UK United Kingdom

Here’s the skinny on fasting for weight loss – the 5:2 diet

The “new” weight-loss strategy known as the 5:2 diet has been receiving much attention in the media since the book The Fast Diet: The Secret of Intermittent Fasting - Lose Weight, Stay Healthy, Live Longer…

People on the diet need to restrict intake of calories on fasting days. Martin Lee

The “new” weight-loss strategy known as the 5:2 diet has been receiving much attention in the media since the book The Fast Diet: The Secret of Intermittent Fasting - Lose Weight, Stay Healthy, Live Longer was launched late last year.

The 5:2 diet allows you to eat as usual for five days and to fast for two days. On fasting days, the dieters need to restrict intake of food to approximately 2000 kilojoules (500 calories) a day for women or 2400 kilojoules (600 calories) for men.

The two days of fasting don’t have to be consecutive and you can decide how you want to spread your food intake on those days as long as you adhere to energy restriction. The food consumed during the two fasting days should have little fat and carbohydrate content and alcohol consumption is not recommended.

During the two fasting days, you are typically allowed protein foods such as eggs, or low-fat yogurt or cheese for breakfast and protein foods such as chicken, fish, lean meat, along with salad or other non-starchy vegetables for lunch or dinner. You are permitted water, green tea, or black coffee. While you can have milk with your beverages, it must be counted toward your caloric intake.

Not a fad?

Intermittent fasting or restricting energy intake for weight loss, which is what the diet is based on, is not a new concept. And there are other kinds of fasting diets around, such as “alternate day fasting”. But while energy restriction in the form of various weight-loss diets has been investigated in both humans and animals, there’s little research regarding the utility of intermittent fasting in humans.

A 2011 study in the United Kingdom that investigated the effects of intermittent energy restriction (to approximately 2266kJ a day for two days) compared to continuous energy restriction (approximately 6276KkJ a day for seven days a week) over six months, in 107 young overweight or obese women. It reported that both diets were equally effective for weight loss, as well as other markers of good health.

But there seemed to be potential difficulties in adherence. At the completion of the study, only 58% of the women in the intermittent fasting group planned to continue with the diet, compared to 85% of those in the energy-restricted group.

This study was one of the largest undertaken in this area so far and the few previous studies in the field have had a much smaller number of participants. Although these smaller studies have been conducted for shorter time periods, the UK study is also considered to be relatively short term.

Weight loss within the first six months is common with a lot of different types of diets. But research studies have shown that the majority of people put much of the weight back on within three to five years.

Many people who tried the 5:2 diet reported weight loss but did the weight stay off? Nata-Lia

Need for caution

Many people who have tried the 5:2 diet report that they have been successful in losing weight but this is the case for most weight-loss diets in the short term. The issue of long-term compliance with the two days of energy restriction remains unresolved, as does long-term weight maintenance because people usually are not able to keep to their new weight.

Difficulties in adherence resulting in weight regain may encourage some people to try another dieting attempt and this can lead to the cycle of weight loss and weight regain being repeated. This happens in most cases of dieting-related weight loss.

The risks or the potential to overeat or gorge on non-fasting days also needs to be investigated. Diet quality is of particular significance for those who fast intermittently to ensure that all nutritional requirements are met and that the intake of some nutrients that have low intakes anyway (such as calcium) is not further compromised.

What’s more, we still need to investigate whether intermittent fasting is a safe weight-loss strategy, especially for people with diseases such as diabetes. Starvation-type diets have side-effects such as dehydration, anxiety, irritability, tiredness and lethargy and whether we should be looking out for these in the 5:2 diet remains to be determined.

Intermittent fasting is reported to be effective among those who have used it for weight loss and it seems to be as effective as an energy-restricted diet in the short term. It may be a viable weight-loss option for some people but we need to research its effects beyond those reported, especially since many of these effects are anecdotal at present.

It’s best to follow healthy eating dietary guidelines and seek advice from your doctor before embarking on intermittent fasting as a weight-loss strategy.

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37 Comments sorted by

  1. Gary Cassidy

    Monash University

    Interesting, thanks.
    Since physical activity is a great thing for our health (exclusive to diet), the effects of 5:2 diet on physical activity should also be considered.
    Does anxiety of energy conservation in the day prior to fasting discourage activity?
    Do the fasting days result in less impuse fo r physical activity?
    Is there carry-over lethargy in the days post fasting which lead to lower pysical activity?
    Are some people already sedentary enough that for them it doesn't matter?

    1. Barry Walker


      In reply to Gary Cassidy

      I have more-or-less followed the 5-2 diet over the last six weeks for the possible health benefits. I'm not religious about the fasting days but usually manage to restrict calories to about 600. I've lost about half a kilo in weight and about a cm or so of belly girth, which is more important to me.

      I have well controlled Type 2 diabetes so I'm watching blood sugar levels with interest. So far I seem to be seeing a small decrease. I'll get a blood test in a few months to check cholesterol and lipids. I'm hoping to drop the statin and metformin regime but I'll be monitoring the results closely.

      Interestingly I find no decrease in energy and fitness levels - in fact possibly the reverse. So in answer to Gary's question, it doesn't seem to be an issue in my case. I'm a little more eager to get out there and walk or exercise.

      Although I don't look forward to the fasting days, I can contemplate continuing indefinitely if I see benefits in the blood chemistry.

    2. Andrea Shoebridge

      logged in via Facebook

      In reply to Barry Walker

      I also have found I have more energy after beginning the 5:2 regimen a couple of months ago and, as per comment below, eat more discerningly on the five days than I used to, not consciously but because that's what my body wants. I have no idea if I've lost weight, but some girth has gone although, as with other comments, it was the preventative benefits for assorted health conditions that enthused me to begin 5:2 rather than weight loss

    3. Peter Horan


      In reply to Gary Cassidy

      I have followed the 5:2 diet since Michael Mosley's (sp?) documentary on 22 April. I have lost about 5Kg without ill effect. Clothing fits better. As yet, I do not know if this rate of loss will slow down and plateau or not.

      The advantage of the 5:2 diet is that the fast day is movable. If one day does not suit, the next day may. Further, on a fast day, I have the same breakfast as on a normal day. This actually means that I can change my mind either way at lunch time. In the long run, I believe…

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    4. Greg Spurgin

      Self employed

      In reply to Gary Cassidy

      I have been on the 5:2 diet for 7 weeks. I find it liberating and so easy to do. I have consistently lost 0.5 kg per week. On my fast days (which get easier the more you do them) I still work, ride by bike, and do other exercises. I hope when my weight is down to my correct BMI I can drop back to 6:1 or one fast day a week, which I feel I can easily maintain for the rest of my life. The article as others have mentioned does not talk about the other health benefits. Its hard to quantify but I feel better as a result of the fasting.

  2. Susan Lawler

    Head of Department, Department of Environmental Management & Ecology at La Trobe University

    This article misses the main point of the 5:2 diet, which is that intermittent fasting decreases the risk of cancer, diabetes and Alzheimer's. Low calorie days give the body a chance to go into repair mode which encourages the growth of new brain cells and alters our body chemistry, a benefit not found in standard calorie restriction diets.

    A review of the science behind this can be found at

    1. Rosemary Stanton

      Nutritionist & Visiting Fellow at UNSW Australia

      In reply to Susan Lawler


      It might be a good idea to read the actual study in your reference. It's available at

      The reference is a review comparing calorie restriction and alternate day fasting (which is more than recommended in the 5:2 diet). It notes the promising effects of alternate day fasting in animals, with studies showing modulation in several risk factors for chronic disease and noting that it may (in animals) have similar effects to calorie restriction…

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    2. Edward John Fearn

      Hypnotherapist and Naturopath

      In reply to Susan Lawler

      Sorry Susan, but your link didn't seem to work.
      The idea that we can reduce our risk of Alzheimer's disease and other forms of cognitive decline by restricting our calorie intake is certainly fascinating.
      The following studies have shown that Calorie restriction reduces Alzheimer's disease type beta-amyloid neuropathology in both mouse and Squirrel monkey models.

      And it also has been shown that caloric restriction improves memory in elderly humans

       In evaluations of midlife obesity, an increased risk of dementia was found for obese individuals.

      Yet paradoxically in later life underweight persons had an increased risk of dementia .

      So while CR does show show promise in this area I think there are still many questions left unanswered.

    3. Dr Graham Lovell

      logged in via Twitter

      In reply to Rosemary Stanton

      Thanks for the improved web reference.

      As you say, this study is a comparison of calorie restriction with alternate day fasting (ADF). It takes as its starting point, "Calorie restriction (CR), defined as a reduction in energy in-take without malnutrition, has been shown to increase life span, improve numerous functional indexes, and reduce metabolic risk factors for chronic disease in several mammalian species".

      On the other hand, the thing that was most intriguing about the BBC presentation…

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  3. Vince Schubert

    logged in via Facebook

    Surinder, i see that your forte is nutrition and dietetics, but you entirely omitted (probably) the most significant aspect of this regime. (i hesitate to call it a diet)
    The research (from the BBC Documentary - emphasised the lowered risk of some cancers and other dis-eases. The lowering of cholesterol and triglycerides, and thus the reduced chance of diabetes may be one of the first benefits achieved.

  4. Chris Booker

    Research scientist

    As someone who's spent more than a decade working in diabetes research it's really encouraging to see an article like this which offers a more balanced perspective than a lot of coverage lately. This intermittent fasting diet looks set to become the next big fad, a la Atkins, low carb, etc. diets.

    There really is very little evidence of any differences in health outcomes as yet, and I'm you noted that almost any diet has been shown to produce short-medium term results (up to about 6 months), but…

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    1. Rosemary Stanton

      Nutritionist & Visiting Fellow at UNSW Australia

      In reply to Chris Booker

      I agree. This method of eating may work for those who can continue to eat moderately for 5 days a week. I don't have a problem with people finding what suits them. I do have a problem with claiming results that have not been shown in any long term studies. And I especially have a problem in extrapolating short term effects (which may be related to weight loss rather than the method of weight loss) and making claims about long-term reduction in serious diseases. If and when long-term, well designed studies show such results, only then is it valid to make such claims.

    2. Dr Graham Lovell

      logged in via Twitter

      In reply to Chris Booker

      Although I participate in the 5:2 diet, I agree that it can probably be assigned to the "fad" category. This is because, like all diets, it involves some measure of restraint, and this is not natural for any of us. In addition, it is a new approach, and it is in the news. How much more "faddy" could it get?

      On the other hand, there are countless millions of people who carefully and voluntarily watch their food intake and maintain a very healthy shape. For them it is not a fad, it is a lifestyle.

      If we were able to find the mechanism to move from "fad" to lifestyle, and to have a healthier population because its members did not over-eat, then that would be a good result, wouldn't it? Perhaps the 5:2 diet provides this mechanism for some. We will have to wait and see.

  5. Claire Edwards

    logged in via email

    I would like to comment on this article, as a participant in the 5:2 'diet'. I have been doing it for several months now, and I have lost approx 6 kilos (with 4 more to go to be in the healthy weight/BMI range). I have also lost weight before and failed to keep it off, with Weightwatchers and by doing a gym challenge which involved unrealistic levels of exercise as well as dieting. I do still exercise regularly and try to eat well on non-fasting days, but do not restrict particular foods at all…

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  6. Dr Graham Lovell

    logged in via Twitter

    I have been on the 5:2 diet for about 6 months. I have lost 3cm around my waist, being half-way to my 6cm loss target.

    Most of all, I like the freedom it gives you to eat on the non-diet days. Not to over-eat, but just to enjoy my food. I often feel hungry, which I didn't when I ate full meals every day, and this is a pleasant sensation.

    However, I found I needed carbohydrates on the calorie-restricted days in order to keep my energy levels up, so I have a single sandwich for lunch, with…

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  7. Carol Chenco

    Research Officer

    I have been doing the 5:2 diet for 7 weeks for the health benefits after watching the BBC Horizon documentary and reading Michael Mosley's book - I realise the science doesn't stack up yet but feel that a couple of days a week of restricting what I eat can only be beneficial given all the problems of overeating fat and sugar in western diets. I don't need to lose weight but have lost 3 kgs in that time and am going to just do it for one day per week so I don't continue to lose weight. I take calcium and also iron tablets as I don't consume meat. My exercise regime has not been affected by the change in eating pattern. The only negative effect apart from missing out on my latte in the morning, is constipation is starting to become a problem and so I have increased my bran intake at breakfast on my 'food' days.

    1. Gary Cassidy

      Monash University

      In reply to Carol Chenco

      "The only negative effect ....., is constipation"

      Perhaps an abundance of very low calorie vegetables on fasting (and non-fasting) days could help?

    2. Carol Chenco

      Research Officer

      In reply to Gary Cassidy

      Yes, I think you're right Gary, thanks. I had increased my consumption of vegetables / salads but reflecting on it, I could increase it further.

  8. Ian Burns

    Company Chairman

    I've been on 5:2 since seeing the Michael Mosley program on SBS (early May). I'm not doing it for weight loss - not necessary - but for the other reasons, the chief of which was the tantalising promise of growing additional neurons! I do two or three long walks a week and play competition badminton weekly, and so far have not noticed any lack of energy. As far as the neurons are concerned...hhmm...what was I talking about...

  9. mhairi fraser

    logged in via email

    I have been on the 5:2 diet for 2 months because I am interested in the health benefits firstly and maintaining healthy weight. I have found the reverse of the concerns expressed in this article: An immediate increase in energy, vitality and clarity of thought; I experience no urge to 'gorge' on the non fasting days in fact I have become more naturally discerning about eating nutritionally balanced food. I have found that what we interpret as hunger is often thirst and so I am drinking more water. Hunger pangs subside very quickly and physical activity on fast days actually appears to be easier!

    All round the 5:2 is easier than I expected and I anticipate it being part of my lifestyle for the rest of my life. My husband who has never gone near a diet in his life has found it extremely beneficial. The weight loss is a nice by product of an approach to health that is very satisfying. I look forward to more research over time with this approach.

  10. Gregor Napier Cutlack


    I'm so relieved to hear of other people who've started the 5:2 eating pattern.
    My wife and i go Monday and Tuesday and adhere to the 600 cals. (i've invented some delicious 600 cal meals), well delicious if you haven't eaten all day.

    With a different outcome after six weeks we both feel a lot better as a result.
    My reasoning so far has been not so much on the weight loss side but in the long term health benefits attributed to the 5:2
    My experiences have varied widely from week to week depending…

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  11. K C Potger

    Healthcare worker

    From an evolutionary biological perspective, it makes perfect sense to subject the body to periods of fasting – and even occasional starvation. It is a recent novelty for our stomachs to always contain food; causing chronically high levels of glucose – and thus insulin – in the blood. There is also a whole suite of hormones that are waiting to be switched on when blood glucose drops.
    We have evolved an endocrinology to deal with the feasting and fasting lifestyles of our forefathers. Likewise, we are ‘designed’ to walk all day – with health benefits seen by regular walking; I would anticipate health benefits from regular fasting.

  12. Roger Hawkins

    ICT Education Project Manager

    One of my main aims for this diet was to reduce Blood Pressure and lose some weight. Prior to this I had been on medication Diovan 80 mg, my BP with medication had dropped and my Doctor suggested I try and manage it with diet and exercise. This I failed to do. Months down the track of excess food (Calorie intake) no exercise and eating food high in fat, processed food, alcohol, salt, I could tell that my BP was very high and would need to get it checked again, knowing I would have to go back onto…

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    1. Rosie Hayes


      In reply to Roger Hawkins

      Roger Hawkins: Your sentence: "The Fast Diet is a mechanism for change from the poor dietary habits which are endemic in our society". This sentence, in my opinion, says it all. I, personally, do not need to wait for proven statistics. If it makes you feel better, if you actually feel real hunger and if it changes your whole outlook - it can only be good. I, too, am on the 5:2 eating plan and I, too, plan to continue.

  13. Raine S Ferdinands

    Education at Education

    This diet is an ancient practice. Practising Buddhists fast once a week all through their lives from the age of 12. My maternal grandmother was a Buddhist and (without fail) fasted on Fridays and ate no meat, fish or poultry on Thursdays (as pre fast days). She kept to this practice all through her life even though she married a meat loving, beer craving Anglican. She was slim, calm and enjoyed life and gardening. Her complexion was spotless. She lived to the ripe age of 98 with no physical or mental ailments; just never woke up one morning. My fun loving grandfather, on the other hand, passed away in his 50s after a sudden heart-attack. Fasting is also a common practice among some Jews and Orthodox Greeks. As for me, I love food and am willing to drop dead at any time I'm called; no regrets.

    1. Michael Leonard Furtado

      Doctor at University of Queensland

      In reply to Raine S Ferdinands

      Raine, not to intrude because I've enjoyed your posts (so long may you live!) but are you perchance related to Keith and Iona Ferdinands?

      (I would have preferred not to ask in an open-forum, but I've no alternative. I promise not to persist with this, beyond receiving Raine's reply, as these were good friends of my family once upon a time and, while fun-loving too, long may they have also lived! My contact details are on my profile.)

      Incidentally, I don't regard these two attributes as mutually exclusive: I know of austerity fanatics who die young and sybarites who lead long and enviously productive and wholesome lives.

    2. Raine S Ferdinands

      Education at Education

      In reply to Michael Leonard Furtado

      Sorry I am unable to assist you in your search for long lost friends, Michael. I have no idea who Keith / Lona Ferdinands are. Good luck in your search, though.

  14. peter davson-galle

    academic philosopher

    Interesting. I adopted a variation on this about 2 months ago; in my case it is a one day (36 hours including 2 sleeps) total food fast each week - but with quite a lot of water drinking.
    I asked my GP about it in passing & he couldn't see what harm it'd do me. I've lost about 9 kilos & notice no lethargy or other ill-effects; indeed, I try to be fairly active on that day. I feel better in a number of ways but, for all I know, that is just the weight loss.
    As observed, the literature seems scrappy; but if all it does is knock 10 kilos off in a (for me, anyway) dead easy regimen, then I'll just keep doing it until the literature waves danger flags relevant to my medical profile.

  15. Joseph Podosky

    Company Director

    I've been on the 5:2 diet for about 8 weeks now and have lost 5 kilos. And I feel better for it.

    I agree that people on diets usually go back to their normal weight if they don't maintain the diet. However, to me, the big point of difference with intermittent fasting is that it is not really a diet rather it's a lifestyle.

    I have every intention of maintaining the 5:2 approach to eating from here on out.

  16. Teresa Sullivan

    HR Asst

    I have been on the 5-2 diet for 3 weeks now and this is first diet I can honestly say I love!. I have been a yoyo dieter for over 30 years and I have found this one easier that any other I have attempted. I have lost 5 kilos in 3 weeks. Yes I know what you may be thinking...too fast, too fast. But I cant help what I am losing. I am sticking to 500 calories on my fast days and normal eating on my feast days. I must admit though I do not want to overeat on my feast days as I don't want to waste…

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  17. Alissa Holder

    Australian Youth Ambassador for Development (Samoa)

    I'd be very interested to hear from proponents of the 5:2 diet about what you eat on a typical fasting day. I've never counted calories and wonder what 500 equates to in actual food terms.

    I'm also curious as to how you sleep the night after fasting? Do you find you wake during the night craving sugary foods (suggesting a drop in blood sugar levels)?

    1. Dr Graham Lovell

      logged in via Twitter

      In reply to Alissa Holder

      I am not a proponent of the 5:2 diet, but a participant (and not a medical doctor!).

      On a fasting day, I eat one weetbix for breakfast, tomato sandwich for lunch, and a salad with a "touch" of tuna or salmon for the final meal, or a vegetable based soup.

      I don't wake during the night (I am a good sleeper), and I don't wake up hungry.

      I don't exercise much on fasting days, as it makes me want to eat for energy (psychological?). Others are happy to continue with their normal exercise routines on fasting days. My wife doesn't notice any difference with her metabolism on fasting days.

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    logged in via Facebook

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  19. iain morrissey

    PDHPE Teacher

    Liked your article, but I think you have missed probably the strongest benefit of this diet and that is, it's gentle and slow modification of your eating habits.
    I have been doing this for a little over a month now, trying to drop from 84 to 78 kg. As an active person, i have restricted my intake on fasting days, to about 4000KJ or for the 'metricly disadvantaged' around 950Cal. I noticed that I am thinking more about what I am eating and on fasting days i'd rather eat the KJ then drink…

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    logged in via email

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