What is an intermittent fasting diet?

Is fasting healthy and is it an effective and safe way to lose weight? We discuss the benefits of fasting and the difference between the 5:2 and 16:8 fasting diets.

Alarm clock on a plate with knife and fork

If you’ve ever considered following a diet, make sure you have all the facts first. BBC Good Food and nutritional therapist Kerry Torrens take a closer look at intermittent fasting (IF) – what it is, the claims behind the headlines, and whether or not it's healthy...

What is intermittent fasting (IF)?

Fasting has been used throughout history and across the world for religious, cultural and spiritual practices. In recent years, the media attention given to diets such as the 5:2 diet have made fasting a popular choice among those wishing to lose weight without having to give up particular foods.

IF focuses around the periods of time that we are not eating – ‘fasting’. The frequency and duration of these fasting periods depends on the specific diet followed and the individual’s schedule. Some popular IF eating patterns include:

  • Choosing a time period in which to eat each day, while refraining from eating outside of this. For example, the 16:8 diet involves fasting for 16 hours per day and eating within an 8-hour window. One of the most common ways to do this is by skipping breakfast and only eating from midday-8pm, so you are fasting for 16 hours per day (between 8pm and midday the next day). Other variations of the diet involve 6-hour eating periods or shorter.
  • Choosing a regular day of the week or month during which to fast for a full 24-hours. For example, if you finish dinner at 8pm one evening, you would refrain from eating until 8pm the next day.
  • Choosing certain days of the week to consume very few calories, while eating a normal number of calories during the rest of the week. For example, the 5:2 diet involves eating only 25% of a normal calorie intake (500 kcals for women, 600 kcals for men) on two days per week, then eating a normal and unrestricted amount on the other five.

There are many proponents of IF diets, each with a slightly different take on the ‘optimum’ way to practice IF. Some of the most well-known include Michael Mosely, whose book The Fast Diet promotes the 5:2 method and David Zinczenko, who’s book The 8-Hour Diet promotes the 16:8 diet.

As well as weight loss, various studies have linked IF with a reduced risk of cancer and heart disease, an extended lifespan, some protection against age-related diseases and a protective effect against cognitive decline. This is still an emerging area of research and these links are as yet inconclusive. It is important to note that some of these studies have only been carried out with animals, and it is unclear at the moment whether all the benefits will be replicated in human studies.

We asked nutritional therapist Kerry Torrens for her view...

Is it safe?

Much of the emphasis of these diets is on the fasting stage, but in order to be a safe, effective and healthy style of eating, the food consumed during your 'eating windows' needs to be of high nutritional value. Aim to include essential fats from oily fish, nuts and seeds, lean sources of protein, wholegrains and starchy carbs and plenty of fruit and vegetables to supply dietary fibre, vitamins and minerals. 

Is it effective for weight loss and is it sustainable?

Studies suggest that when you compare IF with daily calorie restriction it is equally as effective at promoting weight loss in those who are overweight and obese. Results will vary, however, dependent on your individual circumstances and the amount of weight you have to lose. How effective it is in the longer term will depend on your ability to maintain this style of eating going forward. This is because IF is not so much a ‘diet’ but rather a programme of eating, which means sustaining the weight loss is all down to how well you adapt your eating patterns in the long term. 

Woman standing on bathroom scales

How do different versions of IF compare?

There are many variations of IF, with some approaches being more extreme than others. It’s worth noting that there is still a lot for us to learn about this method, including what the optimal fasting pattern and calorie limit is. If you’re considering this form of diet, you should first consult your GP or medical practitioner to confirm it is appropriate and safe for you. Many people find that a moderate fast, finishing all food by 7pm in the evening and then waiting until 8am the next morning before eating breakfast can offer a more sustainable approach, while still reaping some of the perceived benefits.

Who should be more careful when considering fasting?

If you are pregnant, breast-feeding, diabetic or have a condition that requires you to keep a close eye on your blood sugar levels you should avoid fasting. In addition to this, some groups of people are at greater risk of the negative effects associated with fasting. Such effects might include, but are not limited to, headaches and dizziness, an inability to concentrate, flare-ups of a long-standing health condition like gout or an alteration in the way certain medications are absorbed and used by the body. Vulnerable groups may include the elderly, the young (under 18 years of age), those who are on medication, those who have a low body mass index (BMI) and those with emotional or psychological issues around food, including any history of eating disorders.

Please note: if you're considering attempting any form of diet, please consult your GP first to ensure you can do so without risk to health.

Now read...

How to eat a balanced diet
Six things you should consider before starting a diet
All our popular diet guides


This article was published on 16th May 2017.

A registered Nutritional Therapist, Kerry Torrens is a contributing author to a number of nutritional and cookery publications including BBC Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).

All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

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