The Fast 800
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Book review: The Fast 800 (Dr Michael Mosley)

The Fast 800 is one of the most recent diet books by Dr Michael Mosley, author of the famous 5:2 diet. His books focus on the use of “intermittent fasting” for weight loss, blood sugar regulation and general health.

The author

I first heard from Dr Mosley through his piece on high intensity interval training (HIIT, see video below).

I later learned about his 5:2 diet and know a few people who have done it. It wasn’t until I read his bio that I realised that Dr Michael Mosley became a psychiatrist after working in other fields, and then decided to move away from the medical profession. He is mostly known as a science presenter, journalist, producer of films and diet book writer. His books include The Clever Guts Diet, The Fast Diet, Fast Exercise and The 8-Week Blood Sugar Diet.

The Fast 800

The Fast 800 is the name of the book but also the weight loss program based on this book. Both claim to help lose weight, improve blood sugar levels and reduce risk of chronic disease. The number 800 refers to the number of calories to be consumed in the diet (more on this below).

The book


In the beginning of the book Mosley presents the rationale for the Fast 800 program. Based on the author’s research, a dietary intake of 800 calories per day can elicit rapid weight loss (up to 14kg after 8 weeks). This number of calories is more manageable than Mosley’s previous prescription of 500-600 calories for the second (“fasting”) phase of the protocol.

Composition-wise, the prescription is a lower carbohydrate, high protein Mediterranean-style diet. It’s also recommended to practise time-restricted eating to encourage ketosis.

Chapter 1: Why We Put on Weight

Mosley points at high insulin levels due to constant snacking particularly of refined carbs as the culprit for fat deposition. There is a quiz at the end of this chapter to find out if you are addicted to a particular food.

Chapter 2: Intermittent Fasting Comes of Age

This chapter is about the benefits of intermittent fasting on longevity, health, including autophagy, cellular regeneration, cancer prevention and treatment enhancement, dementia prevention, and heart health.

The author also explains three types of fasting: periodic fasting (typically 5 days with no food), the 5:2 approach (2 days of lower energy intake per week) and time-restricted eating (limiting the window in which food is consumed each day).

Chapter 3: The Case for Rapid Weight Loss

According to the author, there is evidence that rapid weight loss does not necessarily lead to gaining the weight back. He also stresses the need for sufficient protein to avoid muscle loss and a lower carbohydrate intake in the beginning to induce mild ketosis, which in turn helps with appetite control. In this chapter there is also a list of people for whom is not a good idea to follow this diet.

Chapter 4: Why I Love the Mediterranean Diet

In this chapter the author presents the rules of the diet:

  • Reduce sugars and starchy carbs
  • Increase consumption of natural healthy fats
  • Eat decent amounts of protein
  • Eat plenty of vegetables, especially dark green and coloured
  • Eat more whole grains and pulses (except for the rapid weight loss and 5:2 stages)
  • Avoid late night grazing
  • Drink mostly tea, coffee and water

This chapter also touches on the importance of the microbiome and the difference between keto, Atkins and the version of the Mediterranean diet advocated in this book.

Chapter 5: Getting Active

This section of the book is about increasing movement to facilitate weight loss, including standing often while working, walking, doing high intensity intermittent training and taking every opportunity for incidental exercise.

Chapter 6: Ways to Beat Stress

This chapter describes stress and poor sleep as contributing factors to hunger. Stress can be mitigated with the Mediterranean diet, exercise and mindfulness.

Chapter 7: The Fast 800 In Practice

Here the author describes the 3 stages of the protocol described:

  • Rapid weight loss: 800 calories per day, low carbohydrate with a recommended 12:12 time-restricted eating
  • The new 5:2: 800 calories for 2 days of the week with a recommended 14:10 time-restricted eating
  • Maintenance

The 800 calories can come from food (there are several recipes included in the book) or shakes.

Mosley also recommends measurements that can be useful to track progress, such as weight, waist circumference, resting heart rate, blood sugar and ketone levels. He recommends weighing yourself often, ideally every day.

Other practical topics include the use of multivitamins and fish oil supplements for nutritional insurance in “fasting” days (same as Valter Longo), potential side effects of the diet, how to choose “fasting” days and a bit of sleep hygiene.

Chapter 8: Supersize Me

The final section of the book contains reflections of when the author gained weight to test the diet, how to appraise scientific literature and other measurements that can be useful to track health, including DEXA scans to measure visceral fat.

Summary and recommendations

In this book, Dr Mosley recommends a combination of energy restriction and time-restricted eating for weight loss and metabolic health. I have some disagreements with the author. I don’t agree with his use of the “fasting” word when he actually means energy or time restriction. I don’t like the use of a fixed number of calories for everyone, but I guess this is by design for the sake of simplicity. I wouldn’t suggest people to rely on shakes to hit the target energy intake – these have a time and a place but don’t teach good eating habits. I don’t think it’s a good idea to weigh yourself every day, especially if you have a history of body image or eating disorders. Finally, I don’t believe DEXA scans are flawless nor harmless due to the radiation.

If you’d like to try this diet, I recommend you buy the book and go over the warnings in chapter 3 first. If you do decide to try it, go through the whole program before you decide it works or not for you.

More information

For more information about the author and the program visit and

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