The ketogenic diet (keto) is a nutritional strategy defined by its high-fat, very low-carbohydrate, and adequate-protein distribution. This eating pattern typically restricts carbohydrate intake to under 50 grams per day, forcing the body to enter a metabolic state called ketosis. In ketosis, the body shifts from burning glucose to burning fat and producing molecules called ketones for fuel. For athletes, this restriction of the body’s primary fuel source raises a fundamental question: does this metabolic shift enhance or hinder athletic performance?
Effect on Sustained Aerobic Performance
The ketogenic diet is often promoted among endurance athletes due to its potential to induce “fat adaptation” and enhance metabolic flexibility. Fat adaptation is the physiological process where the body becomes highly efficient at utilizing stored body fat and circulating ketones as its main energy source. This metabolic shift results in a significantly higher rate of fat oxidation during submaximal, prolonged exercise, which can be up to 2.3 times higher compared to those on a high-carbohydrate diet.
For ultra-endurance events, such as marathons or long-distance cycling, relying on fat can be advantageous because it delays the depletion of muscle glycogen. This “glycogen sparing” allows athletes to maintain a steady energy supply for a longer duration, potentially delaying fatigue. However, fat oxidation is a slower process than carbohydrate oxidation. This slower fuel delivery can lead to a decreased exercise economy, meaning the body requires more oxygen to perform the same work, particularly at intensities above 70% of maximal oxygen uptake. While the diet may maintain performance at moderate intensities, it may impair the ability to sustain higher-intensity efforts required in competitive moments.
Effect on Anaerobic Power and Strength
In contrast to endurance activities, the ketogenic diet presents a challenge for sports demanding high-intensity, short-duration efforts, such as sprinting, heavy weightlifting, or explosive movements. These activities rely almost exclusively on the anaerobic energy system, which requires a rapid breakdown of muscle glycogen for fuel. Since the ketogenic diet severely limits carbohydrate intake, it leads to a substantial reduction in the body’s glycogen stores, sometimes decreased by approximately 45%.
This reduction in available glycogen directly compromises the primary fuel source for anaerobic performance. Research shows that a short-term ketogenic diet can impair performance in anaerobic tests, resulting in decreases in peak and mean power output. For instance, one study found a 7% lower peak power and 6% lower mean power during the Wingate anaerobic cycling test after only four days. The slower process of fat oxidation cannot meet the immediate, high-volume energy demands of explosive exercise, making the diet suboptimal for athletes who depend on bursts of power and speed.
Impact on Muscle Repair and Adaptation
The recovery phase involves muscle protein synthesis (MPS) and the replenishment of energy stores. Adequate protein intake is necessary to stimulate MPS, and the ketogenic diet includes a moderate protein level (typically 15–20% of total calories), which is generally sufficient for muscle maintenance. However, concerns exist regarding whether the very low-carbohydrate state may impair the anabolic response to resistance training.
While muscle can be built on a ketogenic diet, the absence of carbohydrates affects the efficiency of glycogen replenishment. For athletes with high training volumes, the slow restoration of muscle glycogen stores can negatively impact the quality of subsequent high-intensity training sessions. Furthermore, some studies suggest that muscle hypertrophy, or muscle growth, from resistance training may be limited, and there is a risk of losing lean body mass during the initial adaptation phase.
Logistical Challenges of Training Ketogenic
Adopting a ketogenic diet presents several practical difficulties for competitive athletes. The initial transition period, often called the “keto flu,” can cause fatigue, headaches, irritability, and decreased performance, which disrupts consistent training. This phase can last from a few days to several weeks as the body switches its metabolic machinery.
Maintaining the strict carbohydrate restriction of under 50 grams per day is difficult, especially when managing high caloric needs or navigating social environments. Athletes also face the challenge of maintaining proper electrolyte balance, as the diet’s diuretic effect leads to increased loss of sodium, potassium, and magnesium, compounded by sweat loss during exercise. Finally, eliminating carbohydrate-rich foods, such as whole grains and certain fruits, requires careful planning to prevent potential deficiencies in micronutrients like fiber and B vitamins.