The 5:2 diet works — but the side effects are pain, misery and bad breath

The body is a temple. In January, for the most part, it is a rather large and overfed temple. This year, under the pressure of constant lifestyle and dietary advice, I decided on a remedy: the intermittent fasting diet.

The regime, which instructs dieters to feast for five days a week and fast for the remaining two, originally took the States by storm in 2013. It has gradually crept over the pond, with numerous bestselling books (The Fast Diet: Lose Weight, Stay Healthy, Live Longer and The Fast Diet Recipe Book) and celebrity adherents (Benedict Cumberbatch). In fact, it’s not just contemporary ‘celebs’ who are singing its praises: Plato once said ‘I fast for greater physical and mental efficiency’; Aristotle was said to be a faster, too. Martin Luther announced: ‘It is right to fast frequently in order to subdue and control the body.’

So what is it? The diet — more commonly known as the 5:2 diet — dictates that you only need acknowledge it twice a week. On these ‘fasting days’, dieters are advised to consume 25 per cent of the recommended daily calorie intake (that’s 500 calories a day for women, and 600 for men). The 5:2 was originally championed by TV medic Dr Michael Mosley and journalist Mimi Spencer.

There are, famously, two golden rules of dieting: never crash diet and never skip meals. Yet the 5:2 seemingly encourages the former, and, it would seem, does not reject the latter. A website claims that ‘it’s easy to comply with a regime that only asks you to restrict your calorie intake occasionally. It recalibrates the diet equation, and stacks the odds in your favour’. So far, so easy.

Five hundred calories a day seems shockingly restrictive, yet Mosley and Spencer’s book suggests it is easily possible to divide such a meagre allowance into three meals. Combinations include a breakfast of two Ryvitas spread with two teaspoons of Marmite, a ready-made vegetable soup for lunch, and a supper of a small steak or fish fillet with mixed leaves. Dieters are encouraged to split their ‘fast days’; black coffee, Diet Coke, green tea and water (go crazy) are all allowed.

Little is known about the long-term effects of intermittent fasting (IF). The NHS guidelines state that ‘despite its increasing popularity, there is a great deal of uncertainty about IF with significant gaps in the evidence,’ and that possible side effects include difficulty sleeping (it’s true — I woke up regularly with severe hunger pains), bad breath (quite likely), irritability (the most prominent), anxiety, dehydration (thank goodness for the water allowance) and daytime sleepiness.

So far, so unappealing. Yet there is evidence to support intermittent fasting — we don’t have to take Plato’s word for it. A study in 2010 found that intermittent fasters experienced reductions in a number of biological indicators that suggest a diminished risk of developing chronic diseases such as type-2 diabetes. Two years later, a similar investigation produced evidence that the 5:2 diet may help lower the risk of certain obesity-related cancers such as breast cancer.

The science behind intermittent fasting is simple: if you don’t consume enough energy, your body burns off your fat reserves. The more complex explanation is that regular intermittent fasting activates a gene called SIRT1, more commonly known as the ‘skinny gene’, which repairs cells during times of scarcity of nutrients. The average intermittent faster loses one pound a week.

The 5:2 diet does, in truth, totally enervate its disciples. Restricting oneself to 500 calories a day is not easy — despite promises of three meals, it was preferable to stick to sushi for lunch and a bowl of baked beans for supper, washed down with endless glasses of tap water. I found myself eschewing all humans, all conversation, and any thought process that didn’t involve food, so obsessed did I become with the following day when I could eat normally again. Counting calories leaves one morose and forlorn. Life’s great pleasures are suddenly stripped away. But, lo, the weight fell off. Two days of pain began to seem like a reasonable sacrifice — particularly when one considers alternative restrictive options that dominate every day of the week.

I’m not quite in Plato’s league yet — but I certainly felt better after the Christmas slump.

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  • It’s hard to restrict yourself to 600 calories on two days a week. I came up with my own solution which has worked well for me and I’ve lost 14 pounds since August.

    For about six days a week I have abandoned my life long habit of eating a good dollop of carbohydrate with my evening meal such as potatoes, or pasta or rice. I now eat a good sized portion of green frozen green beans witha piece of fish or chicken casserole or some other tasty protein. Green beans in quantity are normally very tedious fayre, but I cook them in stock and then drain and fry them in a dollop of butter, seasoning with pepper and maybe some garlic. This makes them delicious and in a good quantity of the sort of volume that I would have had in a big plate of spaghetti, they are very satisfying and don’t have a lot of calories. They don’t produce lots of wind either and I don’t feel at all hungry. Other meals I avoid high GI like the plague, so I will typically have a slice or two of home made wholemeal bread and butter with a pot of coffee, and maybe another good slice at lunchtime with perhaps smoked mackerel pate. I never eat biscuits or cake and might have a handful of dates now and then instead. I walk a few miles each day and am now dead on twelve stone. What is more, I feel great. Once a week I will have a curry and a couple of good beers. It works and has not been hard to keep up.

    • Scott Brooks

      Oh please. I eat zero calories for 2 consecutive days each week and have for over 4 years.

  • sfin

    Eat less. Move more.

    • starchy

      Bollocks. Read the science. It’s all out there if you care to look.

  • MichtyMe

    Humans are designed for intermittent fasting, prehistoric ancestors having bagged a mammoth would binge, thereafter a restricted diet.

  • Mr Grumpy

    So where does bad breath come in?

    • Fasting induces ketosis and ketosis can cause bad breath.

      • Mr Grumpy

        Would two days a week on 600 calories do that? I’ve had no complaints from Mrs G when I’ve been on 5:2.

      • Scott Brooks

        Except that the diet doesn’t include fasting. It’s a bastardization of the word.

        • Keith Mason

          The way I do 5:2 I have my meal at about 7pm on a normal day, then nothing till a 600 cal meal at about the same time on a fast day. So virtually 24 hours fasting twice a week. Trying to stretch the 600 cals into 3 meals leads to some very odd meal suggestions, and I think it’s probably less effective. Works for me

  • Mary Ann

    Sounds daft to me, diets that require you starve yourself long term will never work, cut down on high calorie food and increase low calorie food, if you don’t cheat it should work.

    • It’s bound to work because weight gain and weight loss are a matter of the laws of thermo-dynamics. More food than you use up in being alive and in your activity turns to fat. You can alter two variables – calories in and calories used. For a twelve stone man a seven mile brisk walk uses 700 calories. For a retired man like me that’s easy to do. Not so easy if you are working long hours though. Ideally, a combination of more exercise and less high calorie food is bound to take excess weight off. Harsh, starving regimes take a great deal of willpower. I think it is best to view weight control as a long term project. It ain’t going to happen fast unless you are a refugee walking across a continent without adequate food. You can replicate that kind of situation by going to Northern Spain and walking the Camino de Santiago. You will cover 500 miles in a month and probably lose 20 pounds unless you eat like an medieval emperor.

    • StarTheory

      If you stay hydrated and eat high protein foods on fast days you won’t be hungry. I’ve lost 4 lbs in 4 weeks on 5:2 so far.

      • trixietimez

        and then what happens when you start eating normally again? This is what befell the people doing that crazy HGC diet… once they stopped restricting calories and starving themselves, they ballooned up.

        • StarTheory

          I guess it depends on the person and their self control. While I’m not doing 5:2 anymore, I lost 6 lbs on it and have kept it off.

        • Andrew Hoffman

          So… if eating “normally” caused me to gain weight, and dieting caused me to lose weight, but I go back to eating “normally” after dieting then I’ll gain weight again? Consider my mind blown!

  • Daily fasting is much easier. Simply restrict your eating window to at most 8 hours daily and eat whatever you like within that window. No having to worry about it, is it a fast day or not and no having to count calories and feel deprived. I’ve lost 70 lbs simply by scheduling my eating to a 5 hour window. It’s called the Fast-5 method.

    • Vitriol

      Yep! I used to do something similar to a 18:6 daily fasting, combined with plenty of walks and I lost 16 kg in 3 months. I had PCOS and the cysts disappeared from my ovaries and I started to have regular, though not perfect, cycles again.
      I stopped to fast so rigorously, but kept my weight for 7 years. Then I wanted to get slimmer, fasted for w while – and finally got pregnant!!! Now, as my little son doesn’t need much milk anymore but I felt quite miserable by the weight I managed to gain during pregnancy and breastfeeding, I started to fast again a month ago and already lost 5 kg, and my cycle started again! I believe it is a really good way to manage PCOS.

      • What a great testimony, Viriol! Fasting is amazing!

    • trixietimez

      You mean it’s basically skipping breakfast.

      • It doesn’t necessarily mean skipping breakfast. Your eating window can be at any time.

    • Mariette Ferreira

      I have tried that, but 5:2 works better for me. On the says I do eat I try to fast for 14hours. There’s no one model that works for all. Fasting for 18hours straight was way too harsh for me. On fasting days I never feel deprived, but that’s just me, I’ve read all about the benefits and knowing I’m helping my body is fine. Good to find something that works for you!

  • A friend of mine (no really) has been on the 5:2 diet for a year or so – and it seems to have helped manage weight gain … that said it might be the cycling to work!
    He has a quick and effective solution for the Bad breath issue you mention – a sturdy ‘AMANO’ metal tongue scraper – 2 or 3 scrapes of the tongue before bed works for him
    (so he tells me).

  • StarTheory

    I’ve been doing 5:2 for 4 weeks now and am down 4 lbs. I could calorie restrict till the cows come home and my scale doesn’t budge. This new way of life is doing the trick and my clothes are getting baggy. It’s not hard at all as long as you stay hydrated and eat high protein foods on fast days.

  • ddemaine

    Surely there’s only one diet that ever works– a balanced diet in moderation (with plenty of exercise)?

  • gzizzle

    5:2 in combination with a low carb diet and worked wonders for me. As a diabetic whose fasting blood sugars were around 180, I can tell you that it has been life changing. My FBS ranges between 90 and 110 almost every morning, regardless if it is during or after fasting. I have lost 30 pounds in 54 days. The bulk of that was in the first month. I have done very little exercise as I am waiting for a little more weight to come off so there will be less wear and tear on my joints.

    Once I fasted a couple times, I really fell right in line with it and ate much less even on my days off.

  • Lagos1

    Fasting two days a week was quite a common practice in pre-reformation England.

  • Scott Brooks

    The author is a pansy.

    I fasted (just water) for 3 consecutive days (Mon-Wed) each week for 10 months. I’ve been doing 2 days/week of fasting for over 4 years. It’s not that hard.

    • trixietimez

      so you’ll do that the rest of your life? How are your kidneys? The problem with the restrictive fasting diets is that they are not sustainable, and eventually you have to deal with eating. Your body needs fuel, you’re just destroying muscle.

      • Liz Carr

        Not true.

    • Mariette Ferreira

      Scott, have you traveled while fasting? How is it with the added movement and sun and sightseeing.

  • lorlex

    The diet is good, but I think it is much better as “save your new weight” rather than lose weight. I lost 12 kilos on dr simeons diet with Anat Stern a couple months ago, and after that I started 5:2 way of eating, because I tend to overeat and eat too much of unhealthy stuff, but still I do one 500 calories day once in two weeks, and it works for me.

  • Gallifrey

    I don’t find two days of fasting difficult on 500 calories at at all. But then I did the Optifast diet for a month – fasting every day on 800 cals – and lost 10 kg. It got easier for me as i went on with the first 3 days easy followed by 4 days of hell then hunger disappeared as ketosis kicked in bigtime. Fasting becomes easy after that and I have to stop myself doing more than 2 days on 5-2 because otherwise my wife gets irritable, not me. She doesn’t like me not sharing meals and losing weight without her.

  • Theraveda

    “… particularly when one considers alternative restrictive options that dominate every day of the week.”


  • I’m not saying that it would work for everyone. But, for me, the 5:2 diet was the only thing that worked for long term. I realize that the 4:3 would succeed in more weight loss, and obviously even more for the every other day IF. But I want something that I can stick to, for years to come. And, honestly, I’d rather starve for 2 days a week than to be slightly hungry every day. (Plus, the 5:2 feeds my slight addiction to wanting to starve myself without sending me to the hospital. Again.) I know the 5:2 doesn’t end in any more weight loss than the standard diet that keeps women like me constantly worried about eating right. That’s stressful. But it does help with mental clarity–something that I desperately need, since I have brain fog as a mental illness D:
    While I wish I could continue the low-carb diet I had been on for 2 weeks, I had to quit, for 3 reasons:
    1.) Keto isn’t supported by a poverty budget.
    2.) Frequent bouts of low blood sugar (along with my sister’s insistence that I eat fruit, because she didn’t want me to end up in a coma)
    3.) My (cringes) odor and complexion were more pleasant when I was eating a variety of fruits, on a regular basis.
    Maybe I have an easier time, dealing with the starving days, because I’m sort of used to it. But I’d rather deal with something that will be over quick, even if it causes some extra pain. I’ve been through worse. Besides! On the days that I fast, my stomach will feel “tight” to where the next 2-3 days it will be hard for me to overeat. Portion control will suddenly feel easy, instead of oppressive.
    Like I said, this diet will not work for everyone. But it seems to be made for others, like me.

  • This is ludicrous. I fast most days until late afternoon/early evening, and I am not enervated, desperate, obsessed, or any other negative thing. Instead, I’m amazed at how few calories I can cope with — happily — as long as I have something nice to drink (in my case: Typhoo tea).

  • I fast every day in that I eat only one meal in the evening and nothing else till the next evening meal, the next day. I am not miserable, and I do not smell. I train daily, I function normally all day, and I feel terrific.

    • polidorisghost

      I lack discipline C, but I have other qualities.

  • GregGrimer

    I eat a large bowl (like a mixing bowl) of salted boiled/steamed spring greens and a tin of red salmon broken up a mixed into it. That get’s me through to 6pm, then I have a single sweet like a sugary chew 30 calories just to boost energy levels, plenty of unsugared black coffee during the day. Not finding the fasting hard. The problem is that I don’t enjoy eating the next day.

  • Biba Baba

    It’s not that bad at all, it’s maybe hard the first few times you do it but I was doing the 18:6 and 5:2 without even knowing it when I was younger and then my boyfriend thought I have to eat dinner and hooked me on eating too much and I’m doing it again… I called it screwing with my system (shock thing – you don’t let it lower you metabolism). I usually eat one yogurt or piece of bread for breakfast (something that’s around 100-150 calories) and then eat lunch which is usually around 400-500 cal.. Not much of a difference if you’re used to 18:6, just smaller portions and no snacks.
    Your body gets used to eating in a set time frame (I eat late breakfast and lunch, sometimes a snack before the time is up – in my country lunch is the size of dinner), if I skip breakfast for 2-3 days, I’m not hungry at that time anymore for instance.
    Anyway, being hungry is no big deal and we have to accept that, it’s healthy even. Our brain functions better, your whole body does. It’s not all about working out (have a hobby, not a chore), running ruined my friend’s heart, another friend’s joints, another friend’s toes etc.
    I started this method by fasting completely for one day and a half and then started 18:6 combined with 5:2 but I only follow the second if I have/want to loose weight (big fancy dinner/going out and you have to eat out of your time frame the day before). People often say they feel tired so let me describe my day: Woke up, 2h pass “I’m hungry”, I distract myself, lunch “I’m not going in the kitchen”, 2 hours pass “I’m sleepy”, I go rest but not sleep and watch a series, 1 hour passes I have company “geez I’m sweating, I never sweat” and after that my energy levels were through the roof, couldn’t even fall asleep for a good time, woke up the same. The hunger pain is much worse when you’re hungry for a short time, it’s just the fantasizing about food that gets you (at least in my experience, once you eat – normal portion – you forget about all the other cravings).
    About lunch (or dinner) depending on you habits I suggest you make yourself some plain hot tea to substitute a hot meal – it makes it easier.
    Humans are designed to need much less (than 4 legged animals for instance) than we consume these days…
    And to those saying cutting out carbs (or something else you like/love) and following a diet is easier, I assure you most people find it easier to get used to a 18:6 (It really isn’t hard, you won’t feel hunger at the fasting times when you get used to it – you get there fast) and/or 5:2 routine than following a specific plan every day you have to force yourself to do each day. With this you’re helping your brain and body and you can eat whatever your little healthy heart desires.

  • I found restricting myself to 500 calories impractical, because of the obsession with food that the author mentioned. You are constantly thinking about hat you’ve eaten, how many calories that represents, how many you have left, what that means you can eat, when would be a good time to eat it, how long you have to wait.

    I found it’s easier to eat nothing. If you can’t eat, the distress is removed. A Cup-a-soup before bed is a pleasant reward that gives you something to look forward to.

  • Mariette Ferreira

    I’ve been doing 5:2 for a few months. Easy, no fuss. I thought it would be hard, but the human body is so adaptable. I’ve lost weight and I’ve been feeling excellent. Fruits works better than vegetables, but you can eat an entire pot of gazpacho soup and still not reach 500 calories – you just gotta plan it.

  • ToddM

    It is the bomb! 2 days of restriction and 5 days of living your life. Weight loss is slow and steady. Very manageable. Lot of greens and veggies and meat on fast days. Down 12 lbs in 7 weeks.

  • ken_lov

    I’ve never experienced any of the side effects the author mentions. Skipping meals two days a week soon became a normal part of my life. Weight, blood pressure and blood sugar have all benefited and most importantly, stayed that way because the 5:2 plan is not a “diet” but a permanent change of lifestyle.