Can You Replenish Glycogen Without Carbs? Understanding Gluconeogenesis and Ketosis

October 8, 2025 •

4 min read

During intense exercise, glycogen stores deplete rapidly, causing a phenomenon known as "hitting the wall," which is a rapid onset of fatigue. But what happens when you intentionally restrict carbohydrates? Surprisingly, your body possesses a powerful metabolic backup plan that allows it to replenish glycogen without carbs, relying on fat and protein instead.

Quick Summary

Your body can replenish glycogen stores without carbohydrates through gluconeogenesis, a process where the liver creates new glucose from non-carb sources like amino acids, glycerol, and lactate. This is crucial for sustaining energy on very low-carb diets, as it maintains the minimal glucose supply needed for vital functions, though it is a slower process than using dietary carbohydrates.

Key Points

Gluconeogenesis is the Key: The body can create new glucose, and subsequently glycogen, from non-carbohydrate sources like protein and fat through a process called gluconeogenesis.
Precursors are Protein and Fat: The main substrates for this process are lactate from anaerobic exercise, glycerol from fat breakdown, and glucogenic amino acids from protein.
Slower, On-Demand Replenishment: Unlike the rapid and robust replenishment from dietary carbs, gluconeogenesis provides a slower, more limited supply of glucose to maintain blood sugar and replenish glycogen minimally.
Metabolic Flexibility on Low-Carb Diets: On a ketogenic diet, the body becomes "fat-adapted," relying primarily on fat and ketones for fuel, which reduces the demand for glycogen.
Impact on Exercise Varies: While low glycogen stores can impair high-intensity exercise performance, a fat-adapted metabolism can be highly effective for steady-state, low-intensity endurance activities.
Adaptation is Required: Initially switching to a very low-carb diet can cause fatigue as the body adapts, but this often improves over time as the body becomes more efficient at using fat and ketones.

The Role of Glycogen and Its Traditional Source

Glycogen is the body's stored form of glucose, primarily located in the liver and muscles. It serves as a readily available energy source, particularly important for fueling high-intensity activities. When you consume carbohydrates, your body breaks them down into glucose. This glucose is then either used immediately for energy or converted into glycogen for storage. For decades, it was believed that adequate carbohydrate intake was non-negotiable for replenishing these vital energy reserves, especially for athletes. However, this perspective has evolved with a deeper understanding of human metabolism.

The Body's Backup Plan: Gluconeogenesis

When dietary carbohydrate intake is very low, the body doesn't simply shut down. Instead, it activates a crucial metabolic pathway called gluconeogenesis, which literally means "the creation of new glucose". This process allows your body to produce glucose from non-carbohydrate sources, ensuring a stable blood sugar level to power critical functions, especially for tissues that rely on a constant glucose supply, such as the brain and red blood cells. While a standard diet relies on external carb sources for glucose, a very low-carb or ketogenic diet shifts the body's reliance to this internal, on-demand glucose production.

Substrates for Gluconeogenesis

Your body can create glucose through gluconeogenesis using a variety of non-carb precursors. These include:

Lactate: Produced by muscles during intense exercise, lactate can be transported to the liver and converted back into glucose through the Cori cycle.
Glycerol: When triglycerides (stored fat) are broken down, they yield fatty acids and glycerol. The glycerol component can be used by the liver as a substrate for gluconeogenesis. Note that even-chain fatty acids cannot be converted to glucose, but glycerol provides a key link between fat and glucose production.
Glucogenic Amino Acids: Certain amino acids, derived from dietary protein or muscle tissue breakdown, can be converted into glucose. Alanine is a prime example, transported from muscles to the liver during the glucose-alanine cycle.

Low-Carb Diets, Ketosis, and Exercise Performance

In a ketogenic state, the body prioritizes burning fat for fuel, producing ketone bodies as an alternative energy source for many organs. While fat and ketones fuel a significant portion of the body's energy needs, gluconeogenesis remains active to produce the small amount of glucose necessary for obligate glucose-dependent tissues. This metabolic adaptation has significant implications for how individuals perform during different types of exercise.

Exercise Adaptation

High-Intensity Exercise: For short bursts of power, such as sprinting or heavy weightlifting, the body relies on muscle glycogen stores. On a low-carb diet, these stores are lower, meaning performance in high-intensity activities can be impaired, especially during the initial adaptation phase.
Low-Intensity Endurance Exercise: The story is different for steady-state activities like jogging or cycling at a moderate pace. After becoming "fat-adapted," the body becomes highly efficient at burning fat. This can spare muscle glycogen, potentially benefiting endurance performance, though some studies still show performance decrements compared to high-carb diets.

Comparing Carb-Based vs. Gluconeogenic Fueling

Understanding the differences between these two fueling pathways is crucial for anyone considering a low-carb approach to diet and exercise.


Feature	Carb-Based Fueling	Gluconeogenic/Ketogenic Fueling
Primary Fuel Source	Dietary carbohydrates (broken down to glucose)	Dietary fat, ketones, and some glucose from gluconeogenesis
Glycogen Repletion Speed	Rapid and efficient, especially with sufficient post-exercise intake	Slower, on-demand process from non-carb substrates
Replenishment Rate	Up to 10 mmol/kg wet weight per hour for the first few hours post-exercise	Limited and less efficient, prioritizing vital glucose needs over glycogen storage
High-Intensity Performance	Optimized, as ample glycogen is available for anaerobic energy	May be impaired due to lower glycogen stores
Low-Intensity Endurance	Uses a mix of fat and glucose, can eventually exhaust glycogen	Relies heavily on fat and ketones, conserving the smaller glycogen stores
Adaptation Period	Immediate, as long as carbs are available	Initial fatigue ("keto flu"), requires time (weeks) for full fat adaptation

Conclusion

Yes, you can absolutely replenish glycogen without carbs, thanks to the body's metabolic flexibility and the process of gluconeogenesis. For those on a very low-carb diet, the liver can synthesize glucose from precursors like glycerol and amino acids. This endogenous production maintains the small but vital supply of glucose needed for certain tissues, allowing the body to function efficiently in a fat-burning state. However, this slower, more limited replenishment method differs significantly from the rapid process driven by dietary carbohydrates. This has practical consequences for exercise, often favoring lower-intensity, longer-duration activities while potentially hindering performance in high-intensity bursts where instant energy is paramount. Ultimately, your body can adapt, but the type of fuel source you provide dictates which metabolic pathways and energy systems will be prioritized.

For more in-depth scientific analysis on how ketogenic diets impact athletic performance, you can refer to the position stand from the International Society of Sports Nutrition.

Frequently Asked Questions

Gluconeogenesis is a metabolic pathway that allows the body, primarily the liver, to synthesize new glucose from non-carbohydrate sources, such as lactate, glycerol, and glucogenic amino acids.

The glycerol component of triglycerides (fats) is a precursor for gluconeogenesis. The liver can convert this glycerol into glucose, which can then be used to synthesize glycogen. The fatty acid chains themselves, however, cannot be turned into glucose.

Not for all activities. While a fat-adapted metabolism is efficient for low-intensity endurance, the lower glycogen stores can limit performance in high-intensity, explosive exercises like sprinting or heavy lifting, which rely heavily on readily available glycogen.

Glycogenolysis is the breakdown of existing glycogen stores into glucose for energy. Gluconeogenesis is the creation of new glucose from non-carbohydrate sources when glycogen stores are low or depleted.

The adaptation period, known as becoming "fat-adapted," can take several weeks on a low-carb diet. During this time, many people experience symptoms of the "keto flu," such as fatigue, as the body transitions from using glucose to ketones as its primary fuel.

Yes. On a ketogenic diet, if protein intake is excessively high, the liver may ramp up gluconeogenesis from amino acids. This can potentially prevent or reduce ketone production, moving the body away from a deep state of ketosis.

While liver glycogen can become very low, complete depletion of muscle glycogen is unlikely because gluconeogenesis ensures a minimal glucose supply for vital functions. Muscle glycogen stores are typically maintained at a lower level but not completely eliminated, particularly in fat-adapted athletes.