The 5:2 diet means eating 'normally' for five days out of seven and then 'fasting' on the other two days out of seven.
During the 'fasting' days, men should consume no more than 600 calories per day and women no more than 500 calories. The proposed health effects of the 5:2 diet include weight loss, improvement in life expectancy, protection against certain cancers and protection against dementia. However, the evidence supporting the 5:2 diet is currently limited. More research is needed to look at the long-term risks and benefits.
What is the basis of the 5:2 diet?
The 5:2 diet is also known as 'The 5:2 Fast Diet™', or 'The Fast Diet', or 'Intermittent Fasting'.
It is based on the underlying principle of eating 'normally' for five days out of seven (not being particularly conscious of calories and not avoiding certain foods) and then 'fasting' on the other two days out of seven.
But 'fasting' does not mean that you eat nothing at all. In the 5:2 diet, it means eating around a quarter of your recommended calorie intake. Men and women have different recommended daily calorie intakes. A woman on the 5:2 diet should eat around 500 calories in total on a fasting day and a man should eat around 600 calories.
The 500 or 600 calories can be eaten throughout the day as snacks, or as one or two meals. And the two days of fasting do not have to be consecutive - they could, for example, be a Tuesday and a Friday.
It is recommended that good foods to eat on a 'fasting' day are foods high in protein and fibre which tend to fill you up more - so things like fish, meat and vegetables.
There are a number of people who support and promote the 5:2 diet. They include Michael Mosley who originally trained as a medical doctor and then became a producer and presenter for the BBC. Mr Mosley famously first followed the 5:2 diet whilst making a BBC Horizon programme in 2012. He has since teamed up with journalist Mimi Spencer and together they have written a book about 'The Fast Diet' and launched a website.
What are the proposed health effects of the 5:2 diet?
Supporters of the 5:2 diet propose that, as well as helping with weight loss, the diet can also improve life expectancy, protect against certain cancers and also protect against dementia.
The theory is that the fasting days lower the levels of a certain chemical in the body known as insulin-like growth factor 1 (IGF-1). It is thought that high levels of IGF-1 in the body accelerate the ageing process and also make ageing-related diseases such as cancer and type 2 diabetes more likely. Low levels of IGF-1 are thought to have reverse effects.
Supporters of the diet also suggest that because you are only fasting for two days out of seven, this means that there is never a long period when you feel like you are depriving yourself of tasty foods. They argue that this makes you more likely to stick to the diet and therefore have successful weight loss.
What is the evidence behind the 5:2 diet?
The evidence supporting the 5:2 diet is limited when compared with some of the other types, or ways, of dieting. Most of the research carried out so far has been around fasting on one or more days of the week (intermittent fasting) rather than the 5:2 diet model exactly.
The other problem with much of the research is that studies have not been carried out over long periods of time. They also tend to have involved small numbers of study participants, or have been based on animals rather than on humans.
One small study published in 2010 showed that women following a 5:2 type diet may possibly have a reduced risk of developing some chronic diseases such as diabetes.
A review of the evidence for intermittent fasting dating back to 2007 showed some promising results in terms of intermittent fasting and protection against heart disease, some cancers and type 2 diabetes. But the study authors concluded that more research is needed for definite conclusions to be drawn. Another review of the evidence published in 2012 suggested that intermittent fasting to control weight may possibly reduce the risk of breast cancer. But again, the authors concluded that more research is needed.
A study on some mice who had changes in their brains similar to those found in humans with Alzheimer's disease showed that mice who were undertaking intermittent fasting had a slower decline in their cognitive function than the mice in the study following a normal diet. (Your cognitive function includes aspects such as perception, thinking, reasoning and remembering.)
A wide-ranging review in 2017 showed that the 5:2 diet helped people to lose weight over a six-month period but was no more effective than diets in which energy was restricted every day. The authors concluded that further research was needed to assess the effects of the 5:2 diet over a 12-month period.
Are there any bad points to following the 5:2 diet?
Some people may find that it is hard to sleep on the 'fasting' days because they feel hungry. Others complain of bad breath, headaches, constipation, irritability and effects on their concentration.
Supporters of the diet suggest that drinking plenty of water can help to reduce the chances of most of these side-effects.
It should also be noted that eating 'normally' for five days out of seven does not mean that you should not eat healthily and stay physically active.
Are there people who should not follow the 5:2 diet?
There are certain groups of people who should not follow the 5:2 diet or carry out intermittent fasting. They include:
- Children and teenagers.
- People who have diabetes and who are being treated with insulin.
- People who are already underweight.
- Pregnant or breast-feeding women.
- People with a history of eating disorders.
- People who are otherwise feeling unwell.
It is recommended that anyone with a chronic illness or anyone taking regular medications (especially the medicine warfarin) should check with their doctor first before starting any kind of diet that involves fasting.
What other options are there to lose weight?
Other types of diet are available - for example:
There are other ways of changing your diet and altering your lifestyle to help you lose weight, such as increasing your physical activity.
You may find other leaflets in this series helpful, including:
Further reading and references
Obesity; NICE CKS, June 2015 (UK access only)
Johnston BC, Kanters S, Bandayrel K, et al; Comparison of weight loss among named diet programs in overweight and obese adults: a meta-analysis. JAMA. 2014 Sep 3312(9):923-33. doi: 10.1001/jama.2014.10397.
Harvie MN, Pegington M, Mattson MP, et al; The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011 May35(5):714-27. doi: 10.1038/ijo.2010.171. Epub 2010 Oct 5.
Hooper L, Abdelhamid A, Bunn D, et al; Effects of total fat intake on body weight. Cochrane Database Syst Rev. 2015 Aug 7(8):CD011834. doi: 10.1002/14651858.CD011834.
Noakes TD, Windt J; Evidence that supports the prescription of low-carbohydrate high-fat diets: a narrative review. Br J Sports Med. 2017 Jan51(2):133-139. doi: 10.1136/bjsports-2016-096491.
Guasch-Ferre M, Babio N, Martinez-Gonzalez MA, et al; Dietary fat intake and risk of cardiovascular disease and all-cause mortality in a population at high risk of cardiovascular disease. Am J Clin Nutr. 2015 Dec102(6):1563-73. doi: 10.3945/ajcn.115.116046. Epub 2015 Nov 11.
Patterson RE, Laughlin GA, LaCroix AZ, et al; Intermittent Fasting and Human Metabolic Health. J Acad Nutr Diet. 2015 Aug115(8):1203-12. doi: 10.1016/j.jand.2015.02.018. Epub 2015 Apr 6.
Headland M, Clifton PM, Carter S, et al; Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients. 2016 Jun 88(6). pii: nu8060354. doi: 10.3390/nu8060354.