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Mountaineering Training | Becoming Bonk Proof

Climbing is a long and demanding endeavor, with a typical summit day on Rainier or Denali stretching for twelve to fifteen hours. Every time you take a step, your muscles require energy in the form of ATP to be able to fire. ATP is created within the muscle cells by mitochondria from two main nutrients: carbohydrates and fat.

For many years, athletes have focused on their carbohydrate intake as the key to performance. Carbohydrates provide a readily accessible and easily digestible energy source for your body, which is the reason that they are the main content in most sports foods; just look at the labels of shot blocks, Gu’s, bars, energy drinks, and the like, and you will see a heavy focus on sugar. There is a good reason for this: your body has a limited ability to digest food while exercising (digestion requires energy of its own), and carbohydrates and sugars are the easiest to digest, requiring little to be done to the glucose components before they enter the bloodstream and are carried to the cells.

The main issue with a reliance on carbohydrates is that your body has the ability to store a finite supply of glucose in the muscle cells and the liver in the form of glycogen. For trained athletes that are efficient with their energy usage, that store still only lasts for about 2 hours of sustained hard effort. If you aren’t familiar with the term “bonking,” it’s that feeling when your performance drops off a cliff; you don’t feel like you are working that hard aerobically, but you can’t possibly go any faster or harder. You’ve run through those glycogen stores and your muscles are out of fuel. Eating while you exercise can help to delay bonking, but your body can only process about 250 Kcal of sugar per hour, far less than you expend over the same period. Even though we are replenishing our sugar fuel, we dip further and further into those reserves as summit day goes on. At the same time, even the leanest among us carries over 24 hours of energy in the form of fat stores. Wouldn’t it be nice to recruit those stores while you are climbing?

Fatty acids are the most energy dense nutrients in our diet and our body stores them readily. They create more ATP per unit than sugars, and our body’s ability to store them can leave us with a huge reserve energy supply. The problem is that when fatty acids and sugars are both present, our metabolisms preference burning the sugars for energy. Julia Goedecke is a sports scientist who has been examining the influence of fat oxidation (metabolism) in endurance athletes. In examining rates of fat oxidation in athletes at different intensity levels, she found a vast difference in overall rates of fat oxidation. Some burned nearly no fat at rest, while others metabolized nearly 100% fat at rest, but while there were differences in overall rates of fat metabolism, those who metabolized more fat at rest derived more of their energy from fat at all intensity levels too. This would suggest that if we can train our metabolism to derive a greater percentage of our energy from fat, it will continue to do that as we up our intensity climbing, and we will use our sugar reserves more slowly, and hopefully avoid the dreaded “bonk!”

Now that we’ve introduced the idea of developing your fat metabolism, stay tuned next week as we get into the details about how to accomplish it.


For more reading Alan Couzens has a number of interesting blogs on the subject. A good one to start with is Improving Fat Oxidation.

Questions? Comments? Do you have experience applying LCHF nutrition to endurance sports? Share your thoughts here on the RMI Blog!

Comments (6)

I have climbed Mt. Rainier twice now.  The first time was not good.  I did not finish. I had misplaced a contact lense.  The second time was much better.  I was a lot stronger with the second climb.  It was a nice day and we all summited.  It was a very nice day.  I have climbed Mt. Hood and have climbed Mt Adams now.  Both were nice climbs. 

Posted by: Mark Brashem on

Those that burn more than “trace” amounts of fat while at rest or during strenuous output are fat adapted. They have gone through the process of fat adaptation by employing a lifestyle of Low Carb, Keto, etc. You don’t get there overnight and in fact some, despite their efforts, never attain it at all. And once your body becomes fat adapted, you have to maintain it too effectively utilize fat as energy. The use of a blood ketone meter is is good tool to monitor your level. That being said, the problem we face as Climbers, is how do we maintain a fat adapted state while travelling? Have you ever attempted to maintain the 75/20/5 (fat, protein, carb) ratio while in India, Pakistan, Nepal, etc? Good luck. Unless you have your food air-dropped in, or pull off the impossible and transport all of your food you will ingest over the next 4-6 weeks from home, you are stuck with noodles, rice, and junk food snacks, all high carb food, which once ingested, rocket you straight out of your fat adapted state. And back to square one. This dilemma is something i am struggling with for 2023 when i next travel to Asia. I know what i can do here on Whitney, Gorgonio and Jacinto while fat adapted and in ketosis, yes, lower elevations than Asia, but if i can some how figure out the food logistics for Asia, i will be Superman! Doesn’t look promising though.

Posted by: Al on


Thanks for your input, and it’s great to hear from people who have been utilizing a more fat adapted diet and training regime for awhile! Your experience is interesting. As athletes we all tend to adapt to different situations a bit differently and nutrition for any endeavor can be very personal. From the research, even athletes that show the highest ability to metabolize fat during exercise, can’t create enough ATP to fully support the effort. Therefore, these athletes metabolize a good percentage of fat and make up the rest with glycogen stores. As stated in the article, glycogen stores are finite, so once an athlete has burned through these their ability to perform will deteriorate significantly unless glycogen stores are replenished. Digestion does take energy, and the body may slow rates of digestion, but won’t fully shut it down. This is also the reason that many “energy foods” are designed to be very easily digestible. While we may gear our training to encourage higher rates of fat metabolism, during an endurance event (triathlon, climb, etc) we preference our foods towards these easily digestible foods that replenish glycogen in order to maintain the ability to perform at a high level.

It sounds like your plan works well for you. We encourage everyone to experiment in their training and find nutrional regimes that work well for them and support sustained performance.

Thanks so much for your engagement with the training blog!

-The RMI Team

Posted by: petevandeventer on

Thanks for the great article Alan. Of course training to become fat adapted and completing an expedition utilizing fat stores are two different things when most guides operate under the flawed view that if you do not eat at a rest break there must be something wrong. Using a “hit and run” method of eating small snacks often while trying to maximize oxygen usage for climbing is bound to lead to problems. Your body needs oxygen and energy to digest food! Your body needs oxygen and energy to climb. In my experiences, when expending energy at lower oxygen concentrations or at high oxygen consumption (high intensity) MY body will preferential stop digestion (the stomach fills but does not empty). That food you eat during a rest break. . . it sits in your stomach and slows down the absorption of water. Those conventional rest breaks. . . not only are we NOT getting extra calories in we are becoming dehydrated; both (minimal liquid processing and increasing demand for energy away from the large muscles and brain) lead to altitude sickness. Become fat adapted and see how long you can go without eating on a heavy training day (12+ hours). From my 35 years of experience, the advice I suggest, “Eat ALL of your food (one meal well before bedtime) when you are done climbing/hiking/training for the day while drinking electrolyte balanced water often.” Additional benefits from this approach, you have more time, less rush, for the important things: safety, climbing, enjoying. . . and less muck mouth.

Posted by: Keith Loritz on

What Doug said! (Above).
Where can I read part 2 of this article? Many thanks.

Posted by: Lynda on

Where do I find part 2 of this article?  Thanks.

Posted by: Doug on

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