Combining Low Carb And Extreme Exercise – The Official Results Of The Great Ketogenic Ironman Experiment

Affiliate Disclosure

Low Carb & Ketogenic Diet, Podcast

Alright folks, this is it.

For the past 12 weeks – in a build-up to racing an Ironman triathlon – I've been combining low carb and extreme minimalist training protocols…

…I've been biohacking every last detail…

…I've been tracking all blood and performance results…

…and now I'm releasing everything to you in one fell swoop.

If you've ever wanted to know exactly what happens to the body when you use ketosis, a low carbohydrate diet or minimalist training protocols, and then try to pull off a feat of physical performance such as an Ironman triathlon or a marathon or a Crossfit competition or anything else remotely insane, you're about to find out what happens – the good, the bad, and the ugly.

I am releasing this entire report as a Premium audio episode – and in addition to detailing all the nitty-gritty blood results, race results, practical fueling tips, workout details, transcribed .pdf of the 1 hour episode and answers to every question about the race submitted on the BenGreenfieldFitness Facebook page, I also reveal some pretty shocking details that you're not going to want to miss, including whether I would ever do something like this again.

If you can't play the audio above, it means you probably don't yet subscribe as a Premium podcast listener – and yes, you're missing out! It's $9.99 for the whole year (yes, the entire year), and includes bonus video and audio podcasts such as this one, as well as “The Naked Truth” episodes with my wife Jessa (in which we drink copious amounts of wine and say shocking things) – and a bunch of other bonus workshops and videos. And don't need a phone app or anything special to access premium content.

Resources mentioned during this audio include:

My “Minimalist Training” Build-Up to Ironman Hawaii World Championships

The video of what I ate during the race.

Downloadable .pdf of WellnessFX blood results

Official Ironman Canada results

Low T3 Syndrome article by Chris Kresser

Article about Sex Hormone Binding Globulin and Cortisol

If you need to catch yourself up to speed, the previous two episodes in this series are: 

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

The Official “Ask Me Anything About Ketosis & Ironman” Premium Podcast

Finally, a quick update:

Just after pushing the “publish” button on this post, I had a chance to speak in detail with Dr. Peter Attia about some additions to the recommendations I make  in this episode – biohacking additions such as ketone esters, caprylic acid, different amino acid ratios and other cool science that will allow for even more success at Ironman Hawaii. Stay tuned for the details on that in the next Premium episode!

Click here to donate to the podcast and, in doing so, directly support Ben's journey to get to the big race in Kona! Questions, comments or feedback? Leave them under the extra photos of the race that I've included for you below (photos credit Fred Cuzner – more photos available on Facebook BenGreenfieldFitness page).





Ask Ben a Podcast Question

34 thoughts on “Combining Low Carb And Extreme Exercise – The Official Results Of The Great Ketogenic Ironman Experiment

  1. Tammy Bateson says:

    Did you find the most ideal time to train when the session will last for at least an hour, up to two or three hours? Was it when your ketone levels were at their highest? Eg. Is it best to train after taking some MCT oil, or when you wake up fasted?

    1. yep, at highest ketone values turned out to be best by far.

  2. Jon says:


    First off thanks for your experiment. While I am not sure I agree with all aspects of Ketogenic in general, I do feel like many of us have relied too heavily on processed or manufactured carbohydrate sources in general. (Nutrition and fueling)

    What are your thoughts on significantly reduced carbohydrates for general nutrition and the addition of “reasonable”. Increase of carbohydrate just around training bouts. Overall total carbohydrate intake still much lower than typical American diet.


    Keep making people think

  3. Jeffrey says:

    Ben, can you dehydrate faster on a low carb diet while training/racing?

    1. Theoretically yes, since carbs retain water. Good reason to listen to thirst!

    2. durianrider says:

      Every single athlete on the planet will consume simple sugars to get the best out of themselves. Even Ben 'Ketogenic' Greenfield.

      This blog is hillarious. First Ben charges 10$ to see the 'magic foods I ate during my ketogenic race' and then we find out it was SIMPLE SUGARS IN REFINED form from COKE and SUPER STARCH!

      Ive not laughed this hard in a few days. This is youtube video material for sure. :)

      1. You should come on my podcast and we can talk about it. If you're game, shoot me an email: [email protected]. I'm off to race Ironman Hawaii next week but could get you on the week after. Cheers! Ben

  4. Bill0001 says:

    I am doing a Ironman in early November and thinking about using UCAN. I have used gels, bars, and bloks in a previous Ironman and Half Ironman’s. During the Ironman I has some GI issues half way through the run but never any problems in long training sessions. I am not fat adapted so was wounding what you would recommend for the different ratios of UCAN, MTC Oil, MAP, X2 Performance, VESPA? The exact same as you or different since I am not fat adapted? I weigh 170 pounds.

    On the run, once you bonked did you completely ditch the UCAN mix or did you continue to drinks along with the coke and gels?

    1. You really need to experiment on yourself – especially if you are not fat adapted. UCAN is not something you can just jump into.

      Once you leave low-carb during a race, there is no going back. I stuck with coke for the rest of the race.

      1. durianrider says:



        No wonder nobody in athletics takes the low carb movement seriously.

  5. Paul says:

    Hi Ben

    Really interesting posts and latest podcast on your experiment, which I can relate to as I switched to ‘Keto’ about 10 weeks ago.

    My question is that currently this is an “experiment” for you and you will take it into Hawaii, but with an open choice would you stick with Keto or go back to low carb (as per your book but with a few improvements from lessons learnt)???

    After 10 weeks on Keto, I have found my performance has not developed as I want and in fact I think less than when I was high carb/low fat. This, together with the lifestyle issues it poses, I am considering just going to low carb similar to your book to try to get more performance and stay healthy….

    Any thoughts?

    1. The experiment was never about "going faster", it was about "going healthier". As I said in this post, I'm easing off the keto but I will always be LCHF for health and longevity.

  6. loveMTBsue says:

    Love it, Very interesting Listen,
    I wouldn't want to do low carb or triathlon myself, however I found the results fascinating :-)

  7. BerkshireRunner says:

    How many hours before the start of the swim did you drink the BulletProof Coffee? And is this your typical (regular daily) timeframe for coffee.
    Thanks for an informative report

    1. I had the coffee between 5 and 5:30am. The race started at 7am but I was buzzing by 5:45. On a regular day, I am rarely awake before 7am… if I can help it!

  8. chuckhazzard says:

    Hello Ben. Interesting experiment. I wonder if you could have squeaked by without coke and other carbs if you had used KetoForce. Might be worth a try prior to Kona.

    1. I doubt it. It wasn't a lack of ketones it was straight up glycogen depletion.

      1. chuckhazzard says:

        My understanding of how KetoForce works is that is slows down the depletion of glycogen. So my thought is that consumption of it during the bike and run could possibly have allowed you to continue on without resorting to quick response carb intake.

        1. Nah. It's just ketones. Amp up brain levels of ketones and attempt to flood the brain to decrease level of perceived exertion and improve focus. Doesn't touch glycogen though…

  9. Kyle Taylor says:

    Hey Ben. I noted some of the changes to nutrition you are planning for Kona. Was also wondering if you are making any changes to your workout / training regimen in the Kona lead up as well. A side from a taper, I am wondering if your Canada experience is causing you to train a little differently now?

  10. beuss01 says:

    Hey ! Ben is a heel striker !

    1. Not much. You'd be surprised at what happens by the time the foot actually makes contact with the ground.

  11. LeafySweets says:

    Congratulations, Ben on your results and qualification for Kona. I need a little extra help in understanding your bonking situation after the first half of the run. Why did the bonk happen if you were taking in SuperStarch, etc.? I have been learning that even the leanest athlete can go for days just on their own adipose even without the SuperStarch. So in my mind, you had an extra bonus with the added SuperStarch. Was this the case because you were doing Z4 and Z5 efforts? What am I missing? Didn't you mention that you were doing Z4 and Z5 without any problems? Thank you for helping me understand this.

    1. I spoke pretty extensively with Peter Attia about this and we determined that based on my bike speed, projected bike power and caloric utilization, I was probably nearly 100% glycogen depleted starting the run – and there fore have three choices: 1) higher exogenous source of fast release sugars immediately or shortly after starting run (potential for stomach issues and not that healthy, but do-able); 2) higher intake of Superstarch per hour during bike (can cause GI distress in some people) 3) "loading up" in final hour of bike with "extra" calories (e.g. 400-600 calories of something like a BonkBreaker bar, etc.) going into run. A combo of the first and third option is probably what I'll do in Hawaii.

      We're also looking into higher intake of amino acids, because a drop in AA's could also cause a bit of a bonk. So those are the basics.

      And Z4/Z5 efforts are just fine in ketosis – unless you try to throw them in after pure glycogen depletion at the end of 8 solid hours of hard exercise – such as attempting to sprint out the last half-marathon of an Ironman.

      1. Jeffrey says:


        What about a nutrition bar that won’t spike insulin that gives extra calories and you can start to maybe eat mid bike. I wish I knew of some good bars that would be acceptable. I’m fueling pretty much the same way you are ben for my next race. Any suggestions?

        1. Jeffrey says:

          Maybe Coccachia bar?

        2. NOTHING is really going to spike insulin when you're exercising, so that's a moot point. It's glucose spikes you're worried about. Just choose something higher fat. Cocochia bar is PERFECT.

      2. Durianrider says:


        Ben bro!! Why did you consume CARBOHYDRATES when doing a KETOGENIC experiment? Why not guzzle down some coconut oil or chew on a fistful of solidified bacon fat?


        Thats like me doing an ironman ‘as a vegan but I eat meat during the ironman..’.

        Come on bro. You can’t be serious???

        Hey, at least you made my morning so much brighter. I havent laughed this hard in a few days. :)

  12. samnicaise says:

    Thanks Ben for the very insightful podcast – albeit without the fun drunk Jessa :)

    I am currently going through a bank of tests to check out my seemingly high cortisol, low T (free and bound), and high SHBG, all on top of having taken supplemental T4 for many years due to central hypothyroidism. While I'm not going to make any moves till I get my labs back in a month, I'm really interested to see what you think of treating SIBO. I might look into it depending on what the labs say.

    On the other hand, why don't all your "stress reducing methods" not help keep your cortisol down? Any insight? Cold training – Tian Chi – circadian "perfection" – breathing exercises… As a further prod, what is your personal stance on meditation/mindfullness and it's use for mitigating cortisol from extreme training.

    Thanks tons!

    1. Because I'm training for Ironman. That sh&* ain't natural. ;)

      Granted, if I weren't also homeschooling my kids, running a corporation and doing a crapload of other stuff, cortisol probably wouldn't be so high…

      1. LeafySweets says:

        Enough said with the high cortisol and homeschooling. From a fellow homeschool parent, it's stressful thinking you might completely screw-up your kids for life. Forget about Ironman.

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