Ketosis For Nordies (and other Endurance Athletes)

A dietary experiment, some (very) simple science, and why carbs are back in, but refined sugar is not.

In April of this year I did an experiment. An experiment where I stopped eating carbohydrates.

Well, almost stopped. I was eating fewer than 50 grams of carbohydrates per day, or about 200 calories worth. That equated to about 2 strawberries, a half cup of zucchini, and some dairy. Since I was eating about 2000 calories per day, that meant I was eating a lot of the other macronutrients. Specifically, I was eating a lot of fat.

I was trying out the ketogenic diet, which in simple terms means I was feeding my body so much fat and so little of everything else that it was forced to burn, well, fat, for fuel. Quick summary of how a body utilizes fuel:

1) We like to burn sugar (glucose). We also like to eat sugar, in the forms of both complex and simple carbohydrates.

2) The body breaks down larger and more complicated sugars/carbohydrates (glycogen) into glucose.

3) We burn the glucose in order to make adenosine tryphosphate or ATP: cellular energy.

4) We ski (run, bike, swim, etc.) fast.

Most of us eat enough carbohydrates to keep our body fueled for all our adventures and training. We know to take a high-sugar snack along for quick energy during periods of sustained exertion. In a real bind, the body will also break down muscle protein to burn as fuel, which is a state we all should strive to avoid because who wants to eat their own muscle!? So what happens when you deprive yourself of your primary source of fuel, and then try to keep training? What happens when you carb-starve your body?

That was what I wanted to find out. I was curious, less about the science of ketosis, more about how it would feel. I knew the science, as much as I needed to anyway. Metabolically, here is what happens in a nutshell: the body breaks down available fat into fatty acids and then into compounds called ketones, which can be burned for fuel in a similar manner to glucose. So the ketogenic diet calls for such a high proportion of fat and such low proportions of protein and especially carbohydrates that the body has no choice but to burn ketones as its primary source of fuel.

But why? Why do it? What’s so cool about it? The ketogenic diet has been hyped for its weight-loss potential, like many fad diets, but that wasn’t what drew my interest. It has been praised by long-distance athletes who swear their energy levels have never been stabler. And there’s something to that. In any diet, fat is a source of prolonged energy, whereas sugar caused energy spikes and crashes. I wanted to see if I would have super long-lasting and stable energy in ketosis. I was curious about how it would feel to train while burning ketones. And I was psyched for the challenge of such a dramatic dietary change.

My diet is already fairly limited due to my food allergies (check out my Foodie Friday posts for some delicious and healthy nut-free recipes!) Trying to get 70% of my daily calories from fat was no picnic. (What, on your picnics you drink 4 tablespoons of heavy cream alongside your egg fried in bacon fat for lunch?) As one gram of fat provides 9 calories to just 4 from protein and carbs, all my meals became very small portions of the highest fat foods I could get my hands on: eggs, high fat dairy (watch the sugar there!), my favorite sunflower seed butter SunButter (remember, no nuts?), unsweetened chocolate, meats like bacon, pepperoni, chicken thigh, and 20% fat ground beef, and olive oil… So. Much. Olive oil.

I tracked my food very closely. I tracked ketones in my urine using ketone strips, which provide a fun metric but don’t tell you your blood ketone concentration. So the test told me I was producing ketones, but didn’t give me a guarantee that I was burning them. I tracked my weight. In the first week I lost a lot of water weight, as expected, since carbohydrates are linked to water retention. I lost a little additional weight since it took me a couple weeks to figure out how to eat enough while keeping my macronutrient ratios in the correct range. My weight loss stopped after about week 3, when I was fully in ketosis and had figured out how to eat enough calories.

I suffered through the infamous brain fog, also known as the keto flu, which is a period of a few days at the beginning of the keto diet in which energy levels and cognitive abilities absolutely tank while the body struggles to keep burning glucose when none is available. It’s as bad as it sounds. But once I got beyond that, I noticed little difference in my daily energy levels during non-exertion. I only noticed it when I was training. I experienced two primary effects, one awesome, one lousy.

1) I could go FOREVER!

Holy smokes, I didn’t need to eat while training! I did a 7 hour run in the Grand Canyon on one hard boiled egg dipped in mayonnaise at the start, and a spoonful of SunButter halfway through. Yes, energy levels were as stable as promised. Endurance athletes fear the dreaded bonk, or a state of shaky delerium in which the body desperately needs sugar to keep going but none is available. (It can ruin a long race, so if you’re going to be racing for more than 60 minutes, you’d better be thinking about bringing food or utilizing feed stations!) It’s not physically possible to bonk in ketosis, and so yes, I could go, and go, and go…

2) But I couldn’t go fast.

I just could not pick up my pace above a standard distance pace. Not even for short stretches. I had about 20 seconds in me before every muscle in my body flooded with that burning feeling of maximum exertion. This meant no intervals, and no racing. I poked around on the internet looking for other endurance athletes who might have experienced a similar thing, wanting to understand why it was happening, and found mixed and inconclusive anecdotes.

Here is my theory as to what was happening: the conversion of ketones to ATP simply takes too long. Putting on a quick burst of speed or maintaining an anaerobic pace for its maximum duration of 60-90 seconds simply demands glucose and glycogen. Ketosis may be great for sustained easy efforts or for daily activity, but it was not great for me.


I choose to try out ketosis during my recovery season, the spring, because I was not training very much or very intensely, I needed fewer calories, and if it totally backfired and wiped out a couple weeks of training, it wouldn’t have mattered. As April transitioned to May and I turned my attention toward next December and the coming race season, the impact the diet was having on my training became a concern.

After struggling through my lack of high gears for a month with no changes, I called it quits on the ketosis diet and celebrated with a brownie. I immediately felt sick and experienced that awful sugar-rush headache. Ketosis as a strict diet was not going to work for me, but many of the dietary patterns I’d picked up during my experiment were things I wanted to continue in my post-keto eating. Here are some of my new dietary trends:

  • First and foremost, my sugar consumption is way down. I add no refined sugar to my foods, and as I already eat almost no processed foods, I end up eating almost no refined sugar at all. I do use maple syrup, very sparingly, in my baked goods and homemade ice cream. I have developed a taste for less sweet food now, and two tablespoons of maple is plenty to take the bitter edge off a 9×13” pan of brownies.

  • I’m pickier about my carbs, preferring vegetables to pasta. This is something I was doing before ketosis, but I’m more mindful now. Living with gluten-free housemates helps a lot; we don’t use flour in our kitchen.

  • I target my carbohydrates, meaning I eat them after workouts, when the body is actively trying to replenish stored muscle glycogen burned during exertion. I eat high protein and high fat meals and snacks before exercise, encouraging my body to continue utilizing fat for a stable energy source during training.

Here’s what I’m curious about: has anyone else tried ketosis, and how did you feel at high exertion? Let me know your thoughts by leaving a comment below or contacting me here. As always you can follow me on Facebook, on Instagram, or sign up for my email list here.

To listen as Outside Magazine reports on a study that found the same thing I did about Ketosis, check out this episode of the Sweat Science podcast.


Standard disclaimer: I receive free product from SunButter in exchange for helping them spread the word about their 100% nut-free sunflower seed butter. All opinions put forth on my social media are my own.

If you’d like to become a sponsor or learn more about the businesses that help make my athletic/nomadic life possible, check out my Sponsors page!


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