Extended Water Fast and Cycling

Hello everybody,

am contemplating to go for an extended water fast 7-14 days during my annual leave and continue cycling outdoors.

I do this for health reasons and if it throws me back in regards to FTP and fitness, so be it.

My question to those who have experienced this before:
-How do you deal with loss of electrolytes and minerals through sweating?
-What training zones are best for 2-3h rides? Will endurance zone be already too tough?
-What power loss did you experience? Like before can ride 200w for endless duration, but now had to reduce to xxxW)
-Did you notice a change over the course of the fast in regards to increasing or decreasing performance?
-Did you notice a long term change in regards to FTP or your power zones? (e.g. better fat adaptation and you can ride longer at higher zones without eating?

Best regards,

Personally? Terrible idea, dude. I certainly wouldn’t be cycling throughout the fast- even if you’re not concerned about fitness, you’re juggling significant health and safety risks for declining performance and recovery.

I am not qualified to comment on the impact to your overall health, however I would strongly encourage you to speak to a medical professional about this.


Hello Toribath,

not my first fast, but my first since started to do sports.

What makes you say that there are health and safety risks?

You’re going to want to make every ride a recovery-ride… 50% tops, and probably not more than an hour. You’re essentially going keto for a week or 2. Most keto initiates experience a drastic reduction in power and tte initially, before bouncing back after several weeks.

Electrolyte loading can help as keto (and fasting) is naturally diuretic (you lose a lot of water, and thus electrolytes). In your case, you’re not even eating, thus missing out on even baseline electrolytes from food exacerbating the situation. So you’ll definitely need to augment copious amounts of sodium.

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Whether or not fasting has any health benefits is not for me to judge.

But I think I’m right when I say you stand to gain nothing by adding cycling to your fast. Positive adaptations just aren’t going to happen.


I did longer fasts (5 days) twice when i was first trying every tool for blood glucose control to see what worked for me. They didnt fit with my lifting goals so I no longer do them but 16s and 24s are still pretty common.

Day 1: 16-24 hours in. If you are used to fasting you can still do whatever. I have done SS and Vo2max workouts on day 1. Helps blow out the glucose stores.
Day 2 and 3: ugh. This is the real answer to your question: You dont get to pick, you are doing Zone 2 at best. Physically you can still pedal all day at easy pace but the RPE is high. Way high. Watching the clock cause it sucks at 150 watts high. I do the workouts outside, cause if i am on the trainer i quit.
Day 3 and beyond: not so bad. You can do some hard short intervals and they dont suck all that bad. 15/15s, 30/30s etc. You just cant do many. Like 1 set not 5. Endurance is still higher RPE than normal but its really not so bad.

Your body is going to tell you what you can do, and you wont be able to override it. Just roll with it. I did mine during recovery weeks and my FTP didnt move (i also didnt improve from that whole block). You do want to supplement salt. I didnt notice anything long term with cycling zones, but I am now able to randomly skip meals for up too a day and not have it impact me at all. I can do my hard hard workouts fasted in the morning if i ate a good dinner. It makes life in general easier and I am glad I did it. Its useful to know you can just not eat for a week and all is good but it wont help your cycling.


I think this is a hugely underappreciated aspect of diet & lifestyle. If you absolutely, positively have to eat something every 4 hours your metabolic health is not in a good place.

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Thanks Mattonabike,

how did you supplement salt? Lots of electrolytes come with carbs. I think as long as it is less than 50kcal this shoukd not
kick one out of fasting state, but woukd like to avoid it alltogether.

a water fast? blimey that’s more like wtf!

Not to sound nasty but please take care with that!

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I used bouillon in some water.

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Nah, not a big deal. Sounds to you as crazy as to most normal people cycling 100+km on a Saturday morning for fun.

Of course will take care. That‘s why I started the thread in the first place, to avoid any surprises.

I am now at the beginning of my 5th day of not eating. So far so good.

Last meal was on Thursday for lunch, rather low carb, as in the days before.

-Friday as first full day was fine. Did an early morning group ride (37km), which was social, so I was well in my lower half of my Z2 as well as commuting to work (8km)

-Saturday morning was the first time when I thought “What the hell am I doing?” Woke up, got up and felt weak and without energy. After drinking 1l of water with 1.0g of Salt I went back to bed. Got up again after an hour and man… did I feel good. Had a black coffee and went out to cycle 50+km

-Sunday… never change a winning team… so lots of water and sodium it is. Since my sweat was super salty on Saturday I reduced it to 0.5g. One black coffee and joined a 60km Sunday coffee group ride. Needless to say I was the slowest one. I try to stay within the lower half of my Z2 and don’t want to push 300w to accelerate once we slowed down. Most amazing thing is however, that during coffee time everybody had sandwiches and cakes around me, but I did not care. Hunger is certainly not an issue.

-Monday… bloody Monday. Joined again our daily morning group ride. On the way to the meeting point I already felt that going in my lower Z2 is more a challenge than the days before. Supposed to be a slow day. But it seems that it attracts those riders, which don’t want to get dropped on the fast days and then they show off… Caught on for maybe around 10km, but the high speed and frequent acceleration put too much a burden on my (fat burning) system. As I intended to finish the ride and not fall from my bike like a Zombie I dropped… and so did my average power every 5km. The ride became a recovery ride from Power Zones point of view. For sure did not feel like this. Wonder if this is because on day 4 the energy system changes more or because I did 200km the previous 4 days and I just need a rest day. (My weekly target is around 200km, recently a bit more cause I have to catch up with my annual goal)

I am really thrilled that cycling is still possible during an extended water fast. 1-2h no problem and if you keep hydration and pace under control even longer rides should not be a problem.

Despite my available power being much lower, I notice that I don’t feel a lack of energy. On rides (even in my Z2) where I had food before, or the day before, I notice that after some time energy goes down. It is not a hard bonk, but its little cousin. This is not the case anymore. I’d assume because the body is exclusively running on fat since Saturday and I have plenty of that to offer.

In regards to weight I lost an average of 0.9kg or 2 pounds a day. For sure most is used up glycogen in the liver and water. But 1-1.5kg should be pure fat. Let’s see how the journey goes on.