The Negative Effects of the 5:2 Diet


Bronto´s halitosis

It seems that after the Horizon programme on the BBC last week, a lot of people have decided to take on the 5:2 diet. A lot of you have posted comments on this blog and have only mentioned a couple of negative side effects, which are:

Bad breath (also called halitosis): this is normal. If the bacteria on your tongue lies undisturbed for long periods of time, it can build up and cause a bad smell. This can be avoided by drinking lots of water herbal tea such as green or peppermint. Also, when the body has run out of its carbohydrate reserves it starts to break down glucose stored in your liver and muscles. After this, your body starts to break down fat stores. Ketones, which are a waste product of this fat breakdown can cause bad breath. Those of you who have tried the Atkins diet may have experienced this before.
• Increased sweating: not sure why this may be, but if anyone knows then please let me know!

The negative and positive effects of any lifestyle change depend completely on…EVERYTHING! The negative and positive effects of the 5:2 diet won’t be the same for a 50 stone man and a 9 stone woman. The effects won’t be identical for a 90-year old active woman and a 17 year old sedentary boy. The following variables can all have an impact on what effects you find when dieting using the 5:2 method:

• Age
• Sex
• The amount of exercise you do
• What you eat on your feed days and fast days
• Your genetic makeup
• Your percentage body fat
• The amount of muscle you have
• How tall you are

…you get the idea! If you’re looking for firm answers about ‘what happens when you go on the 5:2 diet’ or ‘how much weight per week you lose on the 5:2 diet’, you’ll never ever find the answer; because there isn’t one answer for everyone. Just do what you’re comfortable with and what works for you. If you feel like you might collapse when you’re only eating 500 calories on your fast days, then you need to listen to your body and make an educated decision from there.

It’s great to hear people’s comments about this diet, so keep it up! Next week (and twice every week for the next two months) we’ll be hearing from a guest blogger who has taken on the 5:2 diet. Should be interesting to follow someone’s journey through the real thing!


30 responses »

  1. I wrote about the Horizon programme in Learning from Dogs today and have a sequel coming out tomorrow that refers to the excellent science behind the ADF proposition that was put forward in that BBC Programme. You are correct in suggesting that it will affect differently people differently but whoever you are, the benefits that flow from ADF or its 5:2 equivalent are significant.

    • Hi Paul, I agree – the benefits of these are both significant for most people (if not everyone), but at the same time, no two people should expect exactly the same results 🙂

  2. Do the two days need to be in sequence or is it possible to split them into two seperate days within the same week ? Dr Mosely mentioned that he was pretty sporadic with the fasting days week after week but never said if he ever split them.

    • No, the research makes it clear. The original work done by the scientists was based on Alternate Day Fasting (ADF). Then the option of 5:2 was studied and found to be almost as effective as ADF. The two days of fasting must be contiguous.

      I’m really not trying to flag-wave my Blog but I have a post coming out at midnight US Mountain Time (07:00 UTC) tonight that goes into the science that underpinned the Horizon programme – hope you find some value in reading that.

      • Paul, sorry to be controversial but I think you’ll find that the two days don’t have to be continuous. Mosley split his, some of the other blogs support this too. I hope you don’t mind me mentioning this but these forums are so useful I wanted to share the thought.

  3. Perry, of course I don’t mind you raising the question.

    My understanding is that the fasting days must be contiguous but, clearly, it’s not a question that I am qualified to answer. That should be left to the scientists that were involved in the research.

    Both of them are mentioned in my follow up post to the BBC programme, here and that post includes links to the websites where the research may be studied. Plus I’m sure that both Professor Valter Longo and Dr. Krista Varady have publicly accessible email addresses.

    Hope that helps and I would be interested in your findings so I can update my blog post – or feel free to leave a comment directly.

    Best wishes,


    • Paul, like you I’m not qualified to offer advice but I think the missing link here, with regards to my blog, is that I’m following Mosley’s regimen (to be fair I hadn’t made that clear), Mosley is on the 5:2 “diet” and his fasting days are Tuesdays and Thursdays, plus he’s not limiting himself to just one <600 calorie meal per day. I believe he's halving the calories and sharing them over breakfast and lunch with herbal and green teas, plus water during the day. I was impressed by his results, (as per his TV program summing up) after a 5 week period. So, just to clarify, I'm following Mosely's regimen and hope to emulate his results.

      Your observations may very well be true with regards to the US Scientist's comments etc, but I was referring to Mosley and his approach. If you recall, one of the very same US Scientist's featured in the original broadcast confirmed to Mosley at the end of the broadcast that all of his blood counts/readings were down, including the IGF-1, not to mention his weight and body mass index and this was a result of the regime I've referred to above.

      I've not embarked on anything like this before but what I am sure of is that forums such as these are invaluable, it's a great way of sharing information and/or concerns on side effects or advice along the way.

      I hope things are going OK for you, this if a Feed Day 🙂


      • Just into the second day of the fast for this week – second week into the 5:2 pattern. Weight yesterday morning was 161.6 lbs, this morning 161.1.

        But while I agree with the value of forums for a whole range of communications, in this case there is no substitute to reading the original research which is available online. Just my two-pennies worth.

      • Hey Paul, well done on the weight loss! That’s really good news. I think that people should definitely check out any original research for themselves, too. Heather

      • Understood from where? The BBC Horizon programme made it clear, to the best of my recollection, that the two days are to be together. It was the 48 hours of ‘starvation diet’ that caused the body to start repairing damaged cells – for me the whole point of fasting. I have no big need to lose weight.

  4. Oh, and a message to the fair maiden that runs the Blog. Why didn’t you publish my earlier question as to why you called the post The Negative Effects …..?

    I thought it was a fair question. You are correct that not all can approach ADF without thinking about the implications for their own self, but what’s ‘Negative’ about it?

    • Hi Paul. Not sure about what the comment was (apologies if this has been accientally unapproved, but my blog IS set to automatically approve every comment). I am sorry about this! Maybe I should change the name of the post so that it isn’t as misleading. I may indeed change it this evening. Thanks for your input Paul and have a good evening. Heather

      • Ah, at least I can call you by your name! Heather, it’s not an issue – it was just that as a WordPress user I could still see my comment awaiting moderation. Please don’t amend the Post on just my say-so! Paul

  5. Totally inspired by Mosely’s programme and tried the diet week before last – absolutely stuck to it – did not even lose one miserable pound! Am not overweight – 5′ 2″and weigh 122 lbs (8st 10lbs) – would just like to shift about 6lbs. Read Telegraph article yesterday and am all fired up again and am fasting as I type! Will despair deeply if I do not lose at least one lb this time!

    Good luck to you all – Diane

    • Diane (and others), here’s my feedback (pun unintentional!).

      Last Thursday/Friday was the second set of fasting days under the 5:2 pattern. I’m 5′ 7″ (age 67) and while I don’t feel overweight, the tables suggest an ideal weight for my medium body frame would be 149-154 lbs. That feels too low for me but I would like to settle down around the range of 153-157 lbs.

      The primary reason for fasting is extend to one’s healthy life-span for as long as possible.

      Back to my weight reaction on this second week:

      Thursday morning 161.6
      Friday 161.1
      Saturday (first morning after the 2-day fast) 160.8
      Today, Sunday morning, 160.8

      I.e. I have remained the same the following 2 days after the fasting days. A week ago, I continued to lose weight gently on the Saturday/Sunday.

      However, last week (i.e. from Monday to Wednesday) Jean had me on a ‘half diet’, returning to that yesterday, after the fasting days of Thurs/Fri. I mention this because yesterday afternoon I felt very lethargic and low on blood-sugar levels so had a beer and a decent meal in the evening. That perked me up.

      So my conclusion, Dianne, is to let your own body ‘speak’ to you and not worry whether or not you are duplicating the findings of others. As Heather covered in her post, the number of variables between people is vast.

  6. In the horizon program he covered a number of ‘fasting’ methods. So at first I copied and did the 4 days of no food, just cuppa soup and green tea. I ruled out the under recommended calorie guideline, i.e. having 1800 calories a day rather than the 2500 of an adult male as I can’t count calories and find self restraint difficult (Ilo’ll clear my plate and go for seconds!) I thought better of the ADF diet as I cant feasibly see me going every other day on such a low diet and that the 5:2 is just as good. So I usually do Mon and a Wed, or Tues and Thurs.

    I’m not really hitting my 600 calories during the fast days. Like I say, It’s hard for me to count calories. So I go for the best guess and leave a buffer of 100 calories under the 600 say 40g of cereal + milk = about 430 cals. Or I have sugar free jelly and a pro-biotic yoghurt. I’m finding it’s quite hard when everyone else is scoffing a meal and the belly rumbles for hours on end. I’m finding I’m not ‘regular’ on the toilet and have experienced constipation plus onset of ‘piles’ doing this :/.

    I’d really like to know what the effects of taking daily aspirin and multivitamins are on this diet. Or any other (legal) drugs as I take supplements because I rarely hit my ‘5 a day’ during normal days

    • Sam, I have a copy of that Horizon programme and it covers two fasting regimes, not “a number”.

      Those two are ADF: Alternate Day Fasting and the 5:2 pattern. The latter being five days of normal diet and then followed by two consecutive days of 500/600 daily calorie intakes depending on one’s gender. The programme makes it clear that it is the consecutive pattern of those 2 days that allows the ‘starvation’ mode of the brain to kick in that continues it’s work over the other 5 days.

      So unless someone can point me and other readers to the science that says the 5:2 regime is not as written above, I shall stay confident of my understanding.

      Regards to all, Paul

      • Not sure which one you watched then, unless there was two programmes? This was the Live, Fast and Live longer Horizon program and it covered a number of regimes in mice and men. The presenter (Michael mosely) met and talked to an Asian man who eats child portions, his intake to half that of recommended amount – 1250 calories. He then met ‘Joe’ who is a calorie restricter and ate huge piles of fruit each morning – rich in nutrients but low in calories on average 1900 calories. Michael then decided to go on the 86 hour ‘fast’ to jump start his body, having only water and cuppa soups. Then went to cover the ADF diet, feed day/fast day .

        Then onto the 5:2 diet where 2 days out of 5 he restricts calories to under 600. He did state it was 5 of eating normally _then_ 2 days of fasting. But he does contradict this 50 minutes in and says “I managed to fit in 2 six hundred calorie fast days each week, but they tended to be a bit scattered around” and with this went on to get his results. I’ve taken that as 2 days scattered around the 7 and with this, his results were good. I’ve watched the program 4 times now and it’s still vague. Definitely need clarification on this! 🙂

  7. I’ve been on the fast for about 4 weeks with my partner, we are taking 2 days together with 600/500 calories per day spread throughout each day, lots of water, herbal teas etc, I’d like to talk about the side affects:
    Lethargy, short tempered, achy bones, constipation, head aches and hunger, surprise surprise! The one thing we’re both getting is dull headaches which feel a bit like those you get on Detox, I guess this is like a Detox. The upside is the other 5 days we feel energised and enthused and have both shrunk by about 2 clothes sizes, we don’t have scales!

    We are eating very carefully, we have a nutritionist friend who has stressed the importance of taking oil and plenty of water. Great low calorie foods:
    Quinoa, buckwheat, pulses, we like mung beans, papaya, blueberries, fruit etc…. Watercress chopped into Miso soup, yummm. Anchovies are great for adding to soup/stew, just one or two chopped very fine.
    I’ve found ‘calorie counter’ to be very useful, search the food and it gives you a rating from A to E?

    We’re at it till x-mas then we’ll review the ups and downs.


  8. Hello All
    Very interesting to read all the comments re which days to fast. My husband and i have been doing fasting on Tuesdays and Thursdays for 7 weeks now and hope to keep going till xmas and beyond if we feel good to do so.

    I have tried to read as much info as possible on 5 2 dieting. I understand from what i’ve read that the days are NON consecutive. This makes sense to me as the bodies metabolism doesn’t drop as much. If the days are consecutive then the metabolism would be slowing down by day 2.

    I suppose it depends if you’re using 5 2 as a way to lose weight which was not what the program was about, but it was an added bonus whilst fasting to try to live longer. The benefits of anti ageing and protecting against disease was the idea behind 5 2 but unless you’ve had health checks before and after you can never know for sure if you’re benefitting from it.

    Best to just do your best whatever suits you and not get stressed about it as that will shorten your life!! If it feels good do it.

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  10. hi all,
    i have Mosley’s book, ‘the fast diet’ which is an extended version of the Horizon show and he clearly states that the 2 fast days should be NON-consecutive, it is easier to stay on the diet this way, as you know you can eat normally the next day. He suggests you have a 250/300 call breakfast at 7am and a 250/300cal dinner 12 hours later at 7pm, so you are both restricting your calories this day and having the 12 gap. I think the answer is in the questions ADF ‘ alternate day fasting’ suggests that the days are ‘alternate’ and not consecutive.

  11. Hi all, I’ve been on the 5:2 diet now for around 10 weeks. I tend to fast Mondays and Thursdays. I don’t have ease of access to scales but it is clear I’ve lost weight, although my original reasons for doing this diet was the long term health benefits.
    I currently have reactive arthritis and was interested to know what sort of effect if any, this diet has on my condition. It has worsend slightly, but only since Christmas and because there are so many factors that can influence my condition, it makes it hard to pinpoint a particular one.
    I can’t say I have noticed any other negative side effects. I’m very active, despite the arthritis. I play football Saturdays, Sundays and Tuesdays and I try to squeeze a run in as well. Usually around 5/6 miles. So needless to say I’m not suffering from any energy loss. I do not partake in exercise during my fast days and try to drink as much water as possible. I eat at lunch time, usually a low calorie fish/ pasta dish at about 210 calories. I then eat again in the evening, which usually contains 100% vegetables. I plan on continuing the diet, unless the hospital recommends otherwise when I go for my check up in march.
    Anyway, it’s been interesting reading all your comments.

  12. Hi I started the ADF diet after reading an article in the paper. Bought the book and thought it was worth a try. Started on Monday 1st fast day was Tuesday, hard work but got there, 2nd day ate more normally but still watched calories. Day 3 is today second fast day not as hard to do. I weighed 278 lbs when I started and this morning I was 269 lbs so 9 lbs in 2 days is good. Will have to wait and see if it continues as I need to lose weight. I am experiencing tummy pains, but not from hunger is this normal. Or just my body asking for more food.

    • Are the stomach pains on the fast days? I only get hunger pains. But drinking a lot of water seems to have a positive effect on them.

  13. Started this diet 2 weeks ago and lost 4lbs already, well impressed. I don’t expect to lose this much every week but I’ve got to admit its a great start. Because I have limited calories only 2 days a week I think I’ll be able to stick at this one. My problem with other diets has always been the unsociable element where you can’t eat or drink certain things when you are in company. Feel really well so far so hopefully this will become a regular way of life 🙂

  14. My mum started this diet 3 months ago, and up until the last week has had positive results. The issue now is for the last week (every two days) has experienced hours of constant gastric pain, some vomitting and nausea. Doctor’s examination has diagnosed an issue with the gallbladder, and now requires follow up tests. It is interesting when you read about the gallbladder and the effects on Cholesterol/fats within the body, and what happens to bile when fasting, also, whether this type of diet could trigger gallstones or other digestive issues?? Does/has anyone else experienced digestive problems since beginning this diet?

  15. I started the “Fast Diet” 2 weeks ago with fast days Monday, and Thursday. I stocked up on carrots, celery, radishes, and the like. A free digital app from Amazon, Calorie Counter has been helpful.
    My first day was a 500 calorie “fast”, and I noticed immediate elimination of chocolate and other sweet cravings.
    On my non-fast days, I’m trying to stay below 1500 calories, because I am a 66 year old female, and the metabolism is slower than for youngsters. I’ve also added more exercise to my regime slowly.
    I find walking at least 20 minutes in the evening after dinner makes a big difference.

    I’ve had no headaches, but constipation has been a problem, almost as though my body is trying to squeeze every bit of nutrient value out of less food. Rats!

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