Can You Run Fast on a Ketogenic Diet?

My Ketogenic Experiment

In this post I will explore the theory behind a Ketogenic diet for endurance athletic performance, and tell you how I tested the idea for myself using both a Half-Marathon and 5k races as performance markers.

I will attempt to answer the following questions:

  • What is a Ketogenic diet?
  • Why might a Ketogenic diet enhance endurance performance?
  • Will a ketogenic diet work for high intensity performance such as a 5k?
  • What are the downsides of a ketogenic diet?

Ketogenic Diet: The Theory Behind It

In their book, The Art and Science of Low Carb Performance, Jeff Volek and Steve Phinney claim that a ketogenic diet may be beneficial for endurance sports performance.

The idea behind a low carb, high fat ketogenic diet is this: teach the body to use fat as fuel by restricting carbs. By starving the body of carbs, the liver will generate ketones to act as a fuel in place of glucose. Ketones can act in place of glucose as a fuel for the body, especially the brain, which can only run on glucose or ketones.

One advantage of ketones is that they don’t require an active transporter to cross cell membranes; they can easily diffuse to body tissues for energy. They’ve also been shown to treat epilepsy, increase mental focus, slow the onset of Alzheimer’s, help heart attack patients recover faster, and maybe even prevent bonking in a long distance running event.

Advocates of this type of diet point out that it’s probably a much more natural way to eat, since in an ancestral environment, carbs were scarce. Fruit was much smaller and less sugary and grains have only been around in large quantities for around 10,000 years. For much of human history the theory goes, we existed in a state of ketosis, sometimes going days without food, and living off stored body fat and ketones generated from fat stores.

If you are new to the idea of ketosis, this may sound strange, but consider that by the end of a marathon, most runners are generating mild amounts of ketones. If you’ve ever had coconut oil or any coconut products, these are also sources of fat that the body readily converts into ketones.

Breast milk also contains sources of fat that are converted into ketones. These are known as Medium Chain Triglycerides (MCTs). Such fats are not incorporated into body tissues, rather they are shuttled directly to the liver to be converted into ketones for fuel to be used by the body.

So, it seems that ketones are certainly nothing to be afraid of. We’ve evolved to produce them during periods of famine, and they’re generated from MCTs in breast milk, and coconut products.

Since the body stores around 40,000 calories of fat, and only around 2,000 calories of glucose it seems to make sense that we should be well adapted to burning fat as a primary fuel. Only in our modern environment have the tables turned to carbs being the primary source of fuel in the body.

A Natural Experiment

I like to race everything from 5k to Marathon. Perhaps burning fat would work for slow long distance running, but would I be able to run a fast 5k?

I decided to experiment for myself to see what would happen.

I finished reading The Art and Science of Low Carb Performance, 2 weeks before Twin Cities Marathon 2012 and vowed that as soon as the marathon was over I would try the diet. I finished the Marathon in 2:55 and immediately started consuming the high fat low carb ketogenic diet.

The authors say it takes 3-4 weeks for the performance adaptations to occur. I decided to give the diet an 8 week trial period just to be sure. I had 2 upcoming races: a Half-Marathon (3 weeks post marathon) and a 5k Turkey Trot (7 weeks post marathon).

The Diet

The diet is pretty simple. Keep carbs less than 50g per day, keep protein around 0.6 – 1.0g per lbs of body weight, and the rest of your calories come from fat. It is important to keep protein on the lower end, because your liver can readily convert protein into glucose, thus thwarting the need for the liver to produce ketones from fat.

You can eat all the non-starchy vegetables you want, keep fruit to a minimum with the exception low sugar fruit such as berries, and keep protein to a moderate amount. If you tally up your calories, you’ll be consuming around 80% fat, 15% protein and 5% carbs.

My typical daily meal consisted of:

Breakfast (one of the following)

  • Smoothies with berries, spinach, kale, heavy cream, coconut milk, whey protein
  • Eggs with bacon, tomatoes, spinach, avocado
  • Coffee with cream, butter and stevia


  • Big salad with MCT oil/olive oil dressing plus some protein source (salami, fish, or other meat)
  • Dinner
  • Protein, veggies tossed in butter or salad with olive oil dressing

By the third day of drinking smoothies with heavy cream & berries, salads with copious amounts of olive oil, bacon, eggs, and salami, I was finally in ketosis. According to the authors, “nutritional ketosis” is ketone blood levels higher than 0.5 mmol/L.

Training & Racing

In order to know if you are producing ketones, the authors recommend measuring blood ketones (not urine ketones) using an inexpensive ketone/glucose monitor. I bought the Nova Max Plus for around $15 on Amazon, and ketone test strips for around $2 each.

On the third day, curious of what it would feel like to run in such a state, I went out for a 5k time trial. I ran it in 18:37, 2 minutes slower than my best. But, keep in mind I had just ran a marathon 3 days earlier.

During this period I did regular 5k time trials to keep track of progress.


I totally bonked in the half-marathon. I ran the first 6 miles at a pace (5:55/mile) that would have put me around 1:17 for the half, which is the pace of my fastest half-marathon. I can usually easily sustain a sub 6 min pace for at least 10 miles, but by the end of mile 6, I was hurting. I knew I couldn’t sustain the pace I wanted, so I decided to hang back and run the rest of the race with my wife. Perhaps if I had more than 3 weeks to adapt, I would have ran better.

In the 5k I was 4 seconds slower than by best 5k, which I ran the previous year on a high carb diet. At the pace I was running, that would put me about 25 meters behind my best. The conditions were almost identical for each year, about 38 degrees F, with almost no wind.

With so many factors influencing performance on a given day, it is difficult to say if the ketogenic diet was the reason I ran slower. In terms of percentages, 4 seconds is only 0.4% slower. Since so many variables go into racing, it is hard to say if it was the diet that slowed me down.

As far as how I felt during the race: it felt like my hill running and sprint capacity were slightly constrained. I had more energy towards the end of the 5k and when ascending hills while on the high carb diet.

Side Effects

Muscle Cramps

I started getting painful muscle spasms and cramps within the first few days of dropping the carbs and amping up the fat. The muscle cramps came in the form of painful cramps in the arches of my feet, arms and legs.

The authors of, The Art and Science of Low Carb Performance say this is common and recommend taking a magnesium supplement called Slow-Mag. I had been taking this supplement for several weeks prior to my experiment, so I don’t think the cramps can be attributed to a magnesium deficiency. I hoped that my body would eventually normalize and the cramps would go away, but they stayed with me during the entire 8 weeks on the diet.


On the ketogenic diet, my bowel movements slowed to once a day and often required spending more than a few minutes in the bathroom. This was really strange as I am usually in the restroom for less than 2 minutes.

Weight Gain

I was curious about how this diet would affect my weight. On the ketogenic diet, my weight over the 8 weeks gradually crept up from 146 to around 149-150.

Final Thoughts

As far as high intensity athletic performance is concerned, my 5k time was almost identical to the previous year when I was on a high carb diet. For those that believe that you must follow a high carb diet for high intensity exercise, my experience tells a different story.

I do not agree with the authors of The Art & Science of Low Carb Performance, that it only takes 3 weeks to adapt to this kind of diet. My experience in the half-marathon shows that I was not yet adapted, as I can typically hold a sub 6:00 min/mile pace for at least 10 miles.

I was surprised how difficult it was to get my body into ketosis, even while consuming as much as 80% fat! If I ate too much protein, my body would simply convert it into glucose, thus stopping the need for ketone production.

This made me wonder: If my body was working so hard to stay out of ketosis, was this really a healthy thing to do? I now doubt that primitive people lived in a state of ketosis, except during famine. They probably ate enough protein or starchy tubers to keep them out of ketosis.

For me, the side effects of the diet were too great to continue with the diet. Aside from therapeutic benefits for people with Alzheimer’s, and epilepsy, I don’t think a ketogenic diet is helpful for athletic performance. If I had either condition, I would probably just eat a lot of coconut products, and MCT oils which the liver converts to ketones to be used for fuel in the brain.

The diet made going to social gatherings difficult because, unlike everyone else, I had to eat around 80% fat just to keep up with the diet. This made eating at restaurants and at other people’s houses difficult.

As I continued to research the debate on carbs, I learned that it may be prudent to take in some carbs in the diet. There appears to be negative long term side effects of staying on a very low carb diet, such as stomach cancer, kidney stones, and scurvy. I now try to follow the advice contained in Paul Jaminet’s book The Perfect Health Diet, and consume around 30% calories carb (150-200g a day) from safe sources, such as sugary fruit, starchy tubers, and white rice.

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