Exercising whilst fasted

IF fastingAt the end of August 2012 I began experimenting with Intermittent Fasting.  Firstly, one day a week, without exercise (for 10 weeks), and now I’ve moved onto 2 days a week with exercise (see more here for the duration of my fasts and the exercise I’m doing on fasted days).  It’s been a big learning curve for me.

  1. Firstly, I’ve realised that I can work just as well without food (fasted in the morning any day of the week) and I don’t ‘need’ to be fuelled up for a workout which is less than an hour.
  2. Secondly, I’ve realised that I’m able to be hungry and it’s seeming to take me longer to get hungry now.  I’m able to get past dinner time, and I now last longer without feeling like I need to eat a horse.
  3. Thirdly, I’ve realised that I don’t REALLY need to eat when the urge takes me.  Before when I’d get hungry, I’d need to grab a mars bar or banana to keep me going.  I dreaded hunger, but now I’m able to calmly wait until I get a feed.

One of the main reasons for my fasting was the benefits which were shown in Horizon’s: Eat, Fast and Live Longer.  The ‘Live Longer’ part, or longevity really appeals to me.  But there are other reasons I’m doing it too.

Here’s one of them:

  • To get my body to adapt to burning fat more efficiently as a fuel source and reduce my reliance on glycogen during a long run.

Glycogen Depleted Exercise

Now I’ve never ‘hit the wall‘ in a marathon or long run (as I’m always careful to take on fuel and water during my long runs), however I did bonk tremendously one day after I tried a two week food test where I removed Carbs from my diet, then tried to run for 3 hours / 15 miles trail running.  I ran long on the last Saturday of the two week test, was ok on the Sunday and had to take a day off work on the Monday and spend it in bed!

Fasting is different from training with depleted carbs like I did in the two week test though.  The idea is that when you do low intensity workouts at the end of your fast, your body metabolises fat better…and when you do heavy weights workout, your fat metabolism increases too – making it easier to shift stubborn body fat.  (see more here).

The theory behind exercising on low glycogen stores (ie fasted) is that by not having readily available muscle glycogen to burn, you body is forced burn fat. Consequently, your body will become more efficient at using fat as a fuel source.  The real question is, does this theory hold true?


recent study conducted in New Zealand showed that cyclists who completed exercise early in the morning without eating breakfast (fasted state) improved muscle glycogen stores by as much as 50% over the group that ate breakfast before their exercise. Similar studies have made it clear that occasional fasting before exercise can improve glycogen storage and endurance performance.

However, other studies have gone further and tested the effects of training with low glycogen levels for more than one run or for extended periods of time. The research concludes that extended carbohydrate depletion impairs performance and does not enhance fat utilisation.

The research makes a strong case that occasional long runs in a fasted state will improve glycogen storage and fat utilisation, but extended training or multiple long runs in the fasted state will impair performance and does not provide further benefits to fat utilisation.

wallAll that being said, and given my experience ‘bonking’ after not having enough carbs to draw on, I would always make sure I had a big breakfast before a long run or race, although if I’m training for less than an hour, I know I’m ok to train fasted as long as I take on plenty of water and eat (preferably a high protein meal) right after it to help recovery and refuelling.

I won’t be able to prove scientifically if this will happen, but I will be able to try it out and see if it works for me.  🙂  I’ve read a Pyllon’s blog here previously and wondered why you would want to do long runs fasted…. I guess I still do wonder, but I think I’ll do it differently by training for short periods after a fast, and continue to fuel my runs.

Long Fuelled Runs

It’s well documented that actually ingesting carbohydrates improves performance in long (2+ hour) competitions, simply rinsing your mouth with a carbohydrate drink, then spitting it out, seems to help too.

long distance running

Obviously, no significant amount of sugar could make its way into your bloodstream through rinsing your mouth; the performance boost must be coming from the brain sensing that more carbohydrates are on the way. So it’s reasonable to propose that performance will be inhibited significantly before your body actually runs out of glycogen.

Based on my experience, and the simple fact that I’m burning around 100 calories a mile long runs, I will be taking on fuel for any runs which are up to or over an hour and a half.  Your body needs fuel to run long, but I can do fasted training to hopefully help improve glycogen and fat utilisation.

The Pam Reed Phenomenon

You may have heard of the American Ultrarunner Pam Reed, and her book, The Extra Mile.  It’s ‘The story of the woman who shocked the running world in 2002 when she won the sport’s most grueling race the Badwater Ultramarthon —135 miles from Death Valley to Mount Whitney—beating her closest competitor by 5 hours!’  

ExtramileI read it a while back, but one thing really stuck out.  During her early years, Pam suffered badly from anorexia (which might I add I am definitely not condoning!) Throughout the book, it became clear that putting her body and mind through not eating during her younger years, made her more able to deal with exercising for long intensities.

Her body was used to being fasted, so when it became depleted, she could still function and continue to run.  Her brain was mentally strong enough to deal with not eating for long periods of time, and I’m sure this had a great impact on her will power and allowed her to do anything she needed to, to get to the end of the Badwater Ultramarathon and other such long endurance events.

I can’t help but think that this is a clear example of where fasting (albeit in an extreme and perhaps to controlled and long state) has worked for an endurance athlete.

My next experiment

FastFuelUseFatontheRunSo what’s my next experiment?  Well, my next experiment is to see if Intermittent Fasting will allow me to become better at fat utilisation / in less need of glycogen when running long / marathons.

Going back to my challenge:

  • To get my body to adapt to burning fat more efficiently as a fuel source and reduce my reliance on glycogen during a long run.

Or in short:   Fast, Fuel, Use Fat on the Run

roadrunnerbannerMy plan in January is to train for a marathon up to April and continue my Intermittent Fasting.  The idea is that if I train fasted, then hopefully when I come to do my long runs, I’ll be able to last longer, with less reliance on glycogen and more reliance on fat for to fuel my runs.

After a fast I’m pretty hungry and wouldn’t like to do a long run on in that state, so I think  I’ll probably change my fasting days to a Tuesday and a Thursday, so that on a Friday I can will be able to fuel up before a long run. I definitely don’t want to bonk again!

I’ll make sure I eat a good breakfast before my long runs, and experiment with the types and amount of fuel I take on when I run long.  And I’ll see how it all goes. 🙂 Roll on 2013!

Disclaimer: If you are a total beginner marathon runner, I maybe wouldn’t try this at home!  Running a marathon is hard enough on your body, without having to get it to try to work with limited energy.  So please don’t try this if you are not an experienced marathon runner!  Fuelling your body enough is very important when you run long!  For more tips on Running Long, go here.

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2 Responses to Exercising whilst fasted

  1. Pingback: So this is Christmas, and what have we done? | Lorn Pearson Trains…

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