Original episode & show notes | Raw transcript
In the world of endurance sports, athletes constantly seek an edge. One of the most debated strategies in recent years is the adoption of very low-carbohydrate, high-fat (LCHF) or ketogenic diets. The central premise is compelling: our bodies store a relatively small amount of energy as carbohydrates (glycogen)—perhaps 2,000 calories—but hold a virtually limitless supply of energy in body fat. The theory posits that if an athlete can train their body to become more efficient at burning fat for fuel (a state known as “fat-adapted”), they could “spare” their precious glycogen stores for the most critical, high-intensity moments of a race, like a final sprint or a steep climb.
This document will critically examine this hypothesis. We will delve into the fundamental principles of exercise metabolism, analyze the scientific evidence for and against this dietary strategy, and clarify the distinct roles that diet and training play in shaping an athlete’s performance.
To understand the debate, we must first understand how our muscles produce energy. The body’s universal energy currency is adenosine triphosphate (ATP). Both carbohydrates and fats are broken down through complex biochemical processes to regenerate ATP, but they do so in fundamentally different ways.
Carbohydrates (Glycogen): Stored in muscles and the liver, glycogen is the body’s preferred fuel for high-intensity exercise. Its breakdown (glycolysis) can produce ATP very rapidly. While it can be used aerobically (with oxygen), its key advantage is the ability to supply energy anaerobically (without oxygen) for short, powerful bursts. This speed is non-negotiable for efforts above a certain intensity.
Fats (Triglycerides): Stored in adipose tissue and within muscle fibers, fat is an incredibly dense energy source. Its breakdown (beta-oxidation) occurs within the mitochondria and is a strictly aerobic process. While it yields a tremendous amount of ATP, the process is significantly slower and more complex than glycolysis.
The Intensity Dictates the Fuel Mix: The crucial takeaway is that fuel selection is primarily dictated by exercise intensity, which corresponds to the rate at which ATP is required.
Low Intensity (e.g., walking, Zone 2 cycling): ATP demand is low and can be easily met by the slow, efficient process of fat oxidation.
High Intensity (e.g., sprinting, FTP intervals): ATP demand is extremely high and can only be met by the rapid breakdown of carbohydrates. The fat oxidation pathway is simply too slow to keep up.
When exercise begins, the body first relies on instant energy from the phosphocreatine system (for ~10-15 seconds), followed immediately by a ramp-up in glycolysis to meet the sudden high demand for ATP. The slower aerobic systems (using both carbs and fat) then catch up to contribute to sustained efforts.
A fat-adaptive diet forces a metabolic shift. By severely restricting carbohydrates, the body upregulates the machinery needed to break down fats for energy. Studies consistently show that athletes on LCHF diets exhibit remarkably high rates of fat oxidation during submaximal (low-to-moderate intensity) exercise.
However, this adaptation comes with a significant trade-off. The same studies reveal two key consequences:
Reduced Glycogen Stores: Even when measured at rest, fat-adapted athletes consistently show significantly lower muscle glycogen levels than their high-carbohydrate counterparts—often around 50% less.
Impaired Carbohydrate Metabolism: The body, in its efficiency, downregulates the enzymes and pathways responsible for high-rate glycolysis. If you don’t use it, you lose it. The ability to process and burn carbohydrates at a high rate diminishes.
The most compelling question, and the one addressed by the landmark study highlighted in the podcast, is this: Can an athlete train in a fat-adapted state and then “carb-load” just before a race to restore glycogen, thereby gaining the benefits of enhanced fat burning and having a full tank of carbs?
The Study Design: Researchers took a group of elite athletes and put them through two protocols in a crossover design (meaning every athlete did both).
Protocol 1 (High-Carb): Seven days of a high-carbohydrate diet, followed by one day of carb-loading.
Protocol 2 (Fat-Adapted): Seven days of a low-carb, high-fat diet, followed by one day of carb-loading.
The crucial element here is that the pre-race carb-loading successfully equalized the starting muscle glycogen levels in both groups. They both started the test with a full tank.
The Test: The test was designed to mimic the demands of a real-world cycling race: a 100km time trial that included repeated, high-intensity 1km and 4km sprints.
The Results:
Fat Oxidation: As expected, the fat-adapted group burned significantly more fat during the steady-state portions of the time trial.
Overall Performance: There was no statistically significant difference in the total time to complete the 100km. However, looking at individual data, most athletes were slower after the fat-adaptation protocol.
High-Intensity Performance (The Decisive Finding): During the crucial 1km and 4km sprints, the performance of the fat-adapted group was significantly worse. They produced substantially less power (40-70 watts less) compared to when they were on the high-carb protocol, despite having the same amount of glycogen available.
The Conclusion from the Science: The study demonstrates that a fat-adaptive diet impairs the ability to use carbohydrates at a high rate. Even when glycogen is present, the enzymatic machinery to burn it quickly has been downregulated. The fuel is in the tank, but the engine’s fuel injectors are clogged. This strategy compromises the exact type of performance—surges, attacks, and sprints—that decides the outcome of most endurance races.
This does not mean LCHF diets are useless, but their application in performance sport is highly specific and often misunderstood.
Effective for Weight Loss: By promoting satiety and training the body to utilize its own fat stores, many people find ketogenic diets to be an effective tool for weight management.
Potentially Useful for Ultra-Endurance: For events lasting 24 hours or more with very steady, low-intensity pacing (e.g., a 250-mile desert run), a fat-adaptive strategy can be viable. In these scenarios, the intensity rarely requires high rates of carbohydrate use, and the ability to rely on internal fat stores can solve the logistical challenge of consuming thousands of calories.
Not Suitable for Most Endurance Sports: For events like road cycling, marathons, criteriums, or even 24-hour mountain bike races, performance is defined by periods of high intensity. Sacrificing top-end power by impairing carbohydrate metabolism is a significant compromise that is not offset by enhanced fat burning.
The desire to “hack” physiology with a diet is tempting, but the evidence is clear: long-term, consistent training is the most powerful driver of favorable metabolic adaptations.
All forms of endurance training—from long, slow distance to high-intensity intervals—improve a muscle’s ability to oxidize fat. A well-trained, high-carb athlete is already a phenomenal fat-burner.
Training increases mitochondrial density, enhances oxygen delivery, and improves metabolic efficiency in ways that no diet can.
The role of a performance-oriented diet is not to be the primary driver of adaptation, but to enable the highest quality of training. To perform high-intensity workouts that stimulate improvement, you need to be adequately fueled with carbohydrates. Sacrificing training quality to adhere to a restrictive diet is a step backward for any athlete whose sport demands more than just a steady pace. Ultimately, the goal is to become a robust, metabolically flexible athlete who can efficiently burn fat at low intensities and readily access the rapid energy of carbohydrates when the hammer drops. This dual ability is the hallmark of a well-trained endurance athlete.