My first fat adapted cycling failure


Hi All -

Newbie here. I have been cycling for a long time and went Keto a few months ago. I have been checking that I am in keto with test strips and have ordered a meter for greater accuracy after what happened on saturday. I have been doing a lot of distance rides (for me) - two centuries in the past month, a number of 75 miles rides, 24 mile bike commutes. I decided to test whether I was fat-for-fuel adapted enough to do our normal 60 miler without any carbs. Well I learned what a near bonk or bonk feels like about 35 miles out. I was going hard on the hills and was doing great and then got light headed dizzy whenever I started to exert myself. I had to walk up the hills that I have ridden successfully about 100 times. I had had bulletproof coffee, a packet of justin’s almond butter, and a piece of canadian bacon for breakfast. That is actually more calories than I’d normally start out with but very low carbs. The bonk was not related to hydration. It was fuel. I am wondering if anyone has experience with implementing the fat for fuel approach. Do you need to slowly get use to it? Do I need to always use superstarch? Any thoughts are welcome.



Fat-adaptation is not a yes-or-no question. You can be in ketosis for a while but still be relying on glycogen for stronger exertion (actually, I think even those in ketosis for years use some glycogen for exercise, just much less than folks who are not fat adapted or only recently fat-adapted).
Still, the first thing that comes to mind is electrolytes since you don’t mention them. Have you been supplementing (with salt in particular)?
Paging @ianrobo …and until he comes on, you can listen to the podcast he did with Carl and Richard a few weeks ago. I don’t remember the exact episode number but I think “cycling” is in the title.

(ianrobo) #3

well this is the clue. Fat adaption is great at lower levels of power, just fantastic.

However if you are going hard on hills that would be pure glycogen burning and as not replacing it then the bonk was inevitable. Esp if on a fast ride then your body’s no new protein to convert to glycogen making the issue worse.

A lot of this is trial and error and you say only keto for a few months, I reckon it took me 3 months to feel the full impact and go longer and longer fasted. The clue is to go on a fasted 4 hour long steady ride, do that fine, you are fat adapted.

Also you say first 60 miler without any carbs which indicates you have been eating carbs before ?

(ianrobo) #4

oh and @Madeleine thanks for the kind words !!


Thanks for the responses. They get to what I am wondering. I do see that fitness guru Ben Greenfield includes superstarch and protein in his example mix for a triathlons. But then I see others discuss doing long range intense work with no supplementation. So it seems like there are both. Jeff Volek presents many examples of athletes going fat only on 100 runs, others seem to be doing the same. I am sure that you are right - I was pumping up the hills - they are just steep for me. But I was going faster and burning a bit more of my glycogen, for sure. Is there an accepted % of max level that the super athletes are staying below to ensure that they do not run out of gylcogen? Jeff’s youtube video shows results of tests of keto athletes where their ability to tap fat was developed way past the typical levels.

I used the Greenfield mix of superstarch, aminos, protein powder with coconut milk before a century two weekends ago. It was just too much for me - I should have taken it in more slowly - and my system had a hard time for the first 50. For the second fifty I was good and by the end, with no food other than pickle juice I was blowing past many that had past me early on. It felt good. I see what you mean about doing a steady ride. I think that there must be some expansion of the % of max you can burn without glycogen as you train up. I realize I know very little about this subject so thanks much for the info.



I was using electrolytes - no sugars though

(ianrobo) #7

well certainly for some it is different shakes but remeber Ben is Vegan so prob needs the supplementation.

This is a great example for me -

100 miles under 6 hours in zone3/4 HR most of the way round, apparently should be burning all my carbs and bonking, yet all I had was eggs and butter about 2 hours before the ride.

I am a total advocate of Jeff and Stephen Phinney and I have proved it and that includes efforts on the hills because if you do the rest at lower levels your stores will get replenished if fat adapted fully.


That’s amazing. So you just proved that it is possible to be going hard and using fat for a century. Very cool. I would like to be able to burn my own fat for fuel and while I am not a super athlete, I hope to learn how to do it. Do you think you were through your glycogen and making more of it or was your brain and system using ketos for fuel? Although new, I have already purchased Dr. Phinney’s and Professor Volek’s books. Guess I need to study them some more.


(ianrobo) #9

Well Mark I am no super athlete either !!

As to what happened, TBH I do not know, I did a similar ride a week before and same kind of time over roughly the same kind of course.

Their books are a great start but I really swear by the MAF method and training purely at aerobic levels apart from a few effort. If you base is a massive aerobic one burning all fat then when you need the efforts you have plenty in reserve ?

Plus never thought about it before but bonking is your brain reserving the glycogen it needs, it still needs that of course as a Keto athlete but not so much … so can go deeper into glycogen reserves ?


Interesting - Maybe the MAF is doing it for you. I just looked it up. It has to be something like that - training more and more without carbs and probably doing fasted workouts too. Here is the chart showing the results of Volek’s test of fat adapted LCD athletes versus HCD athletes. LCD group has a much greater use of fat with the curve not declining much until about 80% VO2max (


(ianrobo) #11

yep exactly that …

and I started out doing fasted rides of just 2 hours etc and now just extend that to 4 hours and if you can do that then you can do 6/7 hours and century rides fasted


I have been keto for 8 months and believe that I am fat adapted. On days that I do an intense ride with hills I take a small amount of carbs the morning of the ride or the night before. Peter Defty calls this a carb sneak. Eat like half a baked sweet potato buried in butter and sour cream. I have found that the carbs before a ride do not effect ketosis. Have checked my blood afterwards. Fat adaptation is great for long efforts but very intense efforts still require.
A couple months may not be long enough either. Try going for a 3 or 4 hour ride at moderate level and see how you feel.

(John) #13

I have been in keto a long time and did a full on bonk, sitting in a ditch nearly in tears, willing to give my bike to the first person to come along and take me home. It was all from going into the red too much. I have done the same course many times and have no problems, I can smash it up one of the 4 big climbs but that is about it. That trip for some reason I thought I could do 3, each putting me above 180 for a few minutes each.

(ianrobo) #14

maybe John for that ride you was stressed ? hotter weather than normal, a bit of a virus ?

(ianrobo) #15

yep that is exactly what I did extend your fasted rides until every ride at 4 hours feels good then you are it, still take carbs as John says if you expect a lot of efforts, but I did a 305km ride and 5km of climbing all on a bag of jelly babies …

but climbs were taken easy (not too easy !!)

(John) #16

It was hot too, yes.

(ianrobo) #17

each ride is unique in its own way etc got to take that into account

(Tom Seest) #18

I’ll just echo what everyone else here has said.

The craziest thing that I’ve ever done (I felt fine doing it), was fasting for 60 hours and then riding 65 miles completely fasted, but it took 20 months of keto eating, fasting, and so on to get there. I worked my way up to it. I also had access to plenty of body fat during the ride.


What was the ride like? Were you going anaerobic on hills or anything?

(Tom Seest) #20

It was on the Katy Trail, but I was climbing occasional steep grades as we were visiting occasional wineries. It was also all on crushed limestone, so between that and my weight, and the weight of my touring bicycle, I was spinning in my 80’s to 90’s the entire time.

I felt fine the entire time. The only reason I ate at the end, was I was riding with a Registered Nurse who was a diabetes instructor, and she was freaking out because my Blood Glucose level was 46 or 49, and my BK level was 6.8. Not really a problem, but I didn’t want her to panic or cry or scream or anything.