A selection of fruit and vegetables with salmon, beans and nuts

What is the Fast 800 diet?

If you've ever considered following a weight loss diet make sure you have all the facts first. Our nutritionist takes a look at the Fast 800 diet.

Michael Mosley’s new diet has made recent headlines which have linked the regime with various health benefits. We asked nutritionist Kerry Torrens (MBANT) for her expert opinion.

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What is the Fast 800 diet?

Developed by Dr Michael Mosley of BBC Two’s Trust Me, I’m a Doctor, the Fast 800 diet claims to potentially cut your risk of type 2 diabetes, heart disease and cancer and, because stage one is a very low-calorie diet (VLCD), possibly reverse type 2 diabetes. The diet is aimed at those wishing to lose weight, especially if they carry excess abdominal fat, as well as those with blood sugar issues, including pre-diabetes and type 2 diabetes. Michael designed the diet based on the research performed by Professor Roy Taylor of Newcastle University including the DiRECT study, funded by Diabetes UK.

What does the Fast 800 diet involve?

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 recent research relating to intermittent fasting and low carb or low calorie diets.

The Fast 800 is structured in stages and starts with a rapid weight loss phase, which lasts for a minimum of two weeks and a maximum of 12. This involves following a daily eating plan that is restricted to 800 calories – either by using a VLCD meal replacement product which supplies 600 calories (normally equivalent to three shakes) combined with 200 calories of vegetables; or you can choose to eat real food (recipe ideas are included in the book), which comprise lean protein sources and vegetables. 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. Stage one is designed to switch the body from burning sugar to burning fat by prompting 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, lower carb Mediterranean diet for the remaining five days. This stage 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 support metabolism. The plan also suggests time-restricted eating to enhance the benefits of fasting.

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, with a weekly fast day, if needed.

An alarm clock on a plate signifying intermittent fasting

Is the Fast 800 diet healthy?

Whether the Fast 800 will be beneficial for you will depend on your circumstances. One of the goals of the Fast 800 is to target metabolically active abdominal fat – this fat puts people at risk of lifestyle diseases, like type 2 diabetes. 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.  

However, adopters should be aware that this 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 insulin you must discuss the plan with your GP and secure their ongoing support and monitoring for the duration of the plan. 

During first stage you may also 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 stage, micronutrient intake may be low and a vitamin and mineral supplement may be needed to support 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.

Is the Fast 800 diet effective for weight loss?

VLCD, those supplying 800 calories or less, can lead to rapid weight loss, but may not be suitable or safe for everyone and they are not routinely recommended by the NHS. Typically, they are only recommended if you have an obesity-related complication which would benefit from rapid weight loss. The NHS advise that for the majority of us 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 by following the rapid weight loss stage of the Fast 800 you may 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 of the plan offers a more sustainable period from which to progress weight loss at a slower rate.

A woman standing on weighing scales

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

The NHS suggest 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 low-carb Mediterranean diet is followed.

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 diet, continues to monitor participants. The study reported their second year findings which suggest that intensive weight management has the potential to reduce or delay the complications of type 2 diabetes.

Who shouldn’t follow the Fast 800 diet?

It is advisable to refer to your GP or healthcare professional 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. 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, are type 1 diabetics, have had a heart condition or are recovering from surgery. The 800 calories restriction is also not appropriate for anyone participating in endurance exercise.

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.


This article was published on 5 December 2019.

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 BBC Good Food.

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