Attention Uphill Athletes: The information in these articles provides outdated, incorrect, and potentially harmful information. Scientific knowledge evolves as new studies are done and we are on the forefront of tracking and keeping you updated on the current best practices. Since this article was published new scientific and experiential findings directly and definitively contradict the information provided in this article. Uphill Athlete’s long-time registered dietician, Rebecca Dent, is working on updating our community on the new best practices, including what the latest research and coaching experience indicates to be best practices, and why. In the meantime, we want you to fuel all of your training sessions appropriately, to make sure you have enough energy to train and recover ready for your next training session and so you can cope with the training load of the week. This will help to ensure you are eating enough energy day to day to support your health and enable you to reach your full fitness potential ultimately helping you achieve your goal of summit success or crossing that finish line. In the meantime, please make sure you adequately fuel your training with a source of carbohydrate.

Sincerely, Rebecca Dent, MSc Sport Nutrition – Sports Nutrition Diploma – International Olympic Committee and Steve House, Uphill Athlete founder.


This article explores whether eating a low-carb, high-fat diet further enhances ultra-endurance performance beyond training, compared to a diet containing carbohydrates. For our purposes, I define ultra-endurance performance as any exercise lasting longer than 4 hours.

The Benefits of Fat Adaptation

As described in the previous two articles, due to the body’s limited storage capacity for carbohydrates and abundant availability of fat to provide a steady supply of energy, the aim of every endurance athlete should be to train the body to increase its ability to use fat as a fuel source.

Regardless of dietary intake, it has been shown that in well-trained endurance athletes fat oxidation (fat used by the body as a fuel source) is greater compared to recreational/untrained participants. Well-trained athletes are also able to oxidize fat at higher-intensity exercise (1). This highlights that it is the actual endurance training, and the use of strategies such as fasted training, that are of the greatest importance when it comes to fat adaptation and ultra-endurance performance (2).

This raises an important question: Could consuming a low-carb, high-fat diet further enhance the availability of fat in the body to use as an energy source beyond the training effect?

Over the past few years there has been a resurgence in the consumption of a low-carb, high-fat (LCHF) diet among ultra-distance athletes, with anecdotal reports of this dietary method being superior for success in ultra-endurance performance.

What Is a Low-Carb, High-Fat Diet? 

A LCHF diet that induces ketosis (a metabolic response in the body when carbohydrate intake is minimal) is one where >80 percent of the daily energy intake is derived from fat; 15 percent comes from protein; and <20 grams of carbohydrates round things out. In food terms for carbohydrates, that translates to no more than a thin slice of bread or small banana per day. The rest of your daily energy intake comes from high-fat foods including meats, fish, eggs, cheese, oils, nuts, seeds, butter, and avocado. (Remember to take into account the small carbohydrate content of nuts, seeds, fruits, vegetables, and dairy products.)

Studies have shown that well-trained athletes who follow a LCHF diet have higher rates of fat oxidation and use a greater percentage of fat as a fuel source during low-intensity exercise (4–7).

Does Eating an LCHF Diet Improve Ultra-Endurance Performance?

It has been proposed that a LCHF diet may enhance performance in ultra-endurance events by supporting a higher fat oxidation rate (providing more fat as a fuel source) and sparing muscle glycogen.

Despite this premise, there is insufficient scientific evidence to support claims that eating a high-fat diet results in superior performance over eating a high-carbohydrate one (5–7).

A recent study of elite race-walkers eating an LCHF diet showed that it impaired 50-kilometer race performance (7). In another study comparing world-class ultra-distance runners who consumed an LCHF diet to those who included carbohydrates in their diet, higher rates of fat oxidation were shown in the LCHF runners. However, the study did not determine who were the better runners from the two groups (6). This perhaps suggests that, for some, eating an LCHF diet is at least not detrimental to ultra-endurance performance.

What is pertinent to highlight is that it is not the high-fat diet per se that enhances rates of fat oxidation but actually the low carbohydrate intake that augments fat oxidation (2,8).

What about a Lower-Carb, High-Fat Diet?

It is important to note that adapting to a high-fat diet has been shown to impair your ability to perform high-intensity exercise (e.g., speed or hill sessions, hard climbing efforts, strength training). In other words there is potentially a price to pay when consuming primarily fats in your diet. When your ability to use carbohydrates as a fuel source is impaired, you lose your top gear!

To support adaptive changes to endurance training, perhaps it is not one or the other. A lower carbohydrate intake of 2 grams/kilogram of body weight/day may be sufficient to encourage an increased use of fat as a fuel source during low-intensity exercise. This would then allow you to continue to take in carbohydrates around higher-intensity training sessions.


  • Eating an LCHF diet has been shown to enhance fat oxidation in well-trained athletes. However, at present, further research is required to determine if this provides additional benefits to ultra-endurance performance.
  • It is not the high fat intake but the low carbohydrate intake that influences fat oxidation rates.
  • Endurance training ultimately makes us better at ultra-endurance performance. It is the training effect that increases the body’s ability to use fat as a fuel source.
  • An LCHF diet is not a necessity and is not for everyone! There can be some health risks to following an LCHF diet over the longer term if the LCHF diet is not carefully considered and well planned to incorporate all nutrients.
  • It does not need to be one or the other when it comes to choosing between LCHF and including carbohydrates. Adjusting your carbohydrate intake to suit your training could be more beneficial.
  • There are some well-trained ultra-endurance athletes that report to eat and perform well on a high-fat diet. That said, some of the world’s best ultra-endurance athletes consume higher intakes of carbohydrates.


References/Further Reading

  1. Hetlelid KJ, Plews DJ, Herold E, et al. “Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners.” BMJ Open Sport & Exercise Medicine 2015;1:e000047. doi:10.1136/bmjsem-2015-000047.
  2. Jonathan D. Bartlett, John A. Hawley, and James P. Morton. “Carbohydrate availability and exercise training adaptation: too much of a good thing?” European Journal of Sport Science 2015;15(1):3–12. doi: 10.1080/17461391.2014.920926
  3. Jeffrey F Horowitz and Samuel Klein. “Lipid metabolism during endurance exercise.” The American Journal of Clinical Nutrition 2000;72(suppl):558S–63S.
  4. Phinney SD, Bistrian BR, Evans WJ, Gervino E, and Blackburn GL. “The human metabolic response to chronic ketosis without caloric restrictions: preservation of submaximal exercise capacity with reduced carbohydrate oxidation.” Metabolism 1983 Aug;32(8):769–776.
  5. Burke et al. “Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers.” The Journal of Physiology 2017 May 1;595(9):2785–2807.
  6. Volek JS, Freidenreich DJ, Saenz C, Kunces LJ, Creighton BC, Bartley JM, Davitt PM, Munoz CX, Anderson JM, Maresh CM, Lee EC, Schuenke MD, Aerni G, Kraemer WJ, and Phinney SD. “Metabolic characteristics of keto-adapted ultra-endurance runners.” Metabolism 2016 Mar;65(3):100–110.
  7. Louise M. Burke. “Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon?” Sports Medicine 2015 Nov;45(suppl 1):S33–S49.
  8. Hawley JA and Burke LM. “Carbohydrate availability and training adaptation: effects on cell metabolism.” Exercise and Sport Sciences Reviews 2010 Oct;38(4):152–160.
  9. Volek JS, Noakes T, and Phinney SD. “Rethinking fat as a fuel for endurance exercise.” European Journal of Sport Science 2015;15(1):13–20.

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