Brain Fog on fast day?

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  • I’m fasting today (1st day of 2nd week) and I’m extremely drowsy and have a serious case of brain fog. I’ve already made a couple of dumb mistakes at work and finally ate my “emergency apple.” I’m not hungry, that’s not the problem, I just can’t stay awake and can’t think straight. Anyone else ever have this problem? I’ve already had a lot of caffeine for me, really don’t want anymore and it’s not helping anyway. Help!

    hi cherrytomatopie,
    I felt just the same for my first 3 fast days. My 4th day was so much easier and today I’m in my 3rd week and 5th fast and it’s been really good!

    Like you, the hunger wasn’t really the problem but my head was all over the place! I also had to nap in the pm of those first 3 fasts.

    Stick with it. It does get easier!

    I definitely noticed the brain fog. It is a reality. I am lethargic and drowsy and make really ridiculous mistakes. I also cannot find my words! I did choose recently to try a Monday, Tuesday fast versus a Tue, Wed, and I find I do quite well on a Monday due to the extra calories typically consumed on the weekend. Since I had one of those weekends, starting fresh on Monday wasn’t so bad. When I started on Tuesday, I was so irritable and tired all day long. Feeling much better on my second day of fasting for the week today.

    I have the same thing happening and I don’t like it. I keep waiting for that mental clarity and sharpness to kick in! I’ve had 3 fast days so far and I’m hoping my next one will be less “foggy.” Glad to hear I’m not alone in this.

    It does get easier, honestly! In the first few weeks I felt incredibly sleepy by mid-afternoon and also found on occasion I could barely string a sentence together (not great when you’re trying to teach a class of 30 teenagers!). Keep persevering, the ‘brain fog’ (brilliant term!) experienced on fast days does subside when you’ve been following the plan for a while.

    I’m just about to finish my second fasting day and I’m also suffering from this, I’ve found the side effects on my concentration have been far worse than any hunger pangs! I hope you’re right in saying that this will pass because I’m not sure I can get away with being so brainless at work for much longer…

    Ah yes, the brainfog. I’m still suffering from it a bit myself at the moment, but I’m being a bit more extreme than most in my fasting so can’t say that doesn’t make sense.
    Good thing that I just sit behind a computer all day at work and don’t do very difficult work. When I exercise at the end of the day, I noticed my results were a bit lower than I’m used to as well – but that only makes sense because I’m working out at the end of a 22 hour fast! I like to think it makes my muscles munch up my fat though ^^


    Water, water and more water! really helped me deter brain fog. On a fast day, before noon, I usually go through a 2 litre bottle of water with another 2 litre opened and started.

    I am in my 6th week (12 pound loss) and I still drink loads of water or herbal teas and oddly enough, not peeing all day either.

    Plenty of fluids works for me.

    I am in my 5th week now and can honestly say I have not experienced this.

    The only problem I had was a headache on the first couple of fast days and a bit of lightheadedness after exercising on these days. I put it down to dehydration and upped my fluid intake which has cured the problem.

    I actually feel more awake and alert on my fast days ….but then I always did like to be different! ๐Ÿ˜€

    I am on the first fast day of week 3, and I definitely have brain fog. I had a long day at work today — 11 hours. I am home now and my only goal is to stay awake for a couple more hours. I feel less productive on fasting days. I hope the brain fog goes away and I can get more done in the evenings on fasting days.

    I agree . I definately had brain fog and also felt sick the day after my FD. I drank more and more – and as has already said – I didn’t seem to eliminate the large amounts of water that I was absorbing.
    I hope that this will calm down. I have done 2 FDs. It will bd difficult for my work if not.

    i gone through from some type of these headache,tension and stress. i used to take coffee 10 – 11 times a day. then i read a post from one of the forum and they people talking about some kinda supplement called brainkrave. I just order it and using this from few weeks. its working as they mention on their site that it is totally natural. so, please give some reviews about it. thanks in advance !!!

    On my 2nd fast day today and have terrible brain-fog. My concentration is terrible, though I don’t feel particularly hungry. Will try drinking more, but I seem to have been drinking plenty.
    Some of the posters suggest this will get easier as the weeks go by – I hope they are right, in the meantime – any ideas greatly appreciated!

    It does get better.

    I just learnt to accept that I need to drink more on Fast Days. Now, I don’t get brain fog but if I don’t drink enough I can get a slight headache the day after. So, even though I don’t drink the great quantitรฉs I did at the beginning of fasting I still need to remember to drink enough and to make the effort.

    It’s good for me because it’s about taking care of myself .

    I’ve just finished my second proper fast day after starting 5:2 last week. (There was a third attempt on Thursday, but it failed.)

    The calorie restriction was fine, but I felt very tired and foggy, which was not good as I had things I needed to do. I was drinking green tea so there was no caffeine withdrawal, and I made sure to drink plenty of water, but I was so lethargic I could hardly move.

    I had my first fast day on Thursday. It was easier than I thought it would be. I didn’t go the whole entire day without eating though. I ate one boiled egg and yogurt for breakfast at about 9 am and then at 1:30 pm I had another egg with some carrots. I ate a Healthy Choice frozen meal for dinner at 6 pm. I am happy to report that I lost 3 lbs in 2 days!!! I weighed 157 on Thursday morning and 154 on Saturday morning. Anyway, I’m a psychotherapist and need my mind sharp and alert so intuitively I thought that going all day without food would give me brain fog…and my job doesn’t allow me to have that. Doing it this way seemed to work out for me so I’m going to do it the same way tomorrow which is Monday and it will be my second time fasting.

    I’m hoping for encouragement from those of you who have been at this for awhile. Today is the second week of my diet — and my third day of fasting. Like many others, hunger is not my problem. It is brain fog. I’m good at the start of the day but by mid-afternoon, I can’t think clearly enough to work. I would really appreciate hearing how others have managed this.

    Thank you.

    It gets better the longer you do it. Initially I would get headaches and have less concentration. Try and stick with it. For me it took about 6 weeks for this to pass. I think that initially your liver is just so lazy at converting stored fats into glucose and ketone bodies that your brain is screaming for help. Brain can only use glucose or ketone for energy. If your not getting enough food intake then you have to get it from stored fats. Your liver gets better at it. I wasn’t able to ride my bike initially on my fast days, now I can do it with ease. Hang in there it will get better.

    I have such terrible brain fog and sleepiness that I can’t work or think. I was afraid driving to work as well. How long does this take to clear up? I’m in my 1st week. I have to study and work at the same time and don’t know if I can keep doing this if I feel like crap like that. I drink a lot of fluids and caffeine and it doesn’t help :/

    I had this today. Usually I feel brilliant on fast days. I think it is worse because I ate pasta last night instead of something more healthy. I have been eating really well since starting to fast, but that was the second batch of pasta this week and the second day I felt bad. I think the fog may have something to do with switching from glucose to fat metabolism. Perhaps the more glucose you started with the worse the transition. Of course hormones may have played a part too. Dratted things.

    I’ve always found I feel a lot worse the next day if I’ve had alcohol or a big meal before

    The brain fog never went away for me, so I don’t do the 5:2 though I found aspects of it very useful. I cut out eating in the evening (normally after 7pm) and don’t feel hungry again until after 9am the next day so I generally go 14 hours fasting every day. I’ve lost a bit of weight but the main benefit from this has been not feeling hungry all the time. I was a constant grazer and my evening/nights used to be endless trips to the cupboard/fridge to quell pangs that never went away!
    So I’m definitely glad I gave the 5:2 a try – even though I wasn’t able to stay on it I did learn a lot.

    I’m trying to fast on my days where I work from home, which I’m lucky enough to be able to do once or twice a week. Strangely I find it easier to abstain from food when I’m at home than when I’m at work! But I am also more sluggish and tend to do the lower thinking tasks (email housekeeping for example) rather than complex policy or decision making tasks on these days. I also find my day stretches out, so I get about the same amount of work done as I would in the office, where I can eat, but over a longer period. Mainly I think because I’m more easily distracted, and thinking slower. It just seems longer to get things done.

    I hope it gets better too, as my job is very demanding and time focussed, so no quarter given for missed deadlines …


    Hello everyone, I wanted to update that I’m in week 3 and most of the fog is gone on fasting days. I get hungry and think A LOT about food, but so far, I’m powering through. However, I notice now, that I get a bit of fog the day right after I fast. Hopefully this will go away as well…

    India.lochhead, oh yes!

    I kinda dropped the ball this weekend and had a lot of big meals, a lot of sugar and too much alcohol. I could not muster the courage to fast on monday as I usually do – I switched do Tuesday and Thursday for this week. It definitely makes it harder. Maybe it’s because it wrecks havoc on your blood sugar? I’m no physician and don’t know much about it, but it would make sense!

    This is becoming quite an issue for me at the moment which is a shame as I’m liking the diet a lot (through two weeks).

    I also work a very mentally demanding job, so can’t afford this drop in concentration and learning ability. Hunger is not an issue and don’t care much about the headaches, but not being able to focus is no good at all.

    I’m thinking maybe eating the 600 cals during the day might be a better strategy? Then I can do something relaxing at night when I don’t have to use my brain too much. Any ideas?

    If it doesn’t resolve I don’t think I can continue with the diet unfortunately.

    Ironically, one of the reasons I started this fast was my habit of going to the fridge whenever my writing stalled. Now I’m blaming the stall/block on brain fog. It feels very real though and It’s interesting to read about everyone’s experience with the problem. It was/is definitely the biggest problem, more so than hunger.

    Someone else mentioned low blood sugar which I’d also suspected might be the problem. I’m thinking of trying something like an egg or a tangerine (two items I have at hand right now) if it happens later today. Also, rather than eating all of my 500/600 calories perhaps spacing them out might be a better idea.
    Great to have this forum!

    I asked a friend who has been fasting for a year or so what the answer to this was. He said stick it out, it gets better. I hope he’s right. I’ve had to start again from scratch as I gave up on a difficult day and just ate until I felt normal again. Felt awful afterwards for being so weak willed, but now back on the wagon … And trying to be kind to myself and take it easy today. This is fast day two after a break of a couple of months, and will mainly consist of non-thinking tasks (ie housework, maybe some gardening … Not shopping though…) I figure if I fast on a non-work day it might be easier to deal with the fog until my body / brain adjusts. It just means I maybe a bit antisocial for a few weeks! I’ve fessed up to the people who matter … ๐Ÿ™‚

    It’s definitely improved a bit for me now. I’m not at my mental peak on a fast day, but I’m not becoming mentally enfeebled and losing the ability to concentrate anymore. I can work now. Definitely worth moving as much busy work and brainless tasks to the fast days though.

    I found now I start to hit that wall a bit around 15:30 – 16:30 after eating nothing all day, last meal 20:00 the previous day. When I feel that coming on I eat something small, an apple initially was OK, but for me 100g of avocado seems even better. This solves the problem until my main fast day meal 17:30 when I have an evening appointment, or 19:00 when I don’t. I’m basically fine then.

    Still experimenting though, headaches have pretty much gone. Not opposed to trying the calories in the day, but I want the extended fasting effect, and I’d be pretty naggy come bedtime if I went with that approach I think. All in all success though, so far.

    Back again. The fog completely disappeared now that I eat only lunch on my fasting days and have some carb with my lunch.

    I do, however, lose way more slowly, if at all. I guess I can’t have everything ๐Ÿ˜‰

    I drink an extra cup or two of coffee. That seems to help.

    I use unsweetened almond milk in the coffee, about 100 ml for 16 calories.

    This is so refreshing, I’ve been fasting off and on for over a year, and find it a very effective systems for weight management/control. But I do get into a ‘brain fog’ very often. Sometimes, I’m pretty good but I do tend towards more medial tasks. I find that fasting is kind of like being drunk sometimes (especially longer ones 36/48 hour, but even to a degree with 24 hour fasts) but without the fun-drunk part (even though I don’t drink anymore). With fasting, I can get kind of forgetful and a bit wacky… but darn it, it’s the one system that works for me! So, I’ll stick with it… buy my brain sure seems to run better on sugar! Nice to see I’m not the only one. During my divorce two years ago, I didn’t eat due to depression and lost a lot of weight. As the depression lifted my appetite returned. But after I started gaining weight back, I went back to fasting. I’ve done up to 93 hours without food! But, now, with the cognition problems I think I’ll start trying to go back to more cardio/calorie control… at 44 I can’t run like I used to, as knee problems etc… so, it’s a balance, but hard to function mid fast. At least you fit has lots of great cardio machines, elliptical is low impact, so I’m incorporating that. Fundamentally, it’s still lower calories more exercise… is the trick. Thanks for letting me ramble here ๐Ÿ™‚

    The only times I get brain fog is when I’m slipping into ketosis or I’m eating normal and simply eat too much. Normally fasting gives me a clear mind although sometimes it makes me a bit edgy.

    Caffeine withdrawal seems to be my fogging/head aching issue.
    Really naive that I didn’t realise that tea had 50% of the caffeine of coffee – so I went cold turkey – took about 3 days before headaches started. A week on and still with me.
    Trying to fast today, but the headache fogginess will probably distract and destroy.
    Have ordered some nootropic supplements – without caffeine – hoping that they might help with caffeine withdrawal

    Hi everyone..

    I am working with University College London and we are interested in the impact of dieting on our ability to think (i.e. concentration). If you have been on the 5:2 for more than one month and are interested in taking part in a short online study then PLEASE email me!
    You will be sent a unique web address to complete some online tasks which will take around 15-20 minutes and can be completed from the comfort of your own home.
    As a bonus, University College London will be holding a prize draw of a number of Amazon vouchers (up to ยฃ100) as a thank you to all those who complete the online tasks! Thank you very much ๐Ÿ™‚


    This is how I feel. I’m on my 4th day of fasting. I’ve slurred my speech, and forgot particular facts my boyfriend told me, only a few hours later in the day. We are percolating past emotions to the surface. This is our old selves we are saying goodbye to. Well…this is what I’m experiencing.
    The “work” is tiring. I’ve had enough. I don’t want to eat…I want to be clear headed.
    Thanks for my venting moment. We’re doing this together…HOW COOL!

    Yes me too..

    Trying to find my way around the brain fog, anxiety feeling, and low mood as a result of fasting.
    Enjoy the fast weight loss, enjoy the clear feeling in my body after fasting but the stress it places on my mental health is a bit too much.
    Find 16:8 perfect but must eat before 12 or I struggle to concentrate and find words.
    Although don’t notice such a weight loss with this fasting.
    Going to try 5:2 again this week (starting on a Sunday.. to make sure I dont have much to do).
    Find the longer I am doing 5:2 the more tired I and anxious I seem to get…
    I believe this is due to reduced carbs as I am also unable to do any diet that cuts carbs… interesting I think Dr Mosleys other book reduces carbs… some people can do this.. my body type cannot… doesn’t matter how hard I try!

    I’m wondering. All people suffering with brain fog. I got it myself. Is it just lack of glucose to the brain due to glycogen depletion or is it something else. I have quite alot of fillings in mouth. I wonder that when your eating the chemicals that would normally leak from your fillings would normally get stored as fat but when fasting they may be just keep circulating round in the blood causing problems. Just a guess but it seems strange why some people experiance brain fog but not others.

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