Is a High-Fat Diet Good for Runners? The Pros and Cons

October 23, 2025 •

5 min read

The average endurance athlete has long been told that carbs are king, yet a growing movement suggests that prioritizing fat might be the key to unlocking peak performance for runners. Is a high-fat diet good for runners, or is it a misguided trend?

Quick Summary

A comprehensive look at high-fat diets for runners, examining the metabolic shift to fat-burning, performance implications for different distances, and potential drawbacks.

Key Points

Endurance Advantage: High-fat diets can increase fat oxidation and spare glycogen, which may benefit ultra-endurance runners during prolonged, low-intensity efforts.
High-Intensity Impairment: Strict high-fat diets can negatively affect performance in high-intensity workouts and shorter races, as these efforts rely heavily on carbohydrates for fuel.
Fat Adaptation Period: The body needs a significant adaptation period (weeks to months) to become efficient at burning fat, and during this time, runners may experience reduced energy and increased perceived effort.
Metabolic Flexibility: The most modern approach is carbohydrate periodization ('train low, race high'), which strategically uses both fat and carbs to maximize both endurance capacity and high-intensity performance.
Individual Response: The effectiveness of a high-fat diet varies significantly among individuals; what works for one runner may not be optimal for another.
Not a Zero-Carb Diet: A high-fat diet for runners does not mean eliminating all carbohydrates, especially for high-intensity training sessions and races.

The traditional view vs. fat adaptation for runners

For decades, the dominant sports nutrition paradigm has centered on carbohydrates as the primary fuel source for runners. The body stores carbohydrates as glycogen in the muscles and liver, which can be quickly converted to energy for moderate to high-intensity exercise. This strategy, known as carb-loading, has been a staple for runners preparing for races like marathons to maximize glycogen stores and delay fatigue.

However, a newer school of thought, gaining traction particularly among ultra-endurance athletes, suggests a different approach: fat adaptation. This involves training the body to become more efficient at using its vast fat reserves for fuel instead of relying solely on limited glycogen stores. The logic is that since fat reserves are far more abundant, a fat-adapted runner could theoretically sustain a steady pace for longer without "hitting the wall".

How high-fat diets change a runner's metabolism

Switching to a high-fat, low-carbohydrate (LCHF) diet prompts significant metabolic changes. After a few weeks of consistent LCHF eating, an athlete's body increases its fat oxidation capacity, meaning it gets much better at burning fat for energy. Some elite, fat-adapted ultra-runners have been shown to have much higher rates of fat oxidation during prolonged exercise than their high-carb counterparts. This metabolic flexibility is at the core of the high-fat diet argument for runners.

For example, studies have shown that after adapting to a high-fat diet, a runner's respiratory exchange ratio (RER) is lower at various exercise intensities compared to a high-carb diet, indicating a greater reliance on fat for fuel. This metabolic shift, however, comes with a trade-off. While fat-adapted athletes are excellent fat-burners, their ability to perform at high intensities, which requires rapid carbohydrate burning, can be impaired.

The good and the bad: Weighing the evidence

The research surrounding high-fat diets for runners is not one-sided. While proponents point to metabolic flexibility and sustained energy, critics highlight reduced high-intensity performance and potential long-term issues. The suitability of a high-fat diet depends heavily on the runner's specific goals and race distance.

Potential benefits of a high-fat diet for runners

Improved Endurance for Ultra-Distance Events: For ultra-marathons and other long-duration events where intensity is moderate, tapping into the body's vast fat reserves can delay fatigue and reduce the reliance on external fuel sources, potentially minimizing GI distress.
Enhanced Metabolic Flexibility: A fat-adapted body can switch between fat and carbohydrate fuel sources more efficiently, making it less susceptible to energy crashes.
Weight Management: For runners looking to improve body composition, a high-fat diet, when properly implemented, can lead to a reduction in body fat while preserving lean muscle mass.

Potential drawbacks and risks

Impaired High-Intensity Performance: Carbohydrates are the body's preferred fuel for high-intensity efforts, like sprints or tempo runs. A high-fat diet can downregulate the body's ability to burn carbs efficiently, which may hinder performance in shorter races or during crucial race surges.
Increased Perceived Effort: Studies have shown that during the adaptation phase, and potentially long-term, runners on a high-fat diet may experience a higher rate of perceived exertion for the same pace compared to high-carb counterparts. This is because it takes more oxygen to burn fat for fuel than to burn carbohydrates.
Nutrient Deficiencies: Restricting entire food groups, especially carbohydrates from whole grains, fruits, and starchy vegetables, can lead to potential deficiencies in micronutrients and fiber.
Gastrointestinal Distress: While some claim high-fat diets reduce stomach issues during races, eating too much fat at the wrong time (e.g., too close to a run) can cause significant GI upset.

High-fat vs. high-carb diets for performance

The following table compares the typical outcomes of long-term high-fat, low-carb (LCHF) adaptation versus the traditional high-carbohydrate (HCLF) fueling strategy for runners based on current sports nutrition science.


Feature	High-Fat, Low-Carb (LCHF) Diet	High-Carbohydrate (HCLF) Diet
Primary Fuel Source	Fat, especially at lower to moderate intensities	Carbohydrates, especially during high-intensity exercise
Glycogen Stores	Reduced availability, but can be maintained with strategic intake	Maximized and readily available for rapid energy
Max Fat Oxidation Rate	Significantly increased, peaking at higher exercise intensities	Slower than LCHF, fat contributes less to overall fuel
High-Intensity Performance	Can be impaired due to reduced carbohydrate utilization	Optimized due to quick access to glycogen stores
Race Day Fueling	Reduced reliance on exogenous carbs; may still use for surges	Requires consistent carbohydrate intake to avoid "bonking"
Adaptation Time	Several weeks to months for full metabolic shift	Minimal adaptation required, standard approach for many years
Ultra-Endurance Potential	May offer advantages for sustained, low-intensity efforts	Can face challenges with depleting glycogen over very long distances

Fueling strategies beyond the high-fat debate

Rather than committing to an all-or-nothing approach, many sports nutritionists advocate for a more nuanced and flexible strategy known as carbohydrate periodization. This involves strategically adjusting carbohydrate intake based on the type of training session or event. For example, a runner might perform an easy or moderate-intensity run in a low-carb or fasted state to encourage fat-burning adaptations. Then, for a high-intensity interval session or a race, they would fuel up with carbohydrates to ensure peak performance.

This approach, often called "train low, race high," allows the athlete to reap the benefits of both fueling philosophies: developing metabolic flexibility while ensuring that high-intensity efforts are adequately fueled. It provides a middle ground that acknowledges the physiological demands of different types of running without risking the performance impairments associated with a strict, long-term high-fat diet. Ultimately, the key is to experiment safely and find a personalized strategy that works best for individual goals and tolerance. For more insights on this topic, a consultation with a registered sports dietitian is recommended.

Conclusion: Finding the right balance

Ultimately, whether a high-fat diet is "good" for a runner depends on their specific goals and the type of running they do. For the ultra-endurance athlete performing at moderate intensities, a well-implemented fat-adapted approach may offer some advantages in terms of sustained energy and reduced fueling needs. However, for runners focused on performance in shorter distances or those requiring high-intensity bursts, a carbohydrate-centric or periodized approach remains the most evidence-based strategy. A rigid, high-fat diet may compromise the ability to perform high-speed work, leading to reduced training quality and a higher perceived effort. The most modern and flexible approach is to develop metabolic flexibility, using fat and carbs strategically to match the fuel needs of each workout. For most runners, a diet that includes healthy fats alongside periodized carbohydrate intake offers the best of both worlds, ensuring both endurance capacity and the ability to perform at high intensities.

Train Low, Race High: How to Fuel Your Endurance Performance

Frequently Asked Questions

The ketogenic diet is a very specific type of high-fat, low-carbohydrate diet that induces ketosis, a metabolic state where the body primarily burns ketones for fuel. While some runners may experiment with keto, a broader high-fat diet for runners, often called 'low-carb, high-fat' (LCHF), is not necessarily ketogenic and allows for more carbohydrates, often in a periodized approach.

Full fat adaptation can take several weeks to months, depending on the individual and the consistency of their diet. During the initial phase, a runner may experience reduced energy levels and a temporary decrease in performance, sometimes referred to as the 'keto flu'.

For ultra-endurance events, a high-fat diet can help spare muscle glycogen and tap into the body's large fat reserves for a more sustained energy supply, which may help delay or prevent 'hitting the wall'. However, this is primarily relevant for lower-intensity, longer-duration efforts, as higher intensities still require carbs.

For marathons and half-marathons, which are typically run at a higher intensity than ultra-marathons, a high-fat diet may not be the optimal strategy. The body needs rapid access to carbohydrates for high-intensity efforts, and relying solely on fat can impair this ability, potentially reducing race performance.

Runners should prioritize healthy, unsaturated fats such as monounsaturated and polyunsaturated fats. Good sources include avocados, nuts, seeds, olive oil, and fatty fish like salmon, which provide essential fatty acids and can help with inflammation and recovery.

Eating too much fat, particularly saturated fat or high-fat meals right before a run, can slow down digestion and potentially cause gastrointestinal (GI) distress. However, a fat-adapted runner may find that reduced carbohydrate intake during a race lessens the risk of GI issues associated with sugary gels and drinks.

Carbohydrate periodization is a flexible fueling strategy where runners strategically adjust their carbohydrate intake based on their training needs. It is often considered a superior approach to a strict, long-term high-fat diet because it allows the runner to build metabolic flexibility while ensuring they are properly fueled for both low-intensity and high-intensity efforts.

While research is ongoing, some studies suggest potential negative long-term health consequences from severely restricting carbohydrates and eating very high fat, such as potential impacts on cholesterol and micronutrient intake. Most sports nutritionists recommend prioritizing healthy fats within a balanced diet rather than an extreme approach.