How Low Muscle Glycogen Affects Endurance Training
Share
Introduction
Many athletes and fitness enthusiasts aim to improve their performance through smarter training methods. One approach gaining attention is doing endurance exercise with low muscle glycogen levels. A recent study explored what actually happens in your muscles when you work out in this low-fuel state. The results help explain how your body adapts and what it might mean for long-term training outcomes.
What Is Muscle Glycogen and Why Does It Matter?
Muscle glycogen is a stored form of carbohydrate and serves as a key fuel during exercise, especially endurance activities. When glycogen levels are low, your muscles have to rely more on fat for energy. This switch in fuel source can change how your body responds and adapts to training.
The Training Setup
In this study, participants performed a one-legged cycling session in the evening to reduce glycogen in one leg. The next morning, they performed a two-legged cycling workout. This allowed researchers to compare how muscles with low versus normal glycogen levels behaved during the same exercise.
Key Findings
-
Less Glycogen Use, More Fat Use
Muscles with low glycogen used less of it during the workout. Instead, they burned more fat, especially in type I (slow-twitch) and type II (fast-twitch) muscle fibers. This shift in energy use was clear, with large increases in fatty acids bound to carnitine—a marker of fat metabolism.
-
Changes in Muscle Signals
Muscles with normal glycogen showed increased activity in a key muscle-building pathway (called mTOR), which supports muscle repair and growth. In muscles with low glycogen, this pathway was less active, especially in type I fibers, possibly limiting their ability to grow stronger or recover.
-
Protein Breakdown and Repair
Indicators of protein breakdown were higher in the low-glycogen muscles. Also, a process called autophagy, which helps recycle old cell parts, was less active in these muscles after exercise. In contrast, muscles with normal glycogen showed stronger signs of both repair and recycling activity.
What This Means for Your Training
While training with low muscle glycogen can improve fat-burning, it may reduce your muscle’s ability to recover and adapt—especially in slow-twitch fibers. These are the fibers most involved in endurance performance. If your goal is long-term improvement in endurance capacity and muscle function, regularly training in a low-glycogen state might not be ideal.
Conclusion
Exercising with low muscle glycogen changes how your muscles use fuel and respond to stress. While it boosts fat metabolism, it may also blunt important signals needed for muscle recovery and adaptation. For endurance athletes, this means the “train low” strategy should be used with care—perhaps occasionally, but not as a daily routine. Balancing glycogen levels with proper nutrition could help maximize both performance and progress.