Following the success of the 5:2 diet, Michael Mosley's follow-up weight loss plan has once again grabbed the headlines. Discover what the Fast 800 diet is, the claims behind the headlines and whether the plan might be right for you.


What is the Fast 800 diet?

Developed by Dr Michael Mosley of Two’s Trust Me, I’m a Doctor, the Fast 800 diet claims to reduce your risk of type 2 diabetes, heart disease and cancer. For those with type 2 diabetes, because stage one of the plan involves a very low-calorie diet, it may put your condition in to remission.

The diet is aimed at those wishing to lose weight, especially if you carry excess abdominal fat, as well as those with blood sugar issues, including pre-diabetes and type 2 diabetes.

Visit our ‘All you need to know about diets’ page for recipes and more expert advice on weight loss, including low-GI and the Mediterranean diet’

How to follow the Fast 800

The Fast 800 is a combination of intermittent fasting and a Mediterranean-style eating plan, with optional meal replacement shakes.

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The plan starts with a rapid weight loss phase: this lasts between two and 12 weeks, during which you follow a daily 800-calorie eating plan. You do this by following a moderately low-carb, Mediterranean-style diet, with lean protein sources and vegetables. On days when cooking is not possible, meal replacement shakes are available – these are high in protein, fibre and healthy fats. The plan recommends no more than two shakes per day or 10 per week, with the emphasis instead placed on whole, natural foods. Those adopting this stage of the plan typically have either a lot of weight to lose, require fast results, have reached a weight-loss plateau or have type 2 diabetes. This initial stage is designed to switch the body into a mild ketosis.

The second stage of the plan involves intermittent fasting, restricting calories to 800 a day for two days of the week, then eating a healthy, low-carb, Mediterranean diet for the remaining five. This phase of the plan offers an opportunity to embrace home cooking, reduce processed foods and focus on vegetables, wholegrains, nuts, seeds, legumes, healthy fats and lean protein. The plan recognises that sustaining protein intake is important to maintain muscle mass and to support your metabolism and it encourages time-restricted eating to enhance the benefits of an overnight fast.

Once you've achieved your goal weight, you enter the maintenance phase. This phase is designed to fit personal goals and to work with your lifestyle; you'll continue with the Mediterranean-style of eating, following a low-sugar diet with limited amounts of starchy carbs and including a weekly fast day, if needed.

How does the Fast 800 work?

The first stage of the diet works on the premise that when we eat refined, typically 'white' carbohydrates, like white bread, pasta and rice, they are easily broken down in the gut to release sugars which, over time, leads to fat accumulation and possible insulin resistance. The aim of this stage of the diet is to normalise liver and pancreas fat and promote regular insulin response and blood glucose management. Subsequent stages of the plan promote a moderately low carb, Mediterranean-style diet claiming that this will help restore the body's ability to reach and maintain its ideal weight and reduce the risk of chronic disease.

What foods to include

As you progress through the stages of the plan you are encouraged to focus on the following foods:

  • Vegetables, all colours
  • Wholegrains
  • Nuts and seeds
  • Legumes
  • Eggs and dairy (full fat)
  • Fish
  • Lean meats

What foods to avoid

  • Processed foods, including breakfast cereals
  • Starchy, refined 'white' carbs
  • Added sugars
  • Low-fat foods
  • Sweet, tropical fruits
  • Snacks

Is the Fast 800 effective for weight loss?

Very low-calorie diets (VLCDs), such as those supplying 800 calories or less, can lead to rapid weight loss, but may not be suitable for safe for everyone and they are not routinely recommended. Typically, these sorts of diets are only followed if you have an obesity-related complication which would benefit from such rapid weight loss. The NHS advises that, for most people wanting to lose weight in a safe and sustainable way, we should reduce our calorie intake to 1,900 calories for men and 1,400 calories for women.

Dr Mosley claims that following the rapid weight loss stage of the Fast 800 may help you lose as much as 14kg over 8 weeks. This does depend on your start weight, how much you need to lose and assumes you remain motivated and committed throughout this challenging phase. Stage two, it is claimed, offers a more sustainable period from which to progress weight loss but at a lower rate.

Is the Fast 800 diet safe to follow in the long term?

The NHS suggests that, should you choose to follow a VLCD, you should do so for no longer than 12 weeks and under the guidance of a suitably qualified healthcare professional. Dr Mosley also cites 12 weeks as the maximum period for the rapid weight loss stage of the Fast 800 and that, thereafter, a moderately low-carb Mediterranean diet is adopted.

Restricting calories over a long period of time will limit your intake of nutrients and energy, especially if you follow the 'real food' version of the plan. Meal replacement shakes are typically fortified with vitamins and minerals and supply protein and essential fats which may help fulfil your nutrient requirements. However, they are still a tool for short-term use only.

Although there are no reported Fast 800 trials, the DiRECT study led by Professor Roy Taylor, which inspired components of the Fast 800 plan, continues to monitor participants. Following their second year, the DiRECT study suggested that intensive weight management has the potential to reduce or delay the complications of type 2 diabetes.

What is the evidence for the Fast 800?

The Fast 800 diet brings together the concepts of Michael Mosley's previous books, including the 5:2 diet and the Blood Sugar Diet, and combines them with knowledge gained from research relating to intermittent fasting and low-carb or low-calorie diets.

Mosley used data from the research performed by Professor Roy Taylor of Newcastle University including the DiRECT study, funded by Diabetes UK, to support his design of the Fast 800. This study used an 800 kcal liquid diet for a duration of 3-5 months. Likewise, the DROPLET study used total meal replacement shakes providing 810 kcal per day for a period of 12 weeks. Like the DiRECT study, its findings suggested greater initial weight loss and potential 'diabetes remission' for those in the intervention group compared to the control. Follow up of the study group suggested weight loss may persist for a period up to three years but other health improvements, with the exception of blood pressure, were not sustained.

However, there is limited research to support the Fast 800 diet itself and nothing to suggest that is it superior to equivalent energy-restriction diets. While the evidence suggests intermittent fasting diets may be an effective way to lose weight, it appears to be no more effective for weight loss than traditional methods of dieting.

Is the Fast 800 healthy? A nutritionist's view...

One of the goals of the Fast 800 is to target metabolically active abdominal fat – the type of fat that puts people at increased risk of lifestyle diseases, such as type 2 diabetes. If this is relevant to you, you should speak to your GP because the suitability of the Fast 800 diet would depend on your personal circumstances. For the general population, extended periods of time on just 800 calories per day is not appropriate or recommended.

Adopters should be aware that the rapid weight loss stage can be intense and may have a significant impact on blood sugar levels, so if you are prescribed diabetic medication or are on insulin, you must discuss the plan with your GP and secure their ongoing support and monitoring for the duration. During this first stage, you may experience side effects – the most common are headaches, constipation or tiredness. This is typically due to dehydration, so the plan encourages you to increase your intake of calorie-free liquids to 2-3 litres per day.

If you choose not to use a meal replacement product during this initial stage, micronutrient intake may be low and a vitamin and mineral supplement may be needed to support your nutrient intake. Meal replacement products, such as shakes, are designed to help stave off hunger so if you do elect to follow the 'real food' version of the plan, you may also be challenged with hunger pains. It is is also worth noting that, although the Fast 800 diet is based on a moderately low-carb, Mediterranean-style of eating, the latter is typically not low in carbohydrates.

Intermittent fasting: a clock is laid on a plate with a knife and fork next to it

Who shouldn’t follow the Fast 800 diet?

It is advisable to refer to your GP before starting a new dietary regime especially if you are under 18 years old, elderly, have a pre-existing medical condition, including type 2 diabetes or are on medication (especially if that medication needs to be taken with food and at set times).

You may need your GP’s support to make changes to your medication and to monitor your blood sugar or blood pressure levels as you progress through the plan. You should also consult your GP if you’re taking warfarin, are on blood pressure tablets, are pregnant, breastfeeding or undergoing fertility treatment.

In addition, The Fast 800 Diet is not recommended for people who are underweight or have an eating disorder or a history of one, are type 1 diabetics or on insulin, have had a heart condition or are recovering from surgery. The 800kcal restriction is also not appropriate for anyone participating in endurance exercise.

Check the Fast 800 website for a more comprehensive list of people who should not undertake this diet.

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The BBC Good Food podcast - Fasting for weight loss

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 your health.

This article was reviewed on 27 September 2023 by registered nutritionist, Kerry Torrens.

Kerry Torrens BSc. (Hons) PgCert MBANT is a Registered Nutritionist with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including Good Food.


All health content on 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 healthcare professional. If you have any concerns about your general health, you should contact your local healthcare provider. See our website terms and conditions for more information.

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