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The Skinny on Fad Diets

Are they messing with your performance?

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Ella Olsson,; CC BY 2.0

As a climber of two years and registered, practicing dietitian-nutritionist (I studied nutrition education at the Mayo Clinic with a focus on medical nutrition therapy), I’ve seen firsthand with both myself and my clients how nutrition can spell the difference between sending and yet another day of dogging. Much of how your body functions stems from its food sources. But with so many diets out there based on seemingly contradictory principles, it’s tough to know what certain diets are doing for our climbing. While our bodies differ in their needs, knowing how the diets currently getting the most buzz at the cliffs—the ketogenic diet, the paleo diet, and intermittent fasting—generally affect performance can help you decide how to eat. 

Ketogenic Diet

Pediatricians developed the ketogenic diet in the 1920s to alleviate seizures in kids with epilepsy. While effective at its goal, the ketogenic diet remains little understood in its mechanism. It entails tweaking the diet to become low carb and high fat (think avocados, coconuts, olives, nuts, and other high-fat foods and animal protein). This switches you into a metabolic mode called ketosis, which occurs when the body doesn’t have glucose or cellular sugar to use for energy from carb and protein sources, and so switches to burning fat.

While ketone production could save your life in a starvation situation, it won’t necessarily allow you to climb hard. In fact, because carbs provide the most energy in the shortest amount of time, your body uses carb stores as its first source of energy during exercise. A 2017 Nutrition & Metabolism study followed 42 competitive athletes, and found that six weeks of a keto diet had a mildly negative impact on performance in terms of endurance capacity, peak power, and quicker exhaustion. Meanwhile, a review in the 2007 Journal of Applied Physiology found that high-fat diets could increase the perceived effort of training in endurance athletes.

Carbs are the only macronutriet that can provide energy for both anaerobic and aerobic activity, furnishing the quick bursts of energy (power) we need to latch distant holds and the sustained endurance needed for long climbs. Carbohydrate deprivation as seen in the ketogenic diet could limit performance when working through a dynamic crux or powerful boulder problem, enduring long training days, or charging up a big wall. For all climbers trying hard, it’s best avoided.

Paleo Diet

In the 1970s, Dr. Loren Cordain created the Paleolithic-based (paleo) diet to improve people’s alimentation and mimic what he thought humans 10,000 to 2.5 million years might have eaten. This boils down to eating fruits, vegetables, meats, fish, nuts, seeds, and oils while avoiding grains, legumes, dairy, salt, potatoes, and any processed food, including refined sugar. So essentially, it’s a very low-carb diet like the ketogenic diet but with a higher amount of protein versus fat. While this diet involves eating lots of produce and limits processed foods (both good things!), it deprives the body of carbs, cutting off many sources of the nutrients associated with them, such as the thiamin and folate found in whole grains. You could compensate for this with supplements or by diligently including larger amounts of specific allowed foods in the diet. But given the necessity of carbs in peak athletic performance, the paleo diet may, in any case, leave you feeling tired and less capable of summoning power, making those options not worth the effort.

Looking further into the effects of carbohydrate deprivation on performance, a February 2011 study published in Psychophysiology looked at six males performing high-intensity exercise after consuming both a high-carb and low-carb diet at separate times. Across the board, the participants had a greater rating of perceived exertion after eating a low-carb diet than when on a high-carb diet. Out climbing, this can translate into feeling like you’re working harder than if you were embracing healthy, complex carbohydrates.

Intermittent Fasting

Less about diet composition and more about meal timing, intermittent fasting (IF) entered the spotlight for its possible benefits on weight loss, sleep, blood pressure, the gut microbiome, and metabolism. We all intermittently fast while sleeping, not eating for between 8 and 12 hours, but IF extends that period to 16 hours or more daily. Some practice IF by eating the same amount of calories, and some end up eating less given the shorter period of food consumption. While this may result in gradual weight loss through modification of metabolism and possible calorie restriction, exercising in a fasted state will force your body to burn fat instead of carb stores; in some circumstances, you may even burn muscle. This can be detrimental to climbing performance, as the body won’t have available the quick energy from recently eaten food, requiring a longer method of breaking down your body’s fat and valuable muscle (versus ketosis, in which fat from food is burned while muscle mass is preserved).

In 2004, 55 Algerian soccer players were studied during Ramadan, an Islamic tradition that entails fasting from sunrise to sunset—around 14 hours. With the addition of 6 to 8 hours of sleep, most of these players fasted around 20 hours daily. “The phase shift of food intake and disruption of sleep patterns affect actual and perceived physical performance,” stated the results in the 2007 British Journal of Sports Medicine. The study also showed that exercising in an extreme fasted state decreased speed, endurance, strength, and agility.

However, intermittent fasting can be done in a way that will have less, if any, impact on your climbing (see “Intermittent Fasting 101” below). Given that the diet doesn’t modify actual diet composition, it can still promote balanced eating and shouldn’t require supplementation. While not performance enhancing per se, this diet can help with weight loss and the associated health benefits.

there’s no magic bullet …

Above all, listen to your body and your internal wisdom. We climbers can choose from dozens of diets, from vegan, to Mediterranean, to Whole30, to gluten-free, and more. To assess if a diet will maximize performance, take a holistic view: Is an entire food group being excluded, or just one type of food? While you can easily find the nutrients in a single, excluded food elsewhere, an entire group can be hard to replace—this type of dieting should generally be avoided. While supplementation can make some of these diets work, it certainly begs the question as to whether the diet is going to be as effective as simply getting your nutrients from food.

Many popular diets shy away from carbs, as seen with the ketogenic and paleo diets. While these diets may spark weight loss, they won’t support muscle strength, muscle endurance, and quick response time (power). Climbing requires complex carbs to provide quick energy in those dynamic moments (anaerobic effort) as well as sustained, efficient-burning fuel for endurance efforts (aerobic effort). As you probably already guessed, consistent, balanced eating will do more for your climbing than adhering to a specific, restricted diet—there’s no magic bullet! For more clarity on specific diets and individual nutrition needs, consult a registered dietitian.

Intermittment Fasting 101 

If you do try intermittent fasting, use these suggestions to mitigate any impacts on your climbing: 

  1. Stick with a shorter fast. If you fast 12 to 14 hours, you likely won’t see much difference in performance. Select a fasting schedule that allows you to eat for most of your waking hours (8 p.m.–8 a.m., 6 p.m.– 8 a.m., etc.).
  2. Eat the same amount of calories and protein as when you aren’t fasting. This will ensure you have those energy stores available when it’s time to work. 
  3. Fuel before, after, and during (if necessary) workouts for peak performance. Save the fasting for the hours when you aren’t climbing or training.


  1. Impact of a 6-week non-energy-restricted ketogenic diet on physical fitness, body composition and biochemical parameters in healthy adults. Paul Urbain-Lena Strom-Lena Morawski-Anja Wehrle-Peter Deibert-Hartmut Bertz – Nutrition & Metabolism – 2017
  2. “Fat adaptation” for athletic performance: the nail in the coffin? Louise Burke-Bente Kiens – Journal of Applied Physiology – 2006
  3. Carbohydrate and fat utilization during rest and physical activity. Katarina Melzer – e-SPEN, the European e-Journal of Clinical Nutrition and Metabolism – 2011
  4. Low carbohydrate diet affects the oxygen uptake on-kinetics and rating of perceived exertion in high intensity exercise. Lima-Silva AE-Pires FO-Bertuzzi RC-Lira FS-Casarini D-Kiss MA – Psychophysiology – 2011
  5. Impact of Ramadan on physical performance in professional soccer players. Yacine Zerguini-Donald Kirkendall-Astrid Junge-Jiri Dvorak –British Journal of Sports Medicine – 2007