The Fast 800 | Rapid Weight Loss and Intermittent Fasting | Michael Mosley | Book Summary




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The Fast 800: How to combine rapid weight loss and intermittent fasting for long-term health by Dr Michael Mosely

The ultimate guide to fasting, weight loss and long-term good health
The culmination of Dr Mosley’s bestselling dieting trilogy
Features revelatory new science with a higher 800-calorie limit

Six years ago, Dr Michael Mosley started a health revolution with The 5.2 Fast Diet, telling the world about the incredible power of intermittent fasting.

In this book he brings together all the latest science (including a new approach: Time Restricted Eating) to create an easy-to-follow programme.

Recent studies have shown that 800 calories is the magic number when it comes to successful dieting – it’s an amount high enough to be manageable but low enough to speed weight loss and trigger a range of desirable metabolic changes.

Depending on your goals, you can choose how intensively you want to do the programme: how many 800-calorie days to include each week, and how to adjust these as you progress.

Along with delicious, low-carb recipes and menu plans by Dr Clare Bailey, The Fast 800 offers a flexible way to help you lose weight, improve mood and reduce blood pressure, inflammation and blood sugars.

Take your future health into your own hands.


The Fast 800

For rapid weight loss, as long as it is safe for you to do it 800 calories a day, every day, is what you should be aiming at. This is a regimen that has been shown to be safely sustainable for weeks and months. You might want to take this approach if you have a lot of weight to lose; if you are in a hurry; if you have pre-diabetes or type 2 diabetes; if you have a fatty liver; if you want to kick your weight loss journey off with a bang; or perhaps because you have hit a weight loss plateau.

On 800 calories a day you can expect to lose up to 5kg after two weeks, 9kg after four weeks and 14kg after eight weeks, most of which will be fat. Rapid weight loss is often described as ‘crash dieting’ but I want to show you how, done properly, it can be safely used.


A low-carbohydrate Mediterranean diet

The menus at the back of this book offer plenty of filling and tasty recipes for making up 800-calorie days. They are all based on a low-carb, high-protein Mediterranean-style approach.

The reason I am so keen on this way of eating is that it will help you maintain your muscle mass and stop your metabolic rate from crashing as you lose weight.

In the end, the best diet is the one you can stick to and which fits best in your life.



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Other elements of the Fast 800 Programme

I want to introduce you to a relatively new form of intermittent fasting called Time Restricted Eating (TRE).


It involves eating all your calories within a relatively narrow time window each day, usually 8 to 12 hours.


TRE is not an alternative to the 5:2; rather, it complements it.

The importance of ketosis – that is, persuading your body to switch from using sugars to burning fats in the form of ketone bodies to obtain fuel. This is key to the success of intermittent fasting. It also turns out to be surprisingly good for the body and the brain. But it has to be done the right way.


Why losing weight is about more than vanity.


Even relatively modest changes can make a big difference.


Studies have shown that if you are overweight or obese, losing 5% of your body weight will:

  • Reduce your blood pressure and levels of blood fats (triglycerides), which in turn will significantly cut your risk of having a heart attack or stroke.
  • Lower your risk of getting cancer.
  • Sleep better.
  • Cut your risk of developing type 2 diabetes.
  • Boost your sex drive.



The obvious answer, ‘because we eat too much and don’t do


enough exercise’, is too simplistic. It’s like going to a tennis coach to improve your game and being told that all you need to do is ‘win more points than your opponent’. True, but not useful.


So why has there been such an explosion in obesity, worldwide, over the last 40 years?


There are plenty of plausible explanations, including increased anxiety, stress, poor sleep and becoming less active, but top of my list is more snacking and the fact that we are eating lots more junk food – not just more cola, cake and candy, but refined carbs, up by a whopping 20% since 1980.2 These foods are packed with calories and are highly addictive (see page 29). Bursting with sugar and processed fats, they play havoc with our hormones, and one hormone in particular: insulin.

Carbs and insulin

If you’re constantly snacking and doing very little to burn the calories off, your body will become less and less sensitive to insulin. So, your pancreas has to work harder to produce more and more insulin. It’s like shouting at kids. The more you shout, the less attention they pay.


Two bad things now happen:


Your fat cells become large and inflamed, as your body tries to cram more and more energy into them. At some point you exceed your ‘personal fat threshold’. There is no space left to store fat safely, so it begins to overflow into your internal organs, such as your liver.


Despite carrying around too much fat, you still feel hungry all the time. That’s because you now have high insulin levels, which encourage continuous fat storage. Which means there’s less fuel around to keep the rest of your body going.


It’s as if you’re constantly pouring money into your bank account, and then finding it incredibly hard to get it out again. You have money, but you just can’t get at it. High levels of insulin prevent your body from accessing and burning its own energy supply.


So, despite the fact that you are carrying around lots of energy in the form of fat, your muscles and your brain can’t easily access it. Deprived of fuel, your brain tells you to eat more. So you do. But because your high insulin levels are encouraging fat storage, you get fatter while staying hungry.


In other words, if you have a weight problem it may not be because you lack willpower or you’re greedy. It is more likely that, like one in three Americans, you are insulin-resistant and therefore have too much insulin washing around in your blood.


Understanding insulin is crucial to understanding obesity. ‘Insulin shunts sugar to fat. It makes your fat cells grow. The more insulin the more fat.’


‘The increasing amount and processing of carbohydrates in the American diet has increased insulin levels, put fat cells into storage overdrive and elicited obesity-promoting biological responses in a large number of people. High consumption of refined carbohydrates – chips, crackers, cakes, soft drinks, sugary breakfast cereals and even white rice and bread – has increased body weights throughout the population.’


The rise and rise of junk food


The fact that we now eat so many refined and sugary carbs, and eat them so often, isn’t an accident. It was an unintended consequence of the low-fat campaign, the biggest and arguably the most disastrous public health experiment in history.


It all began in 1957, the year the author was born, when the hugely influential American Heart Association decided to mount a campaign aimed at reducing fat consumption. They weren’t, initially, targeting bellies; they were more concerned about hearts. They believed that saturated fat caused heart disease, so it was out with the steak, butter and cheese; in with the pasta, rice, potatoes and veg.


Or at least that was the plan.


Backed by millions of dollars of government money, the low-fat campaign certainly had an effect. Over the next few decades, Americans cut their consumption of animal fats, such as milk, butter and cream, by as much as 20%. They didn’t, however, replace those fats with healthy fruits and vegetables.


Instead, people ate more and more processed foods, which were being heavily promoted by the food industry as ‘low-fat’ or ‘fat-free’. Under the pretence of making food ‘healthier’, the manufacturers stuffed their products with processed vegetable oils (such as margarine) and cheap, sugary carbs. And, as consumption of dairy fats went down and that of sugary carbs went up, obesity rates began to soar.


They don’t teach you much, if anything, about nutrition at medical school – was the effect these foods were having on our body. Eating a boiled potato will push your blood sugars up as quickly as eating a tablespoon of sugar (I’ve tried it!). Ironically, if you eat the potato with fat, such as cheese or butter, the fat will slow absorption and the blood sugar peak will be slower and less extreme.

Why snacking makes us fat


People used to believe in the quaint idea of ‘not eating between meals’. In the 1970s, before the modern obesity crisis, adults would average four and a half hours between meals, while children would be expected to last about four hours. Like flared trousers, those times are long gone. Now the average window between meals is down to three and a half hours for adults and three hours for children, and that doesn’t include all the drinks and nibbles.


The more we snack the more we eat overall.


Food addiction


The modern obesity epidemic wasn’t triggered by a collective breakdown in willpower in the late 1970s. It happened because food manufacturers have found more and more ingenious ways to make us buy their products. Like the tobacco industry, they know how to hook and hold their customers.


Junk food is clearly not addictive in the same way that cocaine is, but it shares some of its qualities. The pleasure you get from it is normally very short-lived. It is about compulsion.


In summary:


The reason we are snacking more and eating more junk foods is no accident: junk food is packed with fat, sugar and salt. It has been designed to make us crave it.


Eating lots of refined carbs – in the form of white bread, rice and pasta, as well as junk food – will keep your pancreas pumping out insulin. High insulin levels keep us hungry, which in turn makes us snack more.



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Cutting your calories for short periods of time, or restricting when you eat your calories, seems to offer many of the benefits of long-term calorie restriction. And it is an awful lot easier and far more convenient.





The list of things that fasting does to your body is long and complex, but one of the more striking benefits comes from activating a process within the body called ‘autophagy’, which literally means ‘self-eat’. Autophagy is an entirely natural process where dead, diseased or worn-out cells are broken down and gobbled up.


Just as we need sleep, so we need time off from constant eating if we are to turn on the repair genes that keep us in good shape. It is only when we are not eating or drinking anything with calories in it that our bodies can begin this process of repair.


Autophagy is triggered by fasting, and becomes more intense as time goes by. It stops once you eat.




Fasting triggers autophagy, which means your body is able to clear away the junk and the debris, i.e. old cells.

Fasting, by triggering autophagy, also creates space for new cells to grow. It’s like a forest fire that burns away the old undergrowth, making space for new plants and trees.


Fasting, followed by feeding, gives the ‘okay’ for the body to go ahead and begin creating new cells.


Flipping the metabolic switch


During the first 24 hours of a fast, big changes go on inside your body. Within a few hours, the sugar (glucose) circulating in your blood begins to fall. If it’s not replaced by food the body turns for energy to a stable form of glucose that is stored in your muscles and liver: glycogen.


Once stores of glycogen begin to run low (around 10-12 hours after your last meal), your body goes through a remarkable change. It switches over into fat-burning mode. It’s called ‘flipping the metabolic switch’. A bit like a hybrid car flipping from using electricity to using petrol when the batteries begin to run low.


When this happens, fat is released from your fat stores and converted by your body into fatty acids and substances called ketone bodies. Like most of your body, your brain will happily use these ketone bodies as a source of energy. In many ways your brain runs better on ketones than on glucose.



For five days a week I would eat healthily and, on my two fasting days, I would cut down to roughly 25% of my normal intake, which would mean I would be eating around 800 calories a day.


I opted for Tuesdays and Thursdays.

Acting before you have a heart attack is far, far better than waiting and hoping.



It has recently become popular, particularly among millennials, body builders and celebrities.


The rules of TRE are very simple: you try to eat most of your calories within a narrow time window, such as 12 hours (also known as 12:12). Once you have decided on your time window (perhaps 9am to 9pm), you don’t eat or drink anything that contains calories outside that period.


You can start doing TRE by simply having your evening meal a bit earlier and your breakfast a bit later (12:12). That way you extend your normal overnight fast (the time when you are asleep and not eating) by a few hours. Once you have got used to this you can move to the 14:10 or even, like Hugh Jackman, to the 16:8 (where you eat all your calories in an eight-hour window, such as between midday and 8pm, and fast for 16 hours).


The optimum fasting window


The research done on animals suggests that fasting for at least 16 hours ensures the greatest benefit,

‘Most of your body fat burning happens six to eight hours after finishing your last meal and increases almost exponentially after a full 12 hours, which means that going longer than 12 hours is likely to be particularly beneficial. Once you’ve achieved your desired weight loss you can go back to an 11-or 12-hour window and maintain body weight.’


And does it matter when you do your fasting/eating?


It is better to eat most of your calories earlier, if you can. That’s because your body is much better at handling sugar and fat in the morning or afternoon, rather than in the evening.


In summary:


There are different ways to do intermittent fasting.


A five-day fast can help reset your immune system and may also enhance the effects of chemotherapy.


Intermittent fasting activates a process called ‘autophagy’, which literally means ‘self-eat’, and puts your body into repair mode.


Studies show that the 5:2 is not only good for weight loss and improving your insulin sensitivity, but may also help boost your brain, reduce your risk of heart disease and lower your blood sugars.


TRE compliments intermittent fasting and can be an effective tool to help you lose weight and keep it off.



One of the other major revolutions in the weight-loss world, alongside intermittent fasting, has been the return of very rapid weight loss diets.


Diets in which you reduce your intake to 800 calories each day, every day, for up to 20 weeks. Diets on which you can expect to lose around 14kg in three months.


We’ve been repeatedly told that rapid weight loss is ineffective and futile: that if you lose weight fast you will put it back on even faster.


The DIRECT study


People who follow a rapid weight loss diet not only lose a lot of weight, fast, but by doing so also clean fat out of their livers and reverse their type 2 diabetes.


The reason most people get type 2 diabetes is that they have too much fat around the tummy. Unlike the fat on your bottom or thighs, the fat on your tummy, visceral fat, infiltrates your liver and pancreas, and stops them ‘talking to each other’. This can, in time, lead to type 2 diabetes.


Just because your pancreas has gone quiet, however, doesn’t mean it can’t be revived. And the best way to do that? Lose a lot of weight. Fast.


Try Meal replacement shakes and soups.


Studies consistently show that early weight loss predicts long-term weight loss.

Won’t my metabolism crash if I go on a rapid weight loss diet?


Whichever way you lose weight, fast or slow, your metabolism will slow down simply because you are now carrying less weight around. That’s why it is so important, as you lose weight, to remain active.



Reduce sugars and starchy carbs


Cut right back on sugary starchy foods, such as cakes, sweets, biscuits, crisps, fruit juices and soft drinks, as these rapidly turn into sugar in your blood, causing sugar spikes, a surge in insulin and weight gain. Aim to have them less than twice a week.


You also need to watch out for foods that get rapidly converted to sugars in your blood, such as:


Potatoes, bread, white rice and white pasta.


Most breakfast cereals and ‘instant oats’ (steel-cut or rolled oats are OK). Sweet, tropical fruits such as mangos, pineapples, grapes, melons and bananas as these are high in sugars (fructose). Instead, opt for berries, apples or pears. Aim for a maximum of 1-2 pieces of fruit a day, ideally eaten after a meal.


Processed foods. More than 70% of processed foods contain added sugars. You have to read the labels, though the problem is that there are more than 70 different names for sugars.


  • Increase your consumption of natural healthy fats
  • Eat decent amounts of protein
  • Eat plenty of green and coloured veg
  • Swap to wholegrains and pulses
  • Avoid snacking between meals or late-night grazing
  • Drink healthily


The Mediterranean diet is more than just a diet. It’s about developing a set of

habits and making permanent changes to your lifestyle.



  • Walk
  • Walking is a cheap and safe way to exercise
  • HIIT (High-intensity Interval Training)
  • Strength training
  • Ways to introduce more activity into your life
  • Buy a bike and cycle when you can. It saves lots of time and money.
  • If your destination is less than a mile away, then why not walk?.
  • Stand while talking on the phone.
  • Use a basket at the shops rather than a shopping trolley.
  • Drink lots of water.



Research has shown that chronic stress leads to increased hunger, comfort eating, self-loathing and disrupted sleep. This, in turn, leads to even higher levels of stress, and more hunger, eating, self-loathing, etc.


The problem is long-term stress, when levels of these stress hormones go up and stay up.



Stage 1: The Very Fast 800 – rapid weight loss

For the Rapid Weight Loss stage of the diet, you eat 800 calories a day. I suggest you stay on this stage for a minimum of two weeks, but you can stay on it for anything up to 12 weeks, depending on your circumstances and how much weight you have to lose.


As well as cutting down to 800 calories a day, I recommend adding in a Time Restricted Eating (TRE) programme. This means, from the start, aiming for a 12-hour overnight fast.


Stage 2: The New 5:2 – intermittent fasting

At some point, it may be at the end of the first two weeks or it may be later (depending on how you get on), you are going to want to switch from the Rapid Weight Loss phase to something which is more gradual. This means switching from 800 calories every day to intermittent fasting, where you will be on 800 calories for a few days a week.


At this stage I’d suggest you try extending your overnight fast by reducing your TRE window from 12 hours to 10 hours. In other words, making your overnight fast last 14 hours. The reason for doing this is that it will help maintain and enhance the benefits of intermittent fasting, in particular autophagy and ketosis.


Stage 3. Maintenance – a way of life

Once you’ve achieved your goals it will be time to go on to the Maintenance Phase.

The good news is that the longer you stick to the maintenance programme, the easier and more natural it becomes.



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Just to remind you, if you have decided to kick off with the Very Fast 800, you are going to be sticking to 800 calories a day, every day, for at least the next two weeks. This will lead to some impressive changes, which many people find extremely motivating. And remember, as I’ve mentioned before, the amount of weight you lose early on is a great predictor of how much you will lose overall.


Managing the first two weeks


The first week or two are likely to be the toughest, as your body adapts to fewer calories and to ‘flipping the metabolic switch’. Your body will be burning more fat and less sugar, which is good, but this can produce side effects.


Lack of protein is one of the major drivers of hunger.


When to eat your calories on an 800-calorie day


We are all different. When I’m doing the 5:2, I prefer eating my calories in just two meals, while putting aside some calories for a small snack. I have a late breakfast, skip lunch and aim to eat early in the evening.


Try the different approaches till you the find one that works for you.


Try to lower or even cut out alcohol altogether while trying to lose weight. Alcohol is bad for dieters for a range of reasons:


It weakens your willpower – I find that once I’ve had a drink, my willpower, always weak to begin with, almost entirely disappears. Alcohol gives you the munchies. When I am drinking I cannot resist crisps.


Alcohol is extremely calorific. Here are a few figures to bear in mind: a large glass of wine (250ml) or a pint of beer comes to around 230 calories, which is similar to eating a small bar of chocolate or an ice cream.



  • Once you have hit your targets, celebrate.
  • Keep junk food out of the house.
  • Weigh yourself regularly.
  • Buy yourself a set of new clothes when you finish the diet.
  • If you do start to put on weight you must take action as quickly as possible.
  • Keep active.
  • Self-monitoring.
  • Join an online community and share your data.
  • Always try to sit down for meals and never eat on the move.
  • Find healthy foods to eat during the day
  • Put less food on your plate than you think you might actually eat
  • Be mindful.
  • Write things down.
  • Prioritise sleep.
  • Above all, remind yourself why you are doing this.




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