant performance drop.