Original episode & show notes | Raw transcript
The central theme of the podcast is a critical examination of the ketogenic diet, particularly in the context of endurance sports. The discussion begins by acknowledging the popular narrative that if burning fat is good for endurance, then maximizing fat-burning through a ketogenic diet must be even better. This logic, as the podcast explains, is an oversimplification that ignores the complexities of human metabolism and the specific demands of high-performance athletics.
The hosts frame the conversation by contrasting the common knowledge that we can burn sugar for energy with the less intuitive process of how our bodies derive energy from food at a cellular level. This sets the stage for a deep dive into the biochemistry of fat and carbohydrate metabolism.
Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood. This occurs when the body has chronically low levels of blood glucose, forcing it to find an alternative fuel source.
Ketone Bodies: These are molecules produced by the liver from fatty acids during periods of low food intake, carbohydrate-restrictive diets, starvation, or prolonged intense exercise. The two primary ketone bodies are:
Acetoacetate
Beta-hydroxybutyrate These molecules are chemically similar and can be used by tissues, including the brain, as a source of energy. The name “ketone” refers to a specific chemical structure (a carbon atom double-bonded to an oxygen atom).
Evolutionary Purpose: The ability to enter ketosis is an evolutionary survival mechanism. It allows the body to function during times of low carbohydrate availability, relying on stored fat and protein from hunted animals or insects. It’s a way for the body to utilize the abundance of acetyl-CoA (a molecule central to metabolism) derived from fat breakdown.
The theoretical appeal of the ketogenic diet for endurance athletes is based on two main ideas:
Glycogen Sparing: Our bodies have a limited storage capacity for carbohydrates in the form of glycogen (around 2,000 kcal). In contrast, even a lean athlete has a virtually unlimited supply of energy stored as fat (35,000+ kcal). The theory is that by adapting the body to burn fat more efficiently, an athlete can spare their limited glycogen stores, leading to better endurance.
Weight Loss: The diet is often associated with weight loss, which can be beneficial for sports where power-to-weight ratio is crucial, such as cycling or running.
The podcast notes that proponents of the diet often use compelling, almost ideological language, like encouraging athletes to “release themselves from carbohydrate dependence.” They present data showing keto-adapted athletes with incredibly high rates of fat oxidation (over 1.5 grams per minute), which seems impressive on the surface.
The core of the podcast’s argument rests on a detailed analysis of a pivotal 2017 study by researcher Louise Burke, which examined the effects of different diets on elite racewalkers. This study is significant because it used highly trained, world-class athletes.
Study Design:
Participants: Elite racewalkers, many of whom competed in the 2016 Olympics.
Intervention: A three-week training camp with three distinct, isoenergetic (equal calorie) diet groups:
High-Carbohydrate (HCHO): 65% of calories from carbs.
Periodized Carbohydrate (PCHO): Alternating between high- and low-carb availability to follow “train-low” principles.
Low-Carb, High-Fat (LCHF/Keto): Roughly 80% of calories from fat, with less than 50g of carbs per day.
Testing: Before and after the camp, athletes underwent a series of tests, including a 10km race, VO2 peak tests, and exercise economy measurements.
Key Findings:
Performance: The two carbohydrate groups (HCHO and PCHO) significantly improved their 10km race times by 2-3 minutes. The keto group showed no improvement; their times remained the same.
Exercise Economy: This was the most crucial finding. Exercise economy refers to the amount of oxygen (O2) consumed to maintain a certain speed.
The carb groups became more economical; they could go faster while using the same or less oxygen relative to their maximum capacity.
The keto group’s economy worsened. They required more oxygen to maintain the same speed they held before the intervention.
The podcast offers two primary biochemical explanations for the loss of exercise economy:
The Oxygen Cost of Burning Fat: While fat is more energy-dense per gram (9 kcal/g vs. 4 kcal/g for carbs), it requires more oxygen to produce ATP (the body’s energy currency).
Per liter of oxygen consumed, burning carbohydrates yields approximately 5.05 kcal of energy.
Per liter of oxygen consumed, burning fat yields only about 4.86 kcal of energy. This means that to produce the same amount of power, an athlete relying on fat must consume more oxygen. In a high-intensity race where oxygen delivery is a limiting factor, this is a significant disadvantage. The body cannot simply “compensate” by breathing harder when already at or near its lactate threshold.
Motor Unit Recruitment: The hosts also speculate that low muscle glycogen levels in the keto group may have forced the body to recruit less efficient, more powerful muscle fibers (fast-twitch fibers) to maintain the required pace, further increasing the oxygen cost.
A common criticism of the first Burke study was that the keto athletes didn’t have a “carb-loading” or reintroduction phase before the final race. The argument was that one must first become “fat-adapted” and then reintroduce carbohydrates to get the best of both worlds.
In response, Burke conducted a follow-up study with a similar design, but this time, all groups underwent a two-week carbohydrate reintroduction and race taper after the initial dietary intervention.
Follow-Up Study Results:
The keto group’s performance did “bounce back” after reintroducing carbs, and they eventually matched the performance levels of the high-carb groups.
However, they did not exceed the performance of the other groups. The three weeks spent on a ketogenic diet provided no additional benefit. It was an unnecessary and complex intervention that merely brought them back to the level they would have reached with a standard high-carbohydrate diet.
The podcast concludes that for most athletes, the ketogenic diet is not a viable strategy for improving performance.
Metabolic Flexibility vs. Specialization: While the human body is remarkably flexible in its fuel use, high-performance exercise has specific demands. Forcing the body to specialize in fat metabolism comes at the cost of its ability to efficiently use carbohydrates, which are essential for high-intensity efforts.
The Demands of Racing: Even in long “submaximal” events like marathons or stage races, the decisive moments—breakaways, climbs, and sprints—are well above threshold and rely heavily on anaerobic glycolysis (the rapid breakdown of carbohydrates). A keto-adapted athlete is metabolically ill-equipped for these critical efforts.
The Verdict: The evidence strongly suggests that a ketogenic diet impairs exercise economy and negates the performance benefits of intense training. The only potential exception might be for ultra-endurance athletes where the intensity is consistently low and fueling during the event is a major logistical challenge. However, even in that niche, many top athletes succeed on high-carbohydrate diets.