Fasting and Running Performance

A week ago, my friend Jason Robillard posted some interesting anecdotes about his training for the Western States 100.  As part of his preparation for the race, he is maintaining his normal training schedule while fasting (going without food) for 12-48 hours at a stretch.  His plan is to mentally and physically train his body to run even in the absence of victuals.

O yes.  It sounds dumb.  But he’s actually a very intelligent, thoughtful person…

He’s just a little crazy is all.  You can see his post HERE.

This is not a commentary on Gruisinger’s theory of anorexia and evolution.  Maybe later….

Here is an excerpt from Jason’s post:

This happened on a training run this last week.  I had fasted for about 24 hours, eaten about 500 calories (beer, Doritos, and jalapeno peppers) [good thinkin Jason...], slept, woke up, ran a three mile tempo run and crosstraining routine in the morning, then did a 7+ mile hard trail run later that night.  By the time I ran the evening run I had consumed 500 calories in 48 hours.  Based on previous experiences, the trail run should have been exceedingly difficult.

But it wasn’t.

It was quite easy.  I managed to maintain about an 8-8:30 pace for the majority of the run.  At about mile 5.5 I slowed down due to some mild fatigue and a sore foot.  Since the Mind the Ducks 12 hour is coming up, I didn’t want to risk injury.  Based on the effort, I should have hit a serious wall very early in the run.  Instead, I felt great.  My energy level was akin to slamming a few Redbulls 10 minutes before the run.

This isn’t the first time I’ve experienced this phenomenon.  I’ve tried to predict when it happens, but that proved to be difficult.  There does seem to be some pattern in regards to pace- the higher the pace the less likely I experience the hard crash… at least for runs under 10 miles.  I’ve never attempted a hard, long run when fasting.

Drum Roll Please….


Allow me to explain.  In the absence of carbohydrate consumption, there is no new glucose being added to the blood.  Every cell in our body needs glucose + oxygen to function.  Take one of those ingredients away and, POOF!  Dead.  Gone….

Since glucose is worth one half of the equation that gives us life, glucose metabolism is very well controlled physiologically.  Hormones like insulin, glucagon, cortisol, growth hormone, and epinephrine all have critically important metabolic effects.

In times of plenty, the hormone insulin causes the body to store excess glucose in the form of triglycerides (1/2 a glucose molecule attached to long hydrocarbon chains called fatty acids) and glycogen (huge macromolecules formed by thousands of glucose molecules all bound together).  Triglycerides are stored in fat cells.  And Glycogen is stored primarily in the liver and in muscle cells.  There they stay until there is not enough new glucose entering the blood from digested foods….

Or until you go on the Atkins Diet….

When food is not plentiful, carbohydrate intake is reduced, and during fasting, insulin levels decline and other hormones dominate the metabolic scene.  Glucagon, Growth Hormone, Cortisol, and Epinephrine are the primary players here.  They break down triglycerides and glycogen, releasing the stored glucose into the blood.  So, when there is no new glucose from consumed food, the cells release stored glucose to sustain the body during times of fasting or carbohydrate restriction.

OK.  Now to answer Jason’s question…

After a short term fast, before glycogen stores have been used up to sustain the basic functions of life, the effect of fasting on exercise performance is to enhance the breakdown of glycogen and triglycerides. In a short-term fast, stored glucose is released more efficiently during exercise – thus the feeling of more energy in a shorter run.

But this also means that the storage tanks will be used up faster.  There may be more energy in the short term.  But try this on a longer run, and you’ll hit the wall sooner.  Endurance exercise can be sustained for longer periods when food is consumed regularly.

Any questions?

Run With Brains,

Scott Hadley PhD, DPT

9 Responses to “Fasting and Running Performance”

  1. Jason Robillard said:

    May 12, 11 at 7:33 am

    Excellent explanation, Scott! Now question #2: Is there a positive training effect by hitting the wall earlier in the run? To put another way- can training to the point of hitting “the wall” lessen the effect of experiencing the wall in a race?

    In my experience, this type of training makes the low of that point of glycogen depletion less severe. During a race, I consume food like a madmad. Still, there’s a net caloric loss. At some point, i still hit a serious low, usually around mile 60-65. With fasting training, that low doesn’t seem as bad.

    Is there a physiological explanation for this? Could it simply be a matter of learning what to expect and developing better coping mechanisms? Or is it just a placebo effect?

  2. The Maple Grove Barefoot Guy said:

    May 12, 11 at 11:06 am

    Great post Scott. The only thing I disagree with is your last sentence. Regular food consumption is not required to maintain athletic performance for long periods of time. You won’t bonk earlier. Your body will just dip into fat stores earlier. And if you train in a fasted state, it will do so with greater efficiency.

    I regularly run multiple hours without any food before or during running, and I actually find that my body feels stronger the longer I go. One pound of fat = 4000 calories. You have enough energy inside your body already in the form of fat to run several marathons. I’m running a marathon on an empty stomach with no food during the race in a few weeks to prove it. Although I’ve already read several accounts of folks doing it before me. The real interesting thing will be when I do the same thing for a 50K and 50 miler.

  3. TrekoScott said:

    May 12, 11 at 12:11 pm

    Thanks MGBG. Let me know how the marathon goes!

    Most of the scientific research on this topic has focussed on shorter distance runs with the standard training approaches in mind. So there is a lack of real evidence on longer distances.

    I agree that fat stores are generally sufficient for longer runs. But the metabolic reactions that convert triglycerides to glucose (via gluconeogenesis) take a bit longer than glycogen metabolic pathways. There are no studies that I know of comparing the ability to simply “keep running” without food versus “running well/fast” without food over longer distances.

  4. TrekoScott said:

    May 12, 11 at 1:51 pm

    Thanks, Jason. I’ll have to do some literature research on this and the question posted by MGBG. The problem is that most of the research done in this area is on shorter distances and moderate/low intensity running….

    Could I compel you to take blood draws every hour or so during the WS100? A backpack filled with ice and blood samples shouldn’t slow you down much!

  5. The Maple Grove Barefoot Guy said:

    May 12, 11 at 3:36 pm

    Thanks for the reply Scott. You can call me Christian, it’s easier. In the bodybuilding circles I used to run in, we did a lot of intermittent fasting because of studies showing that it increased anabolic response to resistence exercise. There have also been studies done on fasted endurance training. Here’s an article explaining some studies that show that fasted training increases endurance and muscle glycogen:

    Who knows whether there is a fall-off point for the benefits of fasted training. I guess I’ll have to find out for myself!

  6. TrekoScott said:

    May 12, 11 at 3:47 pm

    Hey Cristian – thanks for your name! Yes, the benefits of anabolic reactions in resistance training have been well documented.

    And thanks for the link too. I have seen many of those references – but none of them really addresses the issue of “super endurance” training. Perhaps we need to do a study of our own!

    The problem is getting a large enough sample and establishing proper controls. That’s hard – even when researching short-distance endurance exercise. Ultras take research into the realm of “pseudoscience” because of the inability to control confounding factors. But case reports are always fun too!!

  7. The Maple Grove Barefoot Guy said:

    May 12, 11 at 4:40 pm

    I accept your offer to be a case study Scott ;)

    I agree that the effects of fasted super-endurance training are completely anecdotal at this point. What I know is only through random posts on the various forums I haunt. I think it does have some practical application though. I hear that the Kenyans train and race in a fasted state. Someone needs to beat those guys!

    I just saw a post from a gal who beat her PR in the NY Marathon while on day 72 of a juice fast. Compared to that girl, I think Jason and I are tame in our craziness! The body is capable of some amazing things.

    Good talk! Thanks for the replies.

  8. TrekoScott said:

    May 12, 11 at 4:51 pm

    Now juice fasting is a different beast altogether! I totally believe that!

    And I don’t want to discount anecdotal evidence either – it’s just this PhD thing that makes me hesitant to state something without having the hard evidence ;-)

    See you later, Christian.


  9. Chad said:

    May 17, 11 at 6:53 am

    Nice post. iFast.

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