Running Fasted

Last week I mentioned that more often than not, I run fasted. I have been training and racing on an empty stomach for the past two years. I think 2013 was the last time I tried to eat before a race and it just didn’t end so well.

I want to preface this post by saying I’m not a nutritionist, doctor, trainer, professional (in any sense), so please take this all with a grain of salt. If this is something you are truly interested in trying, I highly suggest you consult with someone with a lot more knowledge than myself. But keep in mind, what works for one person doesn’t work for everyone.

Although I will mainly talk about my personal experience, I did want to talk about the science behind “fasted running”. The idea behind running on an empty stomach, or going on “fasting runs” is to train your body to utilize available fuel more efficiently. This is especially beneficial for endurance athletes who want to avoid “hitting the wall” or “bonking”.

Why Run Fasted?

Our bodies rely primarily on carbohydrates to produce energy. Carbohydrates (or our bodies energy) is stored as glycogen which is broken down during a workout or a race to be used as fuel. While this all sounds good – our bodies can only store so much glycogen, so once those glycogen stores are diminished, our bodies turn to breaking down fat, which is a WAY less efficient fuel source. This is when we “hit the wall”.

By fasting before a run, we start teaching our body to burn fat more efficiently. Because we have a lot more available fuel in our body in the form of fat (just not an efficient process), training our body to use that fuel helps with endurance and aerobic performance, and avoiding “the wall”.

How Should I Start?

Start after an overnight fast. This was easy for me because I’m a morning runner. Once you get up and get out the door, your body doesn’t have time to register that it’s hungry for fuel. Because glycogen stores are already low, our bodies are forced to burn fat for fuel. So after time, our bodies become more efficient at utilizing fat for fuel.

You should also start by incorporating this into your easy/recovery runs. Do not start by racing fasted or trying a track workout fasted. For me personally, I have always kept my speed work for the afternoons, after I do have fuel in my body. I find I am more able to hit certain paces and maintain a higher intensity for longer.

So What’s Bad About Training Fasted?

Training in a fasted state usually means you won’t be able to run as fast in training. To break down fat you require more oxygen and it takes longer to break down so therefore you usually end up running slower. If you notice, a lot of my mid-week, mid-distance runs are about 60-90 seconds slower a mile than my race pace. Running hard while fasted is especially taxing, and I really only reserve that type of stress on my body for race day.

And For Me…

I started running fasted for two reasons. 1. I’m a morning runner but didn’t want to get up any early to eat before hand and 2. I have a stomach that does not handle food well before running (especially hard or for long periods of time).

When I first started training for my first marathon in 2011, I would munch on some saltines and peanut butter before heading out for any run lasting more than an hour. What I found was my body felt sluggish, I would cramp up, and I just felt “heavy”. I started to research what are the best things to eat before a run and I stumbled upon the concept of fasted running. I decided to give it a try, so the next morning I woke up, didn’t eat, and went out for a 7 mile run. I found myself feeling lighter, I didn’t cramp, and shockingly, I felt like I had more energy. From then on, I never ate before any training run. I continued to eat before half and full marathons, as I had only been following a “fasting running” training plan for a few months / I still had no idea what I was doing.

From my first marathon in 2011 to my third in 2013, I ate saltines before the race start. Although I didn’t get the same “heavy” feeling as I did with other foods, I still didn’t feel as great as I did on my training runs. But I was too nervous to not eat before such a long distance.

The first marathon I raced fasted was in 2014. I have had a lot of personal success with training and running fasted. It is something that has taken me YEARS to understand and “master”. I have that in quotations because I am still playing with what works best fueling wise. Because I don’t eat before I run, I have to focus a lot on hydration and my nutrition leading up to a race. I tend to drink a ton more water before I head out for a particularly long training run and continue to do so while I’m running. Nutrition wise, I make sure in the days leading up to the race, I’m fueling my body with lots of complex carbohydrates, a good amount of protein, and a good amount of “good fat” – think nut butters, avocado, etc.

Some people have asked if I ever get hungry when running fasted – particularly for my 20+ mile training runs. I am usually up around 6, out on the road by 630, and done by 915. That’s a lot of calories burned and no fuel for a long time. When I initially started training fasted, I would feel hungry while running, it was because I didn’t fuel properly in the days leading up to a long fasted run. Now, I can head out on the road for 3 hours and not once feel hungry. It’s amazing what our bodies are capable of doing.

While this method of training works for me and has only benefited me in my training, it is not for everyone. I know some people who HAVE to eat before they run, and that is okay! It’s so important to listen to your own body and know what it needs to get through a 20 mile run.

I wanted to do a high-level overview as to not get too complex on the science of it all. Although, I do love me some science. If you guys have any other questions, or want me to go into more detail about something specific (science or personal wise!) please let me know!

I also think these articles are great resources for learning more about fasted running:

Should You Try Fasting Before Running

Train Your Body to Fuel Efficiently with Fasting Runs

Training in a Fasted State

The Science of Bonking and Glycogen Depletion


Was this helpful? 

Should I do more informational posts like this?

Anyone have any additional questions for me!?

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45 thoughts on “Running Fasted

  1. This is super interesting! I often go without breakfast when I run to work because I’d much rather have the extra 30 minutes in bed!! But I usually do my long runs after breakfast. That said, I ran a fasted 14 miles a few weeks ago and surprisingly felt absolutely fine! I would definitely be interested in more informational posts from you – you seem to have a lot of experience to share! 🙂

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  2. This is definitely intriguing. I find I can do certain exercises in a fasted state (yoga, weight lifting), but if I try to do something like bootcamp or cycling I’m dyingI guess that is the “training your body” aspect. How does recovery eating look after a long run? Mostly carbs or fat?
    I wonder if this works well with people who have very low body fat percentages, as many runners do.

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  3. Really helpful to read about someone else’s experience! I run fasted too, but take in some type of food on runs longer than 15 miles. Running NYC last year threw me for a loop because of the late start time. Normally I run first thing like you (around 6 a.m.) but not starting until 10:30 for the race I had to eat something. Not sure if that influenced my race or how I felt but I had to break the cardinal rule of “not doing anything new on race day.”

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  4. Thanks for sharing Jamie! I used to run fasted because I like you didn’t want to wake up AND have GI and stomach issues. Then I bonked during my first half and started trying to eat. But I also feel “heavy” when I do, and tend to need the porta-potties, which I like to avoid, so I may start returning to running fasted. I already do during the week (not getting up before 5:30/6 just to eat) but those are 6 miles or less, so it’s more the longer runs and racing.

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  5. Thank you for posting this!!! I guess I sort of unintentionally run fasted on the weekends because I just wake up and head out. Everything I’ve read has been about proper eating/fueling before you head out for a run so it was nice to see you mention this the other week! Do you fuel during your races with chews or GU or anything?

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    1. For a marathon I’ll usually have half a Gu around 16 and then just water and a sip of Gatorade at the aid stations. Very very minimal intake but I think at this point my body is just used to it haha

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  6. That’s really amazing! It just shows how different everyone’s bodies are! If I don’t eat within 30 minutes of getting up I feel sick and nauseous… so I know I need food… but clearly it is working amazing for you!
    Great post! I’d love to read more like this!

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  7. You explained it all really well. I learned about fasted running when I was training for my ultra and I believe that doing those hungry runs was the thing that really got me prepped for my 50 miler. I do have to eat on race day, but I practice fasted runs leading up to it, for sure. They’re magic.

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  8. I do 95% of my runs fasted just because I feel better that way. I never really looked into the science behind it but it does make a lot of sense. I typically do eat something small before I race though because I feel like that little extra energy boost will help. Reading this was super helpful though so I know I’m actually doing it for a reason!

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  9. I can relate to a lot of this – I generally run fasted unless I’m running 14 or more miles, and even then I know I can run fasted if i choose. I mainly know I’m going to need to fuel for the marathon (or want to, I suppose) so “practice” running with fuel so my stomach doesn’t turn on me. I also started running fasted for the same reasons – early morning running and a sensitive stomach. I know what you mean about feeling lighter!

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  10. Great post! I also run fasted in the morning, and I can’t hold anything down. I start to feel really sick after about 5 miles though, so for races I usually just have some fruit and that does the trick. I wish I was one of those people who could down bagels and oatmeal, but I would be pulling over every mile. Yuck. Please do more informative posts like this!

    Liked by 1 person

  11. I never knew there was a term for not eating before a run (or workout).

    I never eat before a morning workout. This started with spin and was because of stomach issues/pains if I did eat. Since that worked for me, I stuck with it for running.

    I actually find afternoon runs really hard because all day I have to be mindful of what I eat.

    All that said, I’m torn as to what to do for the upcoming half since it starts at 9 and I usually prefer to start my runs as early as possible. I’m thinking of having something very small early on, so my body has time to process it.

    I saw someone else ask, but do you ingest any “fuel” during your runs?

    Liked by 1 person

  12. Great post, Jamie! I really think you should do more like this – you are so knowledgeable about the subject and very clear in your explanation. Your description of how you felt before fast runs – sluggish, cramping, and feeling heavy – is exactly how I felt during the middle miles of my marathon. When I ditched the GU and started just taking water and electrolyte drinks, I felt much better. I’ve read a lot of sports nutritionists like Matt Fitzgerald talk about the benefits of training low and racing high in terms of fuel. I really only eat before my runs (just a banana usually) because I’m up at 5:30 but not running until about 7-7:30 – but I’m interested in learning more about it for racing!

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  13. I definitely do not want to get out of bed earlier than I already do to eat! Roll out of bed and run is about all you are getting from me when it is dark out. I started running fasted before I “knew” any better and just works for me. My stomach can be challenging enough without food…just not worth the anxiety it causes eating prior.

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  14. Very interesting! I do almost all of my shorter runs and speed work fasted. Like you, I just felt sluggish and my stomach would get upset when I ate before runs. I do eat a little bit before really long runs though, but nothing more than a piece of toast with PB. I’m sure I could probably do without it, but I’ve had long runs and races where I felt fine energy wise but my tummy was GROWLING at me. Usually toast is light enough that it doesn’t weigh me down but also just enough to top me off for those 16+ milers. It’s a pain in the rear to take in food (Gu, blocks, etc) while I run and I find eating a little beforehand helps me avoid having to eat as much mid-run.

    Great post!!

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  15. Thanks! This was very helpful and a very interesting, concise read. Question – for Boston, with it starting so late, that seemed to go well for you but I’m curious if you were starving by race start? And in general, does the start time of the race change your plan at all? I always run fasted during the week because I wake up so early and I definitely don’t want to eat at that hour! If I run later on the weekends, I am awake for so long and using up energy I’m afraid to start running long without something. But I am totally with you on the PRE-days fueling. For the week/week and a half leading up to a marathon, I have an extra breakfast when I get to work. I feel like this slow build-up is so much better than eating something huge on race day or the night before. I’ve actually never tried that but I think a lot of runners do that thinking the last minute thing is the way to go which just sounds dangerous to me . Ha. I’ll try the weekend long run thing as an experiment and see how it goes!

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    1. I do a high carb high protein meal after my runs. Usually within 45 minutes. And then I’ll eat a more filling comprehensive meal (veggies/fruits) two-ish hours later

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  16. Thanks for sharing! I did a few of my long runs fasted last year, and now I usually don’t eat before my weekday workouts. I do eat before long runs, and always before races. Usually my stomach cooperates but I would imagine if i had alot of issues it would be much easier not to deal with eating anything. I’m planning to take a look at those links you shared!

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  17. I love how you broke it down Barney style 🙂 If I was a morning runner year round, I’d do this all the time. I only eat before a morning run if it’s longer than 17 miles. BUT I tend to not run in the mornings 5 days a week, so obviously I’ve eaten during the day before my early evening runs.
    The idea of racing without fuel worries me though. You mentioned it above, but I feel sluggish and just not quite into it when I have a tempo or otherwise speedy run on deck in the morning. I can’t imagine racing like that – but maybe it would be different on race morning when I have the adrenaline pumping through me. Definitely something to consider!

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  18. I’ve heard of this and the benefits but it is just not for me. I’ve tried a couple of fasted runs and by the end I am just so tired and over it. I’m sure my body would eventually get used to it, but I am just always so hungry! I’m lucky in that I can eat and then go running pretty much right afterwards.

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    1. I will most likely run it fasted, and then take a Gu slightly earlier (around mile 13)… I ran Boston fasted and my start time was 10:55. It seemed to work okay for me. I just make sure I eat later the night before!

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  19. really interesting — thx for posting this. i hate to eat before i run and am also a morning runner by preference, so the couple days a week i end up running after work are the worst for me — i hate the feeling of having had food in me all day. i feel sooooo much better lighter/ on an empty stomach.

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  20. So interesting to me! The same is true if you fast for your regular diet. You “feel” hungry to start but your body adjusts. And it seems like you store fat instead of burning fat if you aren’t eating the right things (at least for me). Thanks for posting! 🙂

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  21. Hi Jamie! I definitely think the key here is that you found that it works for your body. That’s the best thing about training – you have to figure out what works for you. Personally, I don’t run much, but I do NEED fuel before a morning workout. Nice work on being such a dedicated runner, girl! 🙂

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  22. I am definitely intrigued by the theory but I think I’m a bit chicken to try it out! I may have to read up on it a bit more! Thanks for the info!

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  23. I am all about running fasted, up until I hit higher mileage. For most days of training, I work out right when I wake up fasted, but on my long runs that are 10+ miles I usually eat. I have to give myself more time in the morning to eat and then let it settle before running, which I think is a huge reason that I run fasted every other day of the week. There is no way I am waking up any earlier than I have to just to eat! haha. Plus, isn’t food SO much better after having your morning workout done?!

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  24. I do a lot of my shorter runs fasted, but need a little something in me if I am going to up the mileage. This usually involves a scoop of peanut butter or banana. I always drink a cup of coffee before going out on a run… for many reasons. 🙂 I think the most important thing is that you find something that works for you. While there is definitely a physical component, I believe a lot of it is mental as well. I feel mentally better and more prepared for my race or long run if I stick to my routine and what I know. It freaks me out if I do something different. Great post, Jamie!

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  25. Very informative, and something I hadn’t done much research on before. Before I work out, I usually make sure I’m not hungry, but not full – the only time I work out without eating is if it’s first thing in the morning. However, this gives me some new information to think about.

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  26. how does this affect your days when do dont workout in the morning? since i’ve started morning workouts, i’m not hungry for a few hours after i wake up, no matter what time!

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