Glycogen, Carnivore, Ketogenic: The Biological Limit No One Wants to Face

Glycogen, Carnivore, Ketogenic: The Biological Limit No One Wants to Face

by Laurent Glatz – for Athletic Carnivore

A human muscle can store up to 600 grams of glycogen. The liver adds approximately 80 to 120 grams more. Once these reserves are depleted, performance no longer depends on motivation but on biochemistry.

The debate around carb-free diets is consistently skewed by a major confusion: surviving is not the same as performing. The human body has powerful adaptation mechanisms, but these are not designed to optimize force, speed, or high-intensity endurance. They are designed to sustain life.

Glycogen Remains a Limiting Factor

Glycogen is a polymer of glucose stored in muscles and the liver. It is the primary energy source for intense efforts. During weight training, cross-training, or endurance sports with intensity variations, glycogen depletion happens rapidly. Depending on volume and intensity, a workout can consume between 30% and 70% of muscle glycogen stores.

In men, total glycogen reserves generally range between 400 and 700 grams depending on muscle mass. In women, these reserves are lower, often between 300 and 500 grams. These figures vary, but the order of magnitude remains consistent. This reservoir is limited. Crucially, glycogen is not interchangeable between muscles.

Gluconeogenesis Is Not Infinite

When dietary carbohydrates are absent, blood glucose maintenance relies on gluconeogenesis. The liver produces glucose from amino acids, lactate, and to a lesser extent, glycerol. This production has a ceiling. A commonly cited average capacity ranges between 80 and 130 grams of glucose per day.

This point is central: glucose production does not mechanically increase with higher protein intake. Eating more meat does not enable indefinite glucose production. The system is regulated, limited by hepatic enzymes, demand, and hormonal context.

Regarding lipids, the reality is even clearer. Fatty acids cannot be converted into glucose. Beta-oxidation produces acetyl-CoA, which cannot be routed back to gluconeogenesis. Only the glycerol fraction of triglycerides can contribute, and this represents a marginal amount. Relying on fat to directly fuel blood glucose is biologically unfounded.

Strict Carnivore and Strict Ketogenic: Two Different Constraints

A strict carnivore diet, with high protein intake, provides sufficient substrate to sustain functional gluconeogenesis. This allows maintaining a minimal glycogen recharge. However, this recharge is slow. After intense effort that depletes reserves, it often takes several days to restore optimal glycogen levels without dietary carbohydrates.

This imposes a direct constraint on training. Intensity, frequency, and volume must be adjusted. Otherwise, demand exceeds supply.

Conversely, a person on a strict ketogenic diet, with high fat and relatively lower protein intake, further reduces their capacity to produce glucose. They enter an imbalance between consumption and production more rapidly. In this context, maintaining intense sports activity becomes problematic.

The body compensates by increasing cortisol to mobilize energy substrates. Proteolysis may increase. The central nervous system, which depends on glucose despite partial ketone body utilization, begins to be affected.

Nervous fatigue appears. It manifests as decreased concentration, reduced coordination, loss of motivation, sleep disturbances, and persistent fatigue despite rest. This is not a lack of discipline. It is an energy deficit.

Adaptation Is Not Optimality

The narrative that the body “adapts” is technically true but physiologically misleading. Adaptation means the body finds a way to function under constraint. It does not mean it functions optimally.

Studies on low-carb and ketogenic diets show the ability to maintain endurance performance at low intensity. However, as intensity rises, dependence on glycogen becomes dominant again. This is precisely why many studies observe performance improvements in low-carb athletes when they consume glucose during exercise.

Using fast glucose sources, like dextrose, during or immediately after training enables direct muscle glycogen replenishment. In this context, glucose is preferentially taken up by muscles via GLUT4 transporters activated by muscle contraction. Insulin remains moderate, and glucose is directed to active tissues rather than stored as fat.

This mechanism is well known. It explains why peri-workout carbohydrate intake improves recovery, reduces glycogen recharge time, and helps maintain high performance levels.

Rejecting this mechanism in the name of nutritional ideology ignores physiology.

Every Strategy Imposes a Compromise

The problem is not the carnivore diet itself. The problem is the inconsistency between a carb-free diet and a high, repeated energy demand.

Yes, it is possible to train without carbohydrates. But this requires precise effort management, longer recovery periods, and acceptance of potentially reduced performance in certain efforts.

Yes, it is possible to be strictly ketogenic. But this mechanically limits the capacity to produce glucose, and thus to sustain intense efforts.

Extreme discourse does not hold up against biology. Neither one way nor the other.

Physiology shows a simple reality: glycogen is a limiting factor for performance. Endogenous glucose production is capped. Lipids are not a significant glucose source. Metabolic adaptation does not replace a missing substrate.

The question is not about choosing a side. The question is understanding the real constraints of the human body. And accepting that every nutritional strategy involves compromise.