The Ultimate Guide To Combining Fasting and Exercise: Everything You Need To Know.

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Click here for the full written transcript of this podcast episode.

Barry Murray is a guy I have had my eye on for awhile.

Every once in a while he mysteriously pops up on a forum or Facebook thread I'm following, and talks about just having killed some ultra-race out in the middle of nowhere, or completed an epic mountain run…

…on nothing but a bit of butter or coconut oil the day before.

He's doing some crazy workouts in a fasted state, and yet managing to pull off amazing feats of physical performance.

And furthermore, he's not a diet hack – the dude actually knows his stuff – with a degree in Chemistry (BSc) and a Masters (MSc) in Sports Nutrition. He works with professional cyclists, triathletes, distance runners and last year worked as Sports Nutritionist to the BMC Professional Cycling Team. Yes, that BMC Professional Cycling Team.

Like I mentioned, Barry is also a competitive endurance athlete himself and races in ultramarathon events. He represented his country at the 2011 World Ultra Trail Championships, and has won several ultra trail marathons, placed 2nd at one of the UK’s toughest 100mile ultras, and has also set national records for trail routes over the Dublin mountains.

So how does Barry (pictured right) do it?

BM Ultra

You find out in today's audio interview, in which Barry and I talk about:

-How Barry got into fasted workouts…

-An overview of the mechanism, function and exactly what happens inside your body when you combine fasting and exercise…

-The steps to becoming fat-adapted, and how long it takes…

-What types of workouts, athletes and sports fasting and exercise is good for, not not good for…

-Whether elite or world class athletes combine fasting and exercising or racing…

-What happens when fasting and exercise go wrong…

-How today's Primal and Paleo recommendations can mislead people…

-How to get started with fasted exercise, where it can take you, and how far you can go…

Additional Resources:

How To Turn Yourself Into A Fat Burning Machine By Fasting For 24 Hours Then Going Out And Do Monster Workouts Without Bonking. 

A Deep Dive Into Ketosis: How Navy Seals, Extreme Athletes & Busy Executives Can Enhance Physical and Mental Performance With The Secret Weapon of Ketone Fuel.

The Great Ketogenic Ironman Experiment – Can You Go Low-Carb And Be A Fast Endurance Athlete Without Destroying Your Body?

Jack Kruse Tells You How To Live Like A Polar Bear And Eat Like A Great White Shark.

-Barry's website:

Do you have questions about combining fasting and exercise? Comments or feedback from the interview? Leave your thoughts below!

Ask Ben a Podcast Question

34 thoughts on “The Ultimate Guide To Combining Fasting and Exercise: Everything You Need To Know.

  1. Hunter says:

    Hi Ben and Barry, loved the show! Super informative! Ben, you got me into carb back loading and trying to become fat adapted for about 6 months now and I’ve been trying to adhere to your nightly carb refeeds with pretty good success. I’m a CrossFit athlete, 6’2” 175 and I’m around 7-9% body fat. I’ve also gotten into running and did my first 1/2 marathon in February and really enjoyed it. I workout first thing in the morning though, fasted with 90-120 minute sessions consisting of weightlifting, metcon and running and have found lately that I’m starving and really crave carbs and it’s getting harder to push that off until the evening. So Barry what you said about carb refeeds immediately after my workouts excites me! Then switching to low carb the rest of the day. My question is, what carbs would you advise post workout for my volume and intensity? Should I simply consume a simple sugar like chocolate almond milk or coconut water with some protein powder? Or opt for the low GI carbs like oats, sweet potatoes, and Ezekiel bread? Also, should I stay clear of fats for that post WO breakfast or incorporate them in with eggs or some liverwurst? Coconut oil on the sweet potatoes or a nut butter in the oats? Thanks so much!

  2. Donna Cox says:

    Ben, I’m enjoying listening to your podcasts since I saw you with Vishen on Mindvalley.
    I’m a 49 year old female that really wants to be fit and healthy, but I’m a little burned out on hard workouts. I got injured doing Crossfit a few years ago and now I mostly walk, run and do some push ups. I’ve gained 20 pounds and would love to get it back off. Can you give me any advice that is different then what you and your awesome athletes are talking about? Will the 16:8 fasting work for someone like me that isn’t really killing it in the gym?

  3. Sam says:

    Hi Ben,

    I have found your podcasts incredibly helpful and informative. I’ve been intermittently fasting (16:8) daily for the last two weeks and have been trying to find as much research as I can.There seems to be research around fat loss, and retaining muscle mass (Increased growth hormone), however I was wondering if you know of any research in regards to intermittent fasting for high performance CrossFit athletes training upto 6 days a week? Specifically around building muscle and having the fuel required for intense workouts.



    1. Yes, it works fine assuming cyclic carbohydrate refeeds are built in.

  4. wirro says:

    what must eat during the race for triatheles, Swim leg , Bike leg , Running

  5. Mark Ratliff says:

    Stupid question I know; how do I get to your show notes:? I have tried the following url with no success. I wanted to get the show notes from your podcast with Dr. Jason Fung.

    What am I doing wrong? Thank you

    1. These are the show notes are right here, where you are commenting..

  6. Angela says:


    I absolutely love your podcasts! I make excuses to go places because I listen to you in my car with no distractions (except Atlanta traffic). Most of the time you are the highlight of my day (do not tell my husband). I want to do a 10 day water fast (to get rid of toxins, lots of mood fluctuations and just to see how my body responds) and wanted to know if you have a PROTOCOL (i know drink water:-)) on how much water, exercise if any, supplements… My husband is going out of town so this will be a perfect time for ME. I am 49 (ouch 50 is here), a former Ironman, do p90x 1 & 3 consistently, do HIT bike workouts on my trainer, run weekly, I eat 90% organic, eat very little red meat (grass fed only) and wild seafood. Any advice would be greatly appreciated. You always inspire me to reach for the stars and be better! Thank you!!!

    1. Angela this would be a great question for us to cover on the podcast –

  7. Jenahale says:

    Makes it feel like it is ripping it apart and pistachios gassy..

    1. Wow. Yeah, eliminate those for sure. You've got some sort of issue/intolerance going on there.

  8. Jenahale says:

    For the last year I have been teaching about 16-18 group ex classes a week plus training sessions. Granted 7-8 are HIIT classes and the rest are water Areobics. I do my own personal workouts long runs on Saturdays at most 12 miles building up to more. I rest on Sundays from exercise. I have four kids 5,7,8 and 16 so rest is still active. I go to bed most night by 9. However 4 out of 5 days I teach 4 classes in a row 5:45am till 11am… The last 3-4 months I have been waking up having coffee( with whole milk) some prayer time and I head to class. I have been wondering about how to eat on these days..I have been playing with transitioning to a Paleo diet for about a year. I have done a sugar detox several times and felt much better! A lot of these days I do not eat until 11.. When I try to eat in between classes my stomach has trouble digesting… I have also been concerned about not having enough fuel even though I "feel" ok. This podcast was helpful. I many times eat just because I know I burned a bit during the day and want to refuel for the next, but am often not very hungry… I can emotionally eat. Listening to this helps me to see that I may not need to eat to refuel. I eat many good fats, coconut oil, avocados and nuts. Nuts have been starting to bother my stomach though! Any thoughts???

    1. What types of nuts? How do they bother your stomach?

      1. Jenahale says:

        Almonds, pistachios and macadamia… Going from one class to the next… I do run the longer runs fasted and usually do not need much sometimes at about 10miles I feel it, that takes about 1.5 hours

  9. JustinNL says:

    First of all I just want to say thanks to Ben and Barry for this episode. This one was by far one of the best in recent times. It was also great to hear the commentary regarding cyclists. I’m a cyclist myself and found Barry’s ideas and comments to be spot one with my own experiences.

    I’ve been experimenting with my diet over the last two years and like Barry said, it takes time to become more fat adapted. That is the one thing I’ve found to be so very important, it takes time and you have to be really patient. You also have to be willing to experiment and test yourself.

    I’ve gotten to the point where I can do 3 hour rides on nothing but water mixed with Nuun and some Essential Amino Acids. At the end of one of my rides I pulled a solid 7 minute effort, full gas at LT and felt I still had some left in the tank after the effort was over. That was an awesome day for me, after so many months of working up to longer and longer rides, I felt like I finally got it.

    What I’ve learned is the importance of BCAA’s and/or EAA’s, carb backloading and I feel like I’ve got another tool in my belt. By that I mean I now know that I can go out on long rides at a steady tempo without having to rely on the typical sports nutrition bars and sugar drinks. On the other hand I’ve also learned I can do harder rides but with a bit of carbs mixed back in to my nutrition plan. It feels good being able to draw on different fuel sources.

  10. Colin says:

    What are your thoughts on BCAA’s before fasted training? It seems like they would invoke a slight insulin response but would it be beneficial anyway to prevent muscle breakdown or is that a moot point?


    1. Aminos are perfect for fasted training. I use that "trick" all the time to stop my body for cannibalizing my own muscle mass.

  11. Carol says:

    Hi Ben and Barry,
    Thank you for an informative and interesting podcast. My question is this: It seems that the people most often talking about ketosis/fasting and exercise are men. In regards to the possible negative effect on Thyroid levels and increase in cortisol levels, is this method of fasting less effective for women, especially those of us in our mid forties? I have been experimenting with fasting and an early morning workout 2 days a week for several months and I am not sure I see any benefit. Would it be worth it to increase that to 3-4?

    1. Barry says:

      Hi Carol

      Like almost everything, it depends. So many variables and so much individual context. I will say it can work for women no problem. Possibly thyroid and menstrual cycles throw a few more spanners in the works for women.

      A paleo blogger (Stefani Ruper) wrote a post about this describing how IF does not work for women in general.

      Mark Scisson responed to this here:…

      I also came across a more recent review:…

      Bottom line: it can work for women but is not advised for already lean/low body fat physiques.

      You could increase the number of days, or length of time fasting, or even do alternate day fasting. Its a case of going on feel and seeing what works for you.


    2. holybasil says:

      This is a very good question! Any answers??

  12. Andrew says:

    Hello Ben,

    I was curious if you know what types of fuel he uses while actually racing or training hard.

    Thank you for your time

    1. bazmurray says:

      Hi Andrew

      <5hrs running or riding training… I don't use anything… just water

      longer stuff… i'll eat cold meats/cheese and homemade "bonk" type bars.. think Ben has used them in Ironman

      However, I'm not a believer in slow carbs or avoiding carbs during racing any more…if you are ketotic/adapted etc.. and insulin remains dormant during exercise, which it does…and the brain needs to be fuelled … which is key… then simply sugars are absolutely fine in my opinion

      So… when I get brain fog… I'll also eat sugars, natural form as much as possible – fruits, honey shots, rice cakes… but I don't eat these to fuel the muscle, I'm not doing the 60g/hr thing… I take small hits when I feel the brain needs it… and sometimes I even have a beer


      1. HemmoP says:


        I wonder what is your reason to not use ketones (MCT oil, ketone body drink,…) to fuel your brain instead of sugars. I thought ketones were the best fuel for the brain and they would make more sense and be more natural considering that you are 100 percently adapted for fat burning.

        Another question: Even though well adapted, don't you think you are paying a price on your long term health (= shortening your telomeres) by practicing this kind of sport or is it that your body just recovers as if you just hadn't run a 100 mile race?

        Thanks for the interesting podcast!

        1. Barry says:

          If you look at the research on MCT supplementation during exercise, its pretty weak. Converting Lauric Acid to ketones doesn’t happen quickly during exercise and also the dose of MCT needed is high, which causes serious GI issues. So I’m not a fan of MCT Oil during. Ketone Drinks are new, not available to the public yet I think. You can get keto esters but these are something you take pre workout.

          However, I would still favour simple sugars during ultra races. Why ? 1. is practicality… if you’re running a 100mile race.. and you’re at a check point in the middle of the mountains somewhere, and all they have is some fruit and cake… what are you going to do ? Secondly, I still think glucose is a faster fuel delivery source for the brain than ketones… there is also the pleasure receptor factor or feel good sensation that comes with having some sugars if you have been running/biking for several hours and you hit a low.

          In terms of telomeres…. another paleosphere myth not put in the right context as far as I’m concerned. Sure, follow a SAD, do not S&C work, poor sleep, high stress etc… and combine that with running long everyday .. then yeah, maybe. But put the proper support structure in place, then ultra endurance can lengthen your telomeres and also has a huge epigenetic affect.

  13. Ian says:

    Hi Ben, This podcast was great. Explains why i bonk after 2 hours of hard intense fasted state MTB rides in hot weather. I will start carb back loading for sure but how many carbs should i consume in what ratio to fat and protein ? You are an inspiration to me.


    1. Listen to the upcoming podcast that Brock and I do with Jimmy Moore's show…we're going to get into this in detail…

  14. Peter says:

    Ben, I have been trying TianChi, but find the taste awful – are there ways to make it more palatable

    1. Really? I like it. I have tried it with soda water and that is pretty good. Mixing it very well into really cold water also helps.

  15. slederman says:

    Ben, I've been doing alternate day fasting, generally consuming zero calories on alternate days, for the past 6 weeks. I'll make time in the next day or two to listen to this podcast. I'm curious to learn all I can about fasting, fitness, feeding, and fat burning. And, I've shared your post on Facebook. Thanks!


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