Attention Uphill Athletes: The information in these articles provides outdated, incorrect, and potentially harmful information.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.

Note on Scott Johnston. On another website, Mr Johnston, refutes this disclaimer by citing NOW DEBUNKED 10+ year-old studies in an ill-fated attempt to prove that his opinions, which very clearly at odds with modern coaching standards, are still relevant. His views are potentially very harmful. Mr Johnston is 1) Not acknowledging the new research and the evolving process that science is and 2) appears to be unable change his mind when new evidence arises.

Mr. Johnston himself has talked about the problems with overtraining in ultra running. But in the next breath recommends fasted training. Underfueling contributes to health issues related to overtraining. The rest of the coaching community has moved on from the psyeudo-science of fasted training. It is now understood that fasted training, and insufficient fueling in general, can be very harmful and promote under recovery and overtraining and create mulitiple health issues for athletes. Uphill Athlete coaching team and Registered Dieticians have unanimously and enthusiastically moved on from fasted training. We  wish to promote healthy training practices based on the well understood principles of endurance training even when (especially when) our understanding of training and recovery evolve.

I also want to add here that I, Steve House, never practiced fasted training in my career as an athlete. 

We are working to update this information as fast as possible.

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

Becoming a fat adapted athlete is fundamental to proper endurance training. In this article we will explain why the training itself is ultimately more important than your diet’s macronutrient distribution of fat, carbohydrates, and protein. We will also explore what training at a low carbohydrate availability means in practice. Finally we will help you understand what fat adaptation is and how to implement fasted training.

The Benefits of Fat Adaptation to the Endurance Athlete

Carbohydrates and fat are the primary fuel sources used by the muscles during endurance exercise. Broadly speaking three things will determine the contribution of each energy source: 1) Intensity and duration of the exercise, 2) fuel availability, and 3) training status (your level of training/fitness). Endurance exercise lasting longer than 90 minutes results in depleted muscle glycogen (carbohydrate) stores and the need to eat carbohydrates during exercise in order to prolong performance and reduce the time it takes to reach fatigue (1, 2, 3), as well as to speed recovery.

Nutrition strategies have traditionally focused on optimizing carbohydrate storage in the muscles and liver to enhance endurance performance. However, we are only able to store a limited amount of carbohydrates in the body.  Even in the leanest of athletes there is an abundance of fat that can be used to provide a steady supply of energy. Recently there has been a resurgence of interest in fat adaptation training and low-carbohydrate, high-fat (LCHF) diets as means of optimizing the usage of this fat reserve, helping you go farther for longer.

The Role of Training in Fat Adaptation

To try and keep it simple: when training endurance, ultimately it is the actual endurance training that will cause the physiological adaptations that result in being able to perform at higher intensities for longer durations. That includes the body increasing its ability to use fat as a fuel source regardless of carbohydrate intake. The number one driver for fat adaptation is training! However, fat adaptation training—training with low carbohydrate availability, such as when in a fasted state—has shown to further increase these physiological adaptations, including the body’s ability to use fat as a fuel source when training at low to moderate intensities (4), as is the case when in Zones 1 and 2.

As highlighted above, because carbohydrate ingestion before and during endurance exercise of low to moderate intensity (Zones 1 and 2) reduces the body’s ability to utilize fat as a fuel source (5), the goal of every endurance athlete should be to train the body to use a greater contribution of fat as a source of energy (for example, by training in a fasted state). This also has the added benefit of reducing the body’s reliance on eating carbohydrates during endurance exercise, thus sparing muscle glycogen levels. This enables you to tap into your carbohydrate store when you need it the most, during those higher-intensity moments—on steep climbs, when you pick up the pace to get down before dark, and through strenuous moves or cruxes on a long alpine route. This is known as fat adaptation.

Carbohydrates Are Important Too

When preparing for your event, whether an expedition, alpine rock trip, summit attempt, or ultra-distance race, the aim is to optimize availability of BOTH fat and carbohydrates as fuel sources. Research has shown that when you eat a high-fat diet and carry out fat adaptation strategies on all exercise types, this has a “switching off effect” of your body being able to use carbohydrates as a fuel source (6).

The greater the intensity of exercise, the greater the reliance of your muscles on carbohydrates to fuel the movement (7). Therefore it is important to eat carbohydrates around your high-intensity sessions (e.g., strength training, intervals, anything in Zones 3 and 4) to get the most out of that training.

Fat Adaptation and Endurance Training: Summary

Fat adaptation has the following positive effects:

  1. It enhances physiological adaptations to endurance training, enabling you to go farther and faster for longer.
  2. It enhances the body’s ability to use fat as a fuel source during endurance exercise (your endurance training), reducing your reliance on carbohydrates during endurance performance (that summit attempt, ultra race, alpine expedition).

We advocate a lot of fasted training. But on the days you want to go fast by tapping into your maximal intensities, such as during your goal climb or race, you definitely want to eat a high-carbohydrate diet.

This is the first of a series on the role of diet in endurance performance. Be sure to read the second article, “Nutrition Strategies to Maximize Fat Adaptation,” which explains what to eat before, during, and after endurance training sessions, and the third article, “High-Fat Diets and Ultra-Endurance Performance,” which explores the diet’s role during endurance training and looks at the overall effect of a high-fat diet on performance.

Be sure to read the rest of the articles in this series:


  1. Cermak and van Loon. “The use of carbohydrates during exercise as an ergogenic aid.” Sports Medicine 2013 Nov;43(11):1139–55.
  2. Yeo, Wee Kian, Andrew L. Carey, Louise Burke, Lawrence L. Spriet, and John A. Hawley. “Fat adaptation in well-trained athletes: effects on cell metabolism.” Applied Physiology, Nutrition, and Metabolism 2011;36(1):12–22.
  3. Burke, Louise M., John A. Hawley, Stephen H. S. Wong, and Asker E. Jeukendrup. “Carbohydrates for training and competition.” Journal of Sports Sciences 2011;29(Suppl 1):S17–S27.
  4. Hawley, JA, and JP Morton. “Ramping up the signal: Promoting endurance training adaptation in skeletal muscle by nutritional manipulation.” Proceedings of the Australian Physiological Society 2013;44:109-115.
  5. Spriet, LL. “New insights into the interaction of carbohydrate and fat metabolism during exercise.” Sports Medicine 2014;44(Suppl 1):S87–S96.
  6. Burke, Louise M. “Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon?” Sports Medicine 2015;45(Suppl 1):S33–S49.
  7. Jeukendrup, Asker, W.H.M. Saris, and Anton JM Wagenmakers. “Fat Metabolism During Exercise: A Review – Part II: Regulation of Metabolism and the Effects of Training.” International Journal of Sports Medicine 1998 Jul;19(5):293–302.

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