Fasting for Endurance Athletes

I remember spending a lot of time in the library when I was in PT school since we often were presenting projects or writing papers and of course needed to have up to date and vetted research to back up our arguments.

Almost by accident I came across an interesting study. This was 20 years ago, but I still remember being struck by the results of this study. It looked at mice populations and longevity. One group was fed a normal diet, while the experimental group had their calories restricted by feeding just every other day — somewhere around 40% calorie restriction. I recall thinking that the amount of caloric restriction seemed high — I wouldn’t be prepared to cut my calories by that much. But the results of the study were impressive, with the calorie-restricted group living 30% longer! IMG_20151007_182801701

This intrigued me, but a busy school schedule and having to focus on a lot more pertinent (at least as it related to my studies) research caused me to quickly forget and tuck this nugget into the back of my brain.

I didn’t really revisit this research until about a year ago when I began to investigate ways to optimize my help — by sleeping better, recovering faster, having improved digestive health, metabolic health, hormonal balance, etc.

I began to read about fasting as a serious means of improving health in a lot of these areas. But is fasting for endurance athletes even viable? I think it’s not only viable but likely beneficial.

I’ve been experimenting with fasting for a little over a year. My method is pretty simple and I have yet to tackle even the moderately aggressive versions.

Here’s what I do:

I’ll eat a normal dinner one night as early in the evening as I can. I avoid any desserts or snacking after that and then go to sleep as normal. From there I wake up the next day and skip breakfast and just drink some water in the morning. I’ll occasionally use some electrolyte mix, careful to make sure there are no calories, but skip lunch as well.

I aim for 18-24 hours without any calories, and this is surprisingly easy to do, despite what I originally thought. I think that our physiology has likely developed to support going without food for periods much longer than I’m experimenting with. Based on how this has made me feel I think that we might not be meant to eat three or more times a day, every day.

So what have I felt?

Fasting seems to re-set my appetite. I have always been a big eater if I’m not careful. I’ve always been exceptionally active so I’ve never gained much weight to speak of, but I can definitely eat more than I need to if I’m not careful. After a fast, I get fuller sooner and generally eat less for anywhere from 3-5 days.

Fasting also makes my stomach just feel better. I have had some minor irritations in my stomach lining (ulcers) and so I struggle with my stomach feeling good all the time. I have improved greatly with this over the last number of years through changes in diet, but fasting seems to normalize my GI system and even after a few days of eating too-rich food, a short fast makes any discomfort or irregularities go away.

Is it hard to do?

If I eat dinner the night before at around 6 pm I find the most difficult time to get through is about 10:30 am. That’s when my body usually starts to say “Hey, aren’t we supposed to eat something.” Once I get past that point though it becomes progressively easier through the day. By 1 or 2 pm I’ve forgotten all about it (and interestingly I tend to be more efficient at work) and by the time I get off work in the late afternoon I could almost forget to eat dinner.

I get asked whether I exercise on my fast day, and I do. I will often go for an easy run and I don’t have any ill effects from it. Actually I think it helps me settle into the fast and get past my mid-morning hunger sensations.

Tomorrow I’ll break down some of the research about intermittent fasting — what types of fasting are effective and what are some of the benefits can we realize? I think after going through all the research we’ll be able to understand some ways that fasting for endurance athletes can become a staple in your training plan.

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John Weirath

a physical therapist since 1997 and a (very slow) endurance sport competitor for the last 25 years -- full ironmans, solo 24-hour races, ultras -- the author relies on body hacks and research-based techniques to get more out of his training.

He lives with his wife and two kids and runs multiple businesses in Grand Junction, Colorado.