Attention Uphill Athletes: The information in these articles provides outdated, incorrect, and potentially harmful information.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

There is a simple way to implement training strategies to improve your fat adaptation. We have decades of experience with coaching endurance athletes to greater fat burning capabilities and hope to offer a simple path to your improved endurance performance without going to extremes in either diet or training. Extremes can be hazardous and are not necessary if you take a long-term, systematic approach.

We have used the following simple prescriptions with many athletes over the years with good success in every case.

Fat Adaptation Training


We provide the following simple guidelines for our athletes:

  • Shift food intake away from the USDA food pyramid to one that contains about 50–60 percent of calories from fat. (Note that this does not give you license to eat poorly. Except as an occasional treat, limit the bacon and any other highly processed food.)
  • Divide the remainder of your daily caloric intake evenly between carbohydrates and protein.

If you currently have a diet high in carbs, this will probably be quite a demanding transformation. The typical modern diet derives the bulk of its calories from carbohydrates: They are cheap, often need minimal preparation, and are generally seasoned to make them taste good and keep you coming back for more. Keep your intake of processed carbohydrates to a minimum. Eat them mainly while training or racing or immediately afterward (within the 30-minute post-exercise refueling window).

Extreme ketogenic diets will probably help you lose some more fat but too often they impact your ability to train with sufficient frequency and intensity. Our simple suggestion will cause the desired metabolic shift over the course of a few weeks, especially when combined with fasted workouts.

Fasted Workouts

When you awake in the morning you will have presumably not eaten for about 12 hours. Your glycogen stores will already be somewhat depleted, even if you did not engage in a heavy training session the day before (more so if you did).

Glycogen depletion has been shown in numerous studies to be one of the most powerful aerobic adaptation stimuli by signaling an increase in aerobic enzyme activity. So starting a workout in a semi-depleted state kick-starts the fat-as-fuel-adaptation process. Recent studies (see Notes 1–3) show that glycogen depletion also has a major impact on your muscles’ ability to utilize fats. Given all that, you can begin to understand the powerful training effect we are advocating.

Marko Prezelj descends from an acclimatization climb in the Karakoram Range, Pakistan. K7 and K6 loom behind. Photo by Steve House
Marko Prezelj descends from an acclimatization climb in the Karakoram Range, Pakistan. K7 and K6 loom behind. Photo by Steve House

Initially we recommend doing only basic aerobic (Zone 1) workouts in a fasted state. For most people, this will be one of your shorter aerobic workouts each week. Once you can handle these fasted workouts without feeling a craving for carbs, add a second, longer fasted workout in your week. The process can be extended for many people until nearly all aerobic (Zone 1 and 2) workouts are done fasted.

You will still want to fuel on carbs before and after high-intensity and strength workouts so that you can get the best training effect out of this important training. This is an important point that many people miss. Understand that your performance during these workouts is heavily dependent on the available glycogen stores. If they are not topped up prior to that interval session, your training time will not be as effective as it could be.

It’s really as simple as these two changes in diet and exercise. We recommend that you stop reading and start trying it out for yourself. For those who want to know more, Chapter 3 in our book Training for the New Alpinism, “The Physiology of Endurance Training,” makes the case in greater detail.

Title Photo: Kilian Jornet is undisputedly a highly fat-adapted athlete. Here he jogs up the Weingartengletscher en route to a quick ascent of the Täschhorn (4,491 meters) in Switzerland. He didn’t eat during the climb or descent. Note that our 5mm glacier rope is barely visible trailing behind him. Photo by Steve House

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