Quick Fast: My Strategy for Fasting as an Endurance Athlete

June 2020 ยท 8 minute read


I recently ate a ketogenic diet for 4 days, then I fasted through the 3 nights and 2 days that followed. When I resumed eating, 60 hours later, I returned to my “normal” diet. The idea for this weeklong experiment was to push my body into a “therapeutic zone” of ketosis in the fastest way possible, with the least amount of interruption to my day-to-day life. I was successful. My glucose-ketone index (GKI) dropped to ~1 within 24 hours of starting to fast. I stayed in this zone for over 36 hours before breaking the fast.

Rationale and Metrics

I fast for long-term health benefits. I have a family history that includes neurodegenerative disease and several types of cancers, and I would like to ward off the onset of these diseases for as long as possible. Fasting is one way to try to do this - I won’t get into the scientific reasoning, but check out resources that I’ve linked later in this post if you’d like to learn more.

As an ultraendurance athlete, I am aware of the hype around a “fat-adapted” approach to diet and exercise. I do not fast in order to be a fat-adapted athlete, but I consider any resulting increase in my own fat-burning capabilities to be a bonus of periodic extended fasting. During this latest fast, I tracked my heart rate variability and sleep in addition to blood glucose and beta-hydroxybutyrate levels. My heart rate variability metrics suggest that an endurance athlete should not try to train while on a ketogenic diet or during an extended fast. My sleep was relatively unaffected.

I want to say up front that all of my research and references in this article relate to the male body. Fasting for female endurance athletes is not something I am familiar with. At this time, it is very hard to find research and case studies that reflect female, rather than male, experiences on the topic.


Many people aren’t aware that most humans can go for weeks without food. Healthy individuals can restrict caloric intake for days or weeks, and in doing so they will induce metabolic changes in their bodies that allow us to burn fat as fuel.

A 27-year-old man fasted under medical supervision for 382 days as an outpatient. You can read about it here

My History with Fasting

I have been experimenting with fasting since 2015, when I did my first long-term fast of 9 days. I have done an extended (> 2 day) water fast every year since then.

I do not train (meaning, I do not run) while I am fasting. I could probably go for very easy runs of a few miles, but I wouldn’t feel great. I do go for extended walks. I’ll do short bursts of weight training (like a few sets of bodyweight exercises, or a dumbbell circuit workout).

I believe that periodic multi-day fasting will help delay onset of neurodegenerative disease and cancer. This is my primary reason for doing an extended fast at least once per year.

Bang-for-the-Buck Approach

The biggest problem with fasting, at least for me, is that it interrupts my usual routine. I can’t train like I normally do. I can’t go out for meals. I don’t particularly enjoy starving myself for a few days, and I don’t enjoy much social interaction while I’m in a deep state of ketosis.

Given that I dislike the disruptions that fasting introduces to my life, I devised a routine to maximize the beneficial effects of deep ketosis while taking minimal time out from my usual life. First, I eat a ketogenic diet for a few days to ramp up my body’s ability to use fat as fuel. Then, I stop eating and I watch myself very quickly approach a “therapeutic” level of glucose-ketone index, which is the level of ketosis thought to be most beneficial for my long-term goals. After a day or two in this therapeutic zone, I return to life as usual. This entire process takes me less than a week.

Expert Resources

I’m not a doctor, and you shouldn’t take anything I say here as a recommendation or guidance. Do your own research and consult with your medical providers on the topic. For internet-based research, I recommended starting with the resources below. They are produced by well-informed medical professionals that use peer-reviewed scientific research as their bases for arguments and knowledge dissemination:

My Protocol

First Step: Nutritional Ketosis

When I first started fasting several years ago, I quit food cold-turkey. My first few days, as a result, were miserable.

I have since come to realize that the fasting experience is much more pleasant if I accelerate my body’s production of ketones before I stop eating food. This means, practically, that I eat a very low-carb diet for the days leading up to the fast.

Macros to Enter Nutritional Ketosis

During my lead-in period to this fast, I tracked my macros that allowed me to enter nutritional ketosis. I took a lot of recipes from the Primal Kitchen Cookbook, but basically my diet involved absolute minimal carbs. No fruit, no grains, no starchy veggies, and nothing out of a box.

I took my measurements in this phase with the Keto Mojo meter. It was my first time using this meter (I previously used the Abbot Precision Xtra). I liked it.

Nutritional Ketosis Macros, BHB, and Glucose Measurements

Saturday Sunday Monday Tuesday
Total kcal 1977 1729 2133 2226
BHB (mmol/L) N/A N/A 0.7 0.8
Glucose (mg/dL) N/A N/A 86 87
Carbs (g) 23 35 24 30
Carbs (% kcal) 6% 9% 5% 5%
Fat (g) 119 117 144 150
Fat (% kcal) 68% 64% 65% 65%
Protein (g) 104 110 151 154
Protein (% kcal) 26% 27% 30% 30%

As you can see, I was not doing any kind of caloric restriction during this phase. I was, however, severely limiting my carb intake. My BHB and Glucose levels were taken first thing in the morning. Historically, it takes me until day 3 or 4 to see a bump in my BHB concentration, which is why I didn’t measure until day 3 of the diet. You can see that I had an appreciable amount of BHB in my blood on day 3. This means that I successfully completed my first step of entering nutritional ketosis.

Second Step: Starvation

It becomes much easier to abstain from calories once the body is already using fat as fuel. I was still supplementing electrolytes throughout this period of fasting.

During this phase, I took measurements with two meters: the Abbot Precision Xtra and the Keto Mojo. I was curious what the difference in results would be, and they provided quite different results. I’m not sure what to make of that.

Fasting BHB and Glucose Measurements - Precision Xtra

Wed AM Wed PM Thu AM Thu PM Fri AM
Glucose (mg/dL) 70 66 62 56 61
BHB (mmol/L) 0.6 4.6 3.9 4.2 4.1
GKI 6.5 0.8 0.9 0.7 0.8

Fasting BHB and Glucose Measurements - Keto Mojo

Wed AM Wed PM Thu AM Thu PM Fri AM
Glucose (mg/dL) 79 77 71 76 70
BHB (mmol/L) 1.1 3.8 2.9 3.8 4.3
GKI 4.0 1.1 1.4 1.1 0.9

According to both meters, I reached a near-1 GKI within 24hrs of starting my fast. Goal achieved! I maintained this state for the following night, day, and another night. I broke my fast on Friday morning and I ate my normal diet on Friday. By Friday evening, both meters clocked my GKI at over 20… so I was definitely out of ketosis after eating carbs again on Friday.

Sleep and HRV Measurements

I tracked my heart rate variability (HRV) using the Elite HRV app and the Polar H10 bluetooth heart rate monitor. I tracked sleep with my Coros Apex watch and Coros app.

For reference, during the week prior to my ketogenic diet and fasting, my HRV score was an average of 73, with a readiness score of 9 (green light to train hard). My average resting heart rate (RHR) was 45. My sympathetic vs. parasympathetic components of the score were balanced. My average sleep time total was 7 hrs 2 minutes, with deep sleep of 1 hr 13 minutes.

Sun Mon Tues Wed Thurs Fri
HRV 72 72 68 69 66 65
Readiness 9 10 3 6 4 4
RHR 45 44 45 43 46 46
Sleep 6:58 7:12 7:06 7:19 7:47 6:57
Deep Sleep 1:24 1:27 0:38 1:29 1:40 2:09

Curiously, my Readiness Score plummeted when my body entered nutritional ketosis. There was a much larger relative sympathetic component to my score for this period that remained throughout the fast. My HRV also dropped to the mid-high 60s. Based on this feedback, it seems that HRV-based indicators say that it would be unwise to try to do any endurance training while in ketosis. My resting heart rate remained unchanged, as did my sleep patterns (which are somewhat inconsistent anyway). If anything, it seems that I may have achieved a bit more deep sleep while in nutritional ketosis.

After I broke the fast, my HRV returned back to 72 and my Readiness score was back up to 8. My resting heart rate remained unchanged. I got a lot more deep sleep (2:10) on the night after re-feeding.


I achieved precisely what I had set out to do. I wanted to be in a state of deep ketosis (GKI near 1) for at least 24 hrs, and I was able to achieve this state for over 36 hours. It had as minimal impact as possible on my normal schedule and routine. I will continue to employ this strategy of nutritional ketosis prior to water fasts for as long as I continue my habit of periodic fasting.

Thanks for reading!