The Human Fat Engine: The Overlooked Fuel for Performance

The Human Fat Engine: The Overlooked Fuel for Performance

by Laurent Glatz – for Athletic Carnivore

The human body sometimes carries over 50,000 calories of stored body fat. Yet, most modern nutritional strategies continue to focus on fueling a mere 2,000-calorie glycogen reservoir.

This paradox lies at the heart of one of the most unsettling discoveries in modern exercise physiology.

The FASTER Study

In 2016, researchers Jeff Volek and Stephen Phinney published the FASTER Study in Metabolism: Clinical and Experimental. The study compared two groups of elite ultra-endurance athletes. All ran more than 50 kilometers per week for several years and had high VO₂ max values.

The variable that differed: metabolic fuel.

The first group followed a high-carbohydrate diet. The second adhered to a very low-carbohydrate diet, typically under 50 grams per day, for an average of 20 months.

Up to 2.3 Times More Fat Burned

Researchers analyzed substrate oxidation during prolonged exercise at 64% of VO₂ max. The results were striking.

In high-carb athletes, maximal fat oxidation reached about 0.67 grams per minute. In low-carb athletes, it soared to 1.54 grams per minute—more than double.

Some individual values even exceeded 1.8 g/min, representing the highest rates of fat oxidation ever recorded in scientific literature.

Glycogen Does Not Disappear

The most intriguing finding concerned energy stores. Despite a very low-carb diet, resting muscle glycogen levels remained nearly identical between the two groups.

Even more surprising: after three hours of running, muscle glycogen depletion was comparable in both groups.

This contradicts the idea that a low-carb diet drains muscle fuel. Physiology adapts differently.

The Fat Engine

The liver activates gluconeogenesis, producing glucose from amino acids, lactate, and glycerol. Simultaneously, lowered insulin triggers lipolysis in adipose tissue.

Free fatty acids circulate to muscle mitochondria, entering beta-oxidation. This process generates a massive amount of ATP. One palmitate molecule produces about 106 ATP molecules, compared to 30 to 32 from one glucose molecule.

Fat is an extremely dense fuel. And the reserves are enormous.

A Vast, Underused Reservoir

A relatively lean athlete with 10 kg of body fat carries roughly 90,000 kcal of stored energy. By comparison, total muscle and liver glycogen stores hover around 2,000 kcal.

Two reservoirs. One gigantic. The other tiny.

In a carbohydrate-dependent metabolism, insulin suppresses fatty acid mobilization. The body then relies on glycogen and requires frequent refueling. In low-carb adapted athletes, lipolysis remains active and glycogen is used more slowly.

If our physiology can burn over 1.5 grams of fat per minute, why have we built modern nutrition that precisely prevents this mechanism from working?

What this study changes is not just the endurance conversation. It reshapes how we understand the human body. For decades, athletes have been portrayed as carbohydrate boilers: fill up, deplete, refill, monitor the tank, fear the wall, consume gels and sugary drinks to maintain intensity. This logic works in some contexts, especially when seeking very high power in short or repeated efforts. But it becomes reductive when discussing metabolic health, longevity, recovery, and energy resilience.

The fat engine is not an exotic supplement reserved for a few ultra-trail runners. It is a fundamental human capacity. It depends on lowered insulin, fatty acid availability, mitochondrial density, electrolyte status, and the nervous system’s ability to tolerate a more stable, less stimulating, less sugar-dependent energy supply. This is precisely where the carnivore logic makes sense: dense animal foods rich in complete proteins, natural fats, heme iron, zinc, creatine, carnosine, and fat-soluble vitamins can support this terrain without imposing a constant carbohydrate load.

The transition, of course, is not magical. A muscle accustomed to glucose does not become efficient at fat in three days. Beta-oxidation enzymes, transporters, mitochondria, and even the brain need time. That’s why many confuse incomplete adaptation with model failure. The first days or weeks may feel like a performance drop—not because fat is a poor fuel, but because the body hasn’t yet relearned to use it at high rates.

The real question then becomes very concrete: do we want to remain dependent on a limited, fast, nervous, and fragile fuel, or do we want to rebuild access to an almost unlimited reservoir—more stable, quieter, and better suited for endurance?

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