Hi guys! As my title says, I’m currently 40 hours fasted and have become quite lightheaded and a bit dizzy, but I’m not hungry at all!
I’ve got enough electrolytes (been drinking sugar free electrolyte drink) so it can’t be that.
Shall I break my fast or keep going?
This is the longest fast I’ve ever done!
How long have you been eating keto? My guess would be that at 40 hours you’ve burned through all your glycogen stores and your brain is trying to up it’s ketone utilization. Probably not very good at it yet and ketone synthesis may not be all that efficient either. Gluconeogenesis may not be synthesizing glucose efficiently yet. Net: brain running out of fuel. You’re not going to die, but you might get lethargic.
If you want to continue the fast for whatever reasons, you could try drinking a couple of tablespoons of MCT oil. Might get enough more ketones to your brain to eliminate the ketone shortage. If that doesn’t work, take a long nap so you don’t need as much energy for other stuff like staying upright.
Or, just call it ‘end of lesson one’.
The longest fast I’ve done was 5.5 day, and I stopped because I got dizzy when standing. I think that’s because lower insulin = less electrolytes stored.
At the time, I did not supplement much in the way of electrolytes. Now, I would add a lot more.
Personally, I’d take more salt and see what happens. Supposedly, if you put it under your tongue, it absorbs faster. I’ve never been able to actually do that though.
But if that doesn’t stop your dizziness/lightheadedness, I’d quit. That’s not a normal reaction.
Eating keto for only 5 weeks or so, but I’ve been practicing IF for about 5 years. I’ve never fasted for more than 32 hours, but I’ve pretty much done 18:6 every day for a long time.
Right, I’ll try the MCT oil, or perhaps some salt as @ctviggen suggests, thanks!
Energy is ok, but more of a ‘thickheaded’ and slightly dizzy feeling.
I’ll give it another half an hour or so and see how I feel.
And I was thinking the same re a nap, I can have a nap on my lunch break now… the joys of having to work from home!
I agree that electrolytes might be involved as well. But…
I only do overnight IF 2 or 3 nights per week and haven’t done a multi-day fast since I started keto. So I’m speaking as one who eats well daily. In addition to whatever sodium I get in my food, which is likely quite a lot since I like many foods with high sodium content, I also consume 10-12 grams of Himalayan pink salt daily. I get light headed and even dizzy from time to time. Not frequent, maybe a couple times each week, maybe not for a couple of weeks. Nothing serious, just a passing moment of a second or two. Not sure what if anything to make of it. I’ve experienced it all my life pre-keto.
I also get much more frequent so-called ‘ocular migraine’ that displays a slowly expanding, jagged ‘aura’ centered on one or the other eye. Not accompanied by a headache. Just disrupts my vision for 15-30 minutes.
I’ve just had a pinch of pink salt on the tongue, and I feel a bit better already! This could of course be placebo, but I think it’s helped.
Yep, salt is the first thing to do and if you still feel dizzy, eat if you ask me 
Of course, everyone fasts with their own attitude, I personally don’t suffer or stay hungry if I can avoid that.
Not being hungry at 40 hour is pretty normal for me (though I very rarely fast for that long. I had several 40-48 hour fasts, some were accidental and a 120 hour long one, I always ate asap when I got hungry except maybe in the first 24 hours of the long one. most were before fat adaptation, some afterwards, it doesn’t seem to matter to me). If I can skip a day, my body gets disconnected from eating somehow…
But feeling dizzy and weak is a sign to eat for me (I would suspect lack of sodium under certain circumstances but I almost never need to supplement it and never did. most people needs more and more frequently, it seems). I suspect it’s just my body being all dramatic but it doesn’t matter, I won’t feel okay until I have food.
Hunger and need for food are two different things, it’s probably closely correlated in most people, I don’t know but they are pretty much separate in my life and surely many experience it at least sometimes.
Oh and congrats! 
Lightheadedness is a sign of not enough sodium. Your electrolyte drink is probably low in sodium, and getting enough is crucial to managing the other electrolytes. Try eating more salt and drinking to thirst. You might be surprised at the result. If you feel better, you could probably continue the fast; otherwise you should probably stop.