Is it harder to run without carbs?

December 21, 2025 •

4 min read

According to a study on elite race walkers, a low-carbohydrate diet impaired exercise economy and performance compared to a high-carbohydrate diet. This suggests that for many athletes, it is harder to run without carbs, particularly when pushing higher intensities.

Quick Summary

Running on a low-carb diet is challenging due to limited glycogen stores, but the body can adapt to burn more fat for fuel. Performance for high-intensity efforts is often compromised, while lower-intensity endurance can be sustained. Strategies like carb periodization offer a balanced approach for runners seeking to optimize fuel use.

Key Points

High-Intensity Performance Suffers: Running fast requires readily available carbohydrates, and without them, performance at higher intensities is compromised.
Long-term Adaptation is Required: It can take months for a runner's body to adapt to using fat as a primary fuel, during which performance significantly decreases.
Fat Adaptation for Ultras: While not ideal for speed, becoming fat-adapted can be beneficial for ultra-endurance events where fuel needs to last for many hours.
Strategic Carbs are Best for Most: Carbohydrate periodization—eating more carbs around high-intensity workouts and fewer on rest days—offers a balanced and effective approach.
Prioritize Recovery: Low-carb intake can impair post-run glycogen replenishment, leading to slower recovery times and increased fatigue.
Listen to Your Body: The risks of running with insufficient carbohydrates, including reduced performance, increased injury risk, and hormonal issues, outweigh potential benefits for many runners.

Carbohydrates are the body's primary and most efficient source of energy for running, especially at moderate to high intensities. They are stored in the muscles and liver as glycogen and accessed as glucose during exercise. When these stores become depleted—which can happen within 90-120 minutes of hard running—the result is sudden fatigue, often known as 'hitting the wall'. Running without carbs, therefore, bypasses this efficient fuel source, forcing the body to rely more heavily on fat, a less readily available energy source, which makes running significantly harder, particularly for faster speeds.

The Low-Carb Running Experience: Adaptation and Challenges

When a runner transitions to a low-carb or ketogenic diet, their body must adapt to using fat as its primary fuel source. This metabolic switch, known as 'fat adaptation,' takes time and has distinct phases and challenges.

The Adaptation Period

Initial Shock: The first few weeks are often the most difficult. Runners may experience a significant drop in performance, feeling sluggish and weak. This is a common side effect known as the 'keto flu'.
Performance Shift: As the body becomes more efficient at burning fat, performance for low-intensity, longer-duration runs can improve or be maintained. However, high-intensity running (like sprints or tempo work) will likely feel much harder and be slower, as these efforts rely heavily on carbohydrates for quick, high-output energy.
Time Commitment: The process of becoming fully fat-adapted can take anywhere from six weeks to several months, and some athletes never fully adapt. For this reason, a significant change in diet should be undertaken during the off-season, not close to a key race.

Benefits of Running with Limited Carbs

While challenging, a low-carb approach can offer some advantages, particularly for ultra-endurance events.

Increased Fat Oxidation: The body becomes more efficient at tapping into its vast fat reserves for fuel, which can help delay the onset of glycogen depletion during very long events.
Less Fueling Needed: Since fat stores are massive compared to glycogen stores, fat-adapted athletes may require less carbohydrate intake during ultra-long races, potentially reducing the risk of gastrointestinal issues.
Enhanced Metabolic Flexibility: The body becomes more versatile in its fuel use, able to switch more efficiently between carbohydrates and fats depending on the intensity.

Risks and Drawbacks for Runners

The downsides of running without adequate carbs can be significant and impact both performance and overall health.

Impaired High-Intensity Performance: Carbohydrates provide energy much faster than fat. Without them, the explosive power needed for speed work, finishing kicks, or hill repeats is severely compromised.
Slower Recovery: Post-workout, carbs are crucial for replenishing depleted muscle glycogen stores. A low-carb diet can impair this recovery process, leading to prolonged fatigue and sluggish muscles.
Increased Stress and Injury Risk: Chronic low carbohydrate availability can put extra stress on the body, increasing cortisol levels and potentially compromising the immune system. This can increase the risk of illness and injury.
Relative Energy Deficiency in Sport (RED-S): Insufficient energy intake from any source, including carbs, can lead to serious health consequences, particularly in female athletes. This can affect hormonal balance, bone health, and increase the risk of eating disorders.

A Balanced Approach: Carbohydrate Periodization

For many runners, the best approach is not to eliminate carbs entirely but to use them strategically in a method known as carbohydrate periodization. This involves adjusting carb intake based on training demands.

'Train Low, Race High': This strategy involves performing some low-intensity training sessions with limited carbohydrate availability to promote fat adaptation. For key high-intensity sessions and races, however, you would maximize carbohydrate intake to ensure full glycogen stores for peak performance.
Easy Runs and Rest Days: Lower carb intake is appropriate on these days, as energy demands are minimal.
Hard Workouts and Long Runs: On days with intense training, ensure adequate carbohydrate loading beforehand and fueling during the run to support performance and recovery.

Comparison Table: High-Carb vs. Low-Carb Running Strategy


Feature	High-Carbohydrate Strategy	Low-Carbohydrate Strategy (Keto)
Primary Fuel Source	Glycogen (carbs)	Fat (ketones)
High-Intensity Performance	Excellent, supports speed and power.	Severely limited due to lack of quick energy.
Low-Intensity Endurance	Good, requires consistent fueling.	Can be excellent once fat-adapted.
Adaptation Period	Minimal metabolic adjustment needed.	Can take 6+ weeks, includes 'keto flu' symptoms.
Gut Training	Essential for long distances to manage carb intake.	May reduce gastric distress during ultras.
Recovery Time	Optimized with proper post-workout fueling.	Often longer due to impaired glycogen synthesis.
Fuel Transport	Efficient energy delivery to muscles.	Requires more oxygen to produce same energy.
Risk Profile	Lower risk of metabolic and hormonal issues.	Higher risk of stress, illness, and RED-S if mismanaged.

Conclusion: Fueling for Your Goals

For most runners, especially those training for shorter distances or valuing high-intensity speed, relying primarily on fat by eliminating carbs is a suboptimal strategy. While fat adaptation can increase endurance capacity, particularly for ultra-distance athletes, it comes at the cost of speed and power. The body simply cannot produce energy as efficiently from fat as it can from carbohydrates when pushed to higher intensities. A more effective and safer approach for the majority of runners is carbohydrate periodization, which leverages the benefits of both fuel sources. This involves intelligently timing carbohydrate intake to fuel high-quality training and races while promoting metabolic flexibility during less intense sessions. Ultimately, understanding how your body uses fuel is critical to optimizing performance and health. For most, a balanced, well-planned approach that includes strategic carbohydrate intake will lead to more sustainable and successful running. For more detailed information on athletic nutrition, consider resources like the Gatorade Sports Science Institute.

Frequently Asked Questions

Yes, it is possible to run a marathon on a ketogenic diet, but it requires a long adaptation period and performance, especially at higher speeds, will likely be lower than on a high-carb diet. Some runners use a targeted keto approach for races, strategically adding carbs beforehand.

When you run without carbs, your body turns to its limited glycogen stores first and then relies more on fat for fuel. This shift in fuel use reduces the energy available for high-intensity efforts and can lead to fatigue, often referred to as 'hitting the wall'.

Fasted cardio, or running in a glycogen-depleted state, can promote fat adaptation and teach your body to become more metabolically flexible. However, it is not recommended for high-intensity workouts, as performance can be compromised. For optimal performance, fueling before a run is generally best.

Becoming fully fat-adapted typically takes several weeks, with some experts suggesting a window of 6 to 8 weeks, though it can take longer. During this time, runners should expect decreased performance as the body transitions its fuel source.

Yes, consistent, high-intensity training with low carbohydrate availability can cause muscle breakdown through a process called gluconeogenesis, where the body converts protein from muscle into glucose for fuel. This is particularly a risk if calorie intake is also low.

Female athletes face additional risks on low-carb diets, including hormonal disruptions (due to low energy availability), compromised bone health, and a higher risk of developing eating disorders. Adequate carbohydrate intake is crucial for female hormonal health.

Carbohydrate periodization is a strategic approach where runners adjust their carb intake to match their training demands. This might mean consuming more carbs before intense workouts and races, while reducing intake on rest or low-intensity days to promote metabolic flexibility.