Runners – All you need to know about ketosis and fat adaptation

By Amy Tribolini

Many runners have been convinced that they need carbohydrates to fuel for their endurance conquests, but a new question has been circulating in the ultra running community: “Can a high fat diet also be a high performance diet?” 

More elite runners are emerging with claims that fat burning, ketosis, enables them to run more efficiently than their carb-dependent peers. With all the fad-diet advice flooding the mainstream, it is essential to understand how specific fuels are metabolized in the body and what current research is saying.

When training and competing in ultra marathons, proper fuel can be a huge part of your success.  Whether you are consuming carbohydrates or fat, your body will find a way to convert those fuels into energy so you can endure for long distances. 

Carbohydrate is the body’s go-to fuel source. Carbs are quickly and easily converted to glycogen and stored in your cells. When you need energy, your body can rapidly convert glycogen to glucose and release it into your bloodstream to burn. Ketosis occurs when your body is not consuming enough carbohydrates to meet your energy needs, and as an adaptation process, it begins burning fat instead.

There are many proposed benefits of being in ketosis on long runs. Runners state that they don’t experience the dramatic energy spikes and crashes that accompany using high-sugar (high carb) sport supplements, such as gels, bars, and sports drinks. This is due to the fact that fat is a smooth burning fuel, that does not instigate a sugar-insulin cycle. Additionally, even a very lean athlete has around 30,000 calories of fat stored. Compare that to the approximately 2,000 calories of carbohydrate stored in the body. Just by acknowledging the greater storage capacity of fat, you can see why it is a desirable fuel source. 

Now, let’s go back to discussing how carbohydrate is more rapidly converted to energy in the body.  This is true, but in part it is true because the body does not have significant practice in converting fat to fuel. For non-athletes, going into ketosis may never occur. Eating carbohydrate-heavy meals, accompanied by low physical activity, keeps the body from ever transitioning to burn fat as a primary fuel source. In this case, if it were ever necessary for the body to burn fat as a primary fuel, it would be an uncomfortable process and the body would likely feel fatigued. 

The good news is it doesn’t have to stay this way. An athlete that commonly enters ketosis on long runs has more practice and has thus become more efficient at burning fat. Once athletes become well adapted, they may not feel a significant difference burning fat versus carbohydrate. The main distinction may be that they no longer feel the desperation to replenish their lost sugar stores frequently, during a run.     

Attempting to live full-time in ketosis is an extreme lifestyle change and can require cutting out entire food groups, but the lessons learned from ketosis can be applied in a more moderate manner through a method called ‘fat adaptation’. You may not have heard of fat adaptation, but if you’re an ultra runner, your body is likely to be no stranger to it. 

If you have ever ran out the door on an empty stomach and decided to do a longer run than planned, your body may have had no choice but to turn fat into fuel.  Since one pound of body fat contains 3,500 calories, the average 150-pound person could run for three hours and burn a mere half a pound of stored fat. Ultra runners can find peace of mind knowing that their bodies already contain the necessary fuel for long runs. I am not advocating for runners to starve themselves for better results, quite the opposite. I am encouraging endurance athletes to fuel with healthy fats and limit sugary, high-carbohydrate supplements as a means to ultimately perform more efficiently on long runs.       

What is fat adaptation?

I’ll start by explaining what fat adaptation is not. It is not a low-calorie starvation diet; it is also not like the Atkins diet. Fat adaptation is a ‘state of being’ where the body is comfortable, efficient, and content burning fat as fuel. This method works by understanding food’s macronutrient content: carbohydrate, protein, and fat. 

Fat adaptation requires two things: decreased intake of carbohydrate and increased fat consumption.  These dietary changes coupled with the right kind of physical activity can be the magic combination.  Since converting fat to fuel is a slower metabolic process than converting carbohydrate to fuel, especially in the untrained body, practicing this technique with lower-intensity physical activity is where athletes want to start.

How do I become “fat adapted”?

The easiest way to jumpstart fat adaptation is by skipping your morning breakfast and going for a run first thing in the AM. **Gasp** Isn’t this what every nutritionist says not to do? Yes, breakfast is an important meal to fuel your body, especially if you primarily burn carbohydrates as fuel. But, if you are an endurance athlete looking for that edge in long races, this is for you. 

When first trying out this technique, your body may hurl hunger cues to your brain, desperately demanding a bagel, orange juice, cereal, or other high carbohydrate foods. If you feel you need to eat before heading out on a run, selecting high fat/low carbohydrate foods can provide satiety without sabotaging your fat adaptation goals.   

I like to make coffee in the morning and put a large scoop of coconut oil in it. The bonus with coconut oil is that the fat structure (medium-chain-fatty-acids) increases energy expenditure and ultimately allows your body to burn fat more rapidly. If you are a big breakfast eater, this may seem hard at first because your body is so conditioned to burn carbohydrates as fuel.

Dietary tips to enhance fat adaptation

  • You don’t have to remove all carbohydrates for fat-burning to initiate, what is more beneficial is removing grains.
  • Continue to eat fruits, vegetables, proteins, and a lot of healthy fats such as: avocado oil, olive oil, coconut oil, nuts and seeds.
  • Exercising on an empty stomach (or a high-fat snack) in the morning can be the most effective way of entering the fat adaptation zone. 

How should I train to aid fat adaptation?

Start with moderate intensity running (approximately 70% effort). While your body is adapting to converting fat as fuel, remember, this is a slower process at first. Be prudent, lay off the high intensity running until your body has more time to adjust. For example: my comfortable running pace is a 9-minute/mile. I know this because I can carry on a conversation, run long distances at this pace, and not feel exhausted when I’m finished. If initially I tried to enhance my fat adaptation while doing 7-minute/miles, I would feel exhausted, struggling, and desperate for sugary, high-carbohydrate snacks when my workout was done. This is because the body requires carbohydrate for high intensity workouts. Whatever your comfort zone is for running, utilize that as your pace while adapting to fat burning. 

Just weeks into your training phase for fat adaptation, your body has likely adjusted to burning fat more efficiently. You may notice that you feel less hungry during and after runs. This is due to the stability of your blood sugars. Burning fat does not give you the severe highs and lows in blood sugar, it allows your blood sugar to remain steady despite burning significant calories. 

If you want to re-introduce high-intensity training, such as hill repeats and speed workouts, you can re-introduce more carbohydrates into your diet. Carbohydrates are not harmful; they simply turn off or decrease your fat burning for the time being. High-intensity exercise benefits from carbohydrate burning because of how rapidly your metabolic process can convert it to energy. Using the naturally rapid metabolism of carbohydrates coupled with your newly acquired fat burning efficiency, you should be more equipped to handle any pace, distance, or course. 

What are the benefits of fat adaptation?

Research is showing that fat-adapted athletes are able to race endurance events with just a fraction of the calories typically consumed. They are achieving these goals with stable blood sugars and minimal crashes in energy. Additionally, they are not suffering the typical gastro-intestinal malaise often caused by sugary, high-carbohydrate sports supplements. Consuming fewer calories, while feeling steadier levels of energy, may allow ultra-runners to reach higher levels of performance.

Research also shows that lactic acid, a compound produced when glucose is broken down and burned as fuel, is decreased in athletes burning primarily fat as fuel. Once built up in the body, lactic acid can produce painful, burning sensations. Fat adaptation and a heavier reliance on fat as fuel during a race, can cut back on lactic acid formation and decrease overall discomfort in the body.  


Fat-adaptation can come in extremely handy during long endurance efforts such as ultra marathons.  I like to think of it like a ‘get out of jail free’ card.  If my stomach turns sour or I simply don’t want to take as much time eating during a race, I know that my body is well trained to adapt.  This is because my body has become more self-sufficient using itself as a fuel source.  Just like ultra runners count on their physical training to get them through a hard race, I can count on my body to do what it has practiced: to efficiently burn fat as fuel.


About the author
Amy Tribolini currently works as both a Registered Dietitian and Nutrition Professor. She lives, trains, and competes as an ultra runner out of Colorado Springs, Colorado. Amy specializes in fueling endurance athletes, athletic performance, and plant-based diets. Amy holds both a Bachelors Degree in Dietetics and a Masters Degree in Human Nutritional Science from the University of Wisconsin.   

You might like to read these other articles on the topic of fat burning:
Plant based carbohydrate recipes for fat burning strategies
Nutrition for stripping fat and building lean muscle mass for race readiness
How to lose fat ultra running


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Volek JS, Noakes T, Phinney SD. Rethinking fat as a fuel for endurance exercise. Eur J Sport Sci. 2015;15:13–20. doi: 10.1080/17461391.2014.959564

Spriet LL. New insights into the interaction of carbohydrate and fat metabolism during exercise. Sports Med. 2014;44:S87–S96. doi: 10.1007/s40279-014-0154-1.

Brooks GA, Mercier J. Balance of carbohydrate and lipid utilization during exercise: the “crossover” concept. J Appl Physiol. 1994;76:2253–2261.

Arciero, P. J., Miller, V. J., & Ward, E. (2015). Performance Enhancing Diets and the PRISE Protocol to Optimize Athletic Performance. Journal of Nutrition and Metabolism, 2015, 715859.

Cox, P. J., & Clarke, K. (2014). Acute nutritional ketosis: implications for exercise performance and metabolism. Extreme Physiology & Medicine, 3, 17.

Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism. 2016;65(3):100–10.

Photo credit: romanboed Amsterdam Meet: 3K via photopin (license)

"There are many proposed benefits of being in ketosis on long runs. Runners state that they don’t experience the dramatic energy spikes and crashes that accompany using high-sugar (high carb) sport supplements, such as gels, bars, and sports drinks"

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