“Don’t graze unless you are a cow or want to be the size of one.” - Dr Zoe Harcombe

In this chapter we discover how having periods without food is something that the human body has developed to both endure and appreciate. And how, if we are to reverse Britain’s decline into ill health, intermittent fasting is something that most people should partake in.

During the Second World War, when food was rationed, a common saying gained traction: ‘eat little but often’. It might have been born out of necessity, but you still occasionally hear people saying it today. Sadly, from a medical perspective, it now appears to be very bad advice indeed. If you subscribe to our views on evolution – that we must both eat what we were designed to eat and eat at a frequency we have evolved to digest – then ask yourself whether caveman ate little but often! Of course not. He was constantly going from feast to famine. As it turns out, our body is not designed to eat little but often.



Dr Jason Fung 

Dr Jason Fung

As a healing tradition, fasting has a long history. Hippocrates of Kos (c 460 – c 370 BC) is widely considered the father of modern medicine. Among the treatments that he prescribed and championed were
the practice of fasting and the consumption of apple cider vinegar.


Hippocrates wrote, “To eat when you are sick, is to feed your illness.” The ancient Greek writer and historian Plutarch (c 46 – c 120 AD) also echoed these sentiments. He wrote, “Instead of using medicine, better fast today.”


Eating Regularly is Not Normal

We aren’t designed to eat regularly. Caveman and the hunter-gatherer didn’t have fridges or freezers. They couldn’t store an apple for a year like the oxygen-free warehouses the big supermarkets use today. They had no preservatives or tin cans. When they caught an animal, they had a feast, after which they might go days or even weeks without eating anything substantial.

It’s not just our ancestors who fasted, various faiths and religions to this day still participate in different ceremonial fasts. Muslims celebrate Ramadan, the ninth month of the Islamic calendar, with a month-long fast known as Sawm. Christians participate in Lent and the Greek Orthodox Church asks that its followers fast for more than 180 days a year. Saint Nikolai Velimirović wrote, “Gluttony makes a man gloomy and fearful, but fasting makes him joyful and courageous. And, as gluttony calls forth greater and greater gluttony, so fasting stimulates greater and greater endurance. When a man realises the grace that comes through fasting, he desires to fast more and more. And the graces that come through fasting are countless”.


What Happens To Our Metabolism When We Intermittently Fast?

Doesn’t fasting mess with our metabolism and slow down our metabolic rate? Let me first explain metabolism and metabolic rate. Dictionary.com says, “In metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesised”. So metabolism is the breaking down of either our incoming food or our stored body fat to use as energy. But imagine what would have happened to our caveman ancestor if, on days when he couldn’t catch anything, his metabolism slowed down. His energy levels would drop and his chances of catching his next meal would, just like his physique, get slimmer and slimmer. It would all be one huge downward spiral and before long he would starve and perish. When we fast our metabolism does not drop as many would have us believe, but in fact increases.

The idea of ‘don’t skip breakfast because you need to kick-start your metabolism’ is fiction, probably started by marketers for some hugely profitable cereal-producing company. According to Dr Michael VanDerschelden in his wonderfully researched book, The Scientific Approach to Intermittent Fasting, “Studies conducted right after a fasting period have shown a metabolic rate increase of 3.6 – 14% for up to 48 hours”. He then goes on to say that our body does not see a slowdown in metabolic rate for a period of three or four days after our last meal.


Dr Jason Fung 

Dr Jason Fung

Most people expect that a period of fasting will leave them feeling tired and drained of energy. However, the vast majority of people experience the exact opposite. The increased adrenaline levels invigorate us and stimulate the metabolism. During a short-term fast, your body has enough glycogen available to function.


During a prolonged fast, your body can make new glucose from its fat stores - a process called gluconeogenesis (the ‘making of new sugar’). Fat is burned to release energy, which is then sent out to the body. It is the fat-storage process in reverse.


Dr VanDerschelden also writes, “Think what it must have been like for humans in the hunter-gatherer days. These desirable traits of mind enhancement and energy would allow them to effectively search for food and kill prey, increasing survival. With that said, after several days of not eating, these intelligent adaptations would do more harm than good. We would not want our body to sustain a high metabolism and keep burning fuel three or four days after eating for fear of starvation”.

Intermittent fasting plan

So, there you have it – fasting doesn’t decrease our metabolic rate but actually increases it. Therefore, let’s assume we normally consume 2,500 calories a day spread across breakfast, lunch and dinner, but - if we fasted and ate them all in one meal we would start to lose weight because our metabolic rate would be marginally higher. Plus, what I experienced right from the very beginning with intermittent fasting is that we tend to eat way fewer calories in one meal than we would across three.

Let’s also remember that the only reason humans carry fat is to feed the body when there is no food available. So, if we are overweight or obese, doesn’t it make sense to use some of our existing fat reserves to power our body? Many times throughout this book you will read the statement, ‘that’s what we are designed to do’, and put simply our body was designed to go through periodical cycles of feast and famine. Don’t fear intermittent fasting – embrace it.

Intermittent Fasting Prevents Numerous Diseases and Cures Others

The problem with eating little but often is that we constantly keep putting sugar back into our bloodstream. If we live constantly in feast mode, our liver and pancreas never get a break. As we read earlier, insulin is produced to carry poisonous sugar to our belly, bottom or thighs - and while it’s active we simply cannot burn fat. But when we fast, insulin levels drop significantly and we turn into a fat-burning machine. The news gets better still. Our growth hormones (known as HGH) go through the roof when we fast. And as they are natural, they are far more powerful and beneficial than the synthetic ones that many athletes take to enhance performance.

What’s more, our nervous system sends a little army of norepinephrines to our fat stores and they start breaking fat down into fatty acids that can be consumed as energy while we’re fasting. It’s kind of an either/or situation – either our body is focusing on creating insulin to deal with poisonous sugars or it is creating wonderfully beneficial HGH. They are kind of mutually exclusive. They really don’t get on together and hate being in the same room. For those who don’t fast, it’s one of the reasons why getting a good long sleep is important, as it’s about the only time they can produce HGH. A report by the American College of Cardiology stated that fasting triggered a 1,300% increase in HGH secretion for women and a whopping 2,000% increase for men.

In periods of fasting our body goes into a mode of repairing, rebuilding and renewing our cells – virtually all of them. Plus, one of the biggest advantages is that we don’t keep spiking our insulin levels, and as a result there is a greatly reduced risk of becoming diabetic, getting cancer or suffering from heart conditions. There is also growing evidence that it enhances several different brain functions and helps to prevent both Alzheimer’s and Parkinson’s disease. But there’s also one factor that played a huge role in making intermittent fasting part of my lifestyle, and that is that fasting has now been scientifically proven to slow down the ageing process.

body repair


Dr Jason Fung 

Dr Jason Fung

Regular fasting, by routinely lowering insulin levels, has been shown to significantly improve insulin sensitivity. This finding is the missing piece in the weight-loss puzzle. Most diets restrict the intake of foods that cause increased insulin secretion but don’t address insulin resistance. You lose weight initially, but insulin resistance keeps your insulin levels and body set weight high. By fasting, you can efficiently reduce your body’s insulin resistance since it requires both persistent and high levels. Insulin causes salt and water retention in the kidney, so lowering insulin levels rids the body of excess salt and water. Fasting is often accompanied by an early, rapid weight loss. For the first five days, weight loss averages 1.9 pounds (0.9 kilograms) per day, far exceeding the loss that could be expected from caloric restriction.


I think the most fascinating thing I read in Dr Michael VanDerschelden book The Scientific Approach to Intermittent Fasting was under the heading ‘Top 10 Causes of Death’ where he says, “Who would have thought that by doing an intervention like intermittent fasting, you could actually significantly go on to reduce your risk of the top two causes of death in the world, which are heart disease (cardiovascular disease) and cancer”. Dr VanDerschelden then talks about how intermittent fasting can also prevent strokes (the fifth biggest killer) and of course we have already mentioned that it helps prevent Alzheimer’s, which is the sixth largest cause of death. In addition, along with cutting out CARBS and other sugars, we know there is every chance we can avoid type 2 diabetes, which is the seventh biggest killer in the modern world.


There is another concept that I would like you to take to heart and it’s called ‘autophagy’. It is mentioned in many of the books I have researched, but one of the best and certainly simplest explanations I found was online, written by Nick English in July 2016: “It’s a natural process called autophagy (literally ‘self-eating’), and it’s the body’s system of cleaning house: Our cells create membranes that hunt out scraps of dead, diseased or worn-out cells; gobble them up; strip ‘em for parts and use the resulting molecules for energy or to make new cell parts”. Think of autophagy as our body’s instinctive recycling programme. It sends faulty parts to the dustbin and at the same time stops cancerous growth.


The good news is that there are three ways to get our body to perform autophagy, and they are completely aligned to the principles of living primally:
1. Consume a diet high in quality fats and low in carbohydrates.
2. Embrace intermittent fasting.
3. Do high impact intensity training (you will read about our approach to exercise in the next chapter).

While autophagy is the process, the actual garbage collectors are called lysosomes. They travel around the body constantly picking up the trash and performing a natural detox. But there is a small problem with lysosomes the older we get. They slow down and don’t pick up the trash as efficiently as they did when we were younger. In our younger years, lysosomes are very much our heroes. But in our older years they become irresponsible and play a pivotal role in the ageing process. The single best solution in order to get them to do their detoxing job properly is intermittent fasting.

Why is this? My theory goes something like this. If a caveman was sitting in his cave constantly feasting, having all of his food brought to him on a plate and not having to go out to hunt or gather, then nature would be misled into thinking that everything was wonderful in his body and therefore there would be no need to deploy a task force to make repairs. However, the reality of a caveman’s life was very different. He was constantly going from feast to famine. When he was eating, nature sat back and let him enjoy the spoils of his hard work, but when there was no food available and he started to feel hungry, nature began to get concerned. His body asked, ‘Why has he not caught anything today? To ensure he catches dinner tomorrow, I better go and make sure I put everything into good working order’. The two main benefits of intermittent fasting are:

  1. It gives the body a break from food, allowing it time to enter a self-repair mode.
  2. It helps us to safely lose weight.

Author Patrick Holford 

Author Patrick Holford

There is a process of massive cellular repair, called autophagy, which means ‘self-eating’, which is triggered when you fast and switch to burning your own body fat. It can be triggered by low calories or also high fat, but it won’t be triggered if you are doing a ketogenic diet with lots of meat and cheese and while you are building muscle. Because when you do weight training, you are telling your body to go into a growth phase and dairy products promotes a hormone called IGF-1, insulin-like growth factor, so while a high fat ketogenic diet with dairy products will help you lose weight, reverse diabetes and do all of these good things, it won’t trigger autophagy. There is some great work by Professor Valter Longo in California. He has found that if you have five consecutive days, just five days of eating lower calories and a ketogenic diet; eat very low carbs and also low protein - there are certain things you can’t eat because protein promotes growth - you can trigger cellular self-repair. Autophagy cleans up all our messed-up energy factories, called mitochondria, and it gobbles up cancer cells, and it gobbles up damaged proteins. It’s like putting your car in for a service.


You don’t have to do it forever, it’s a short, sharp self-repair process. So maybe once a quarter or once a month if you are not well, do a five-day period, where you purposely trigger this self-repair process. It will be relatively high fat ketogenic, it will be very low carb, it can’t have too much protein, it will be pretty much vegan, but you can have a little bit of fish. (I then asked Patrick what foods would keep us in autophagy and during the conversation, the following were mentioned: kale, seaweed, olives, cinnamon, coconut oil - specifically MCT C-8 - almond milk, almond butter, cacao powder, eggs, kimchi, asparagus, glucomannan, turmeric and vitamins).

monthly plan


But If I Intermittently Fast Won’t I Lose Muscle Mass?

People considering fasting may ask themselves, ‘Okay, so it doesn’t slow down our metabolism, it reduces our chance of getting numerous diseases and is even considered by many as a way of curing type 2 diabetes, but don’t those who intermittently fast lose muscle?’. Not true! If our caveman started to lose muscle on days when he couldn’t catch lunch, then he would never catch an animal again. If it was true, the human race would likely be extinct, and you would not be reading this right now. After measuring my muscle mass after a 4-day fast, it had actually increased rather than decreased. According to research, for our body to consume our muscles as a source of energy when intermittent fasting, we would have to have less than 4% of our body as fat. Even elite sports people are rarely below 8%, so we most likely have quite a long way to go before burning up any muscle!

Remember that fat doesn’t really consume much energy, but muscles do. So, we want to keep strong and healthy muscles because that way we burn more energy and the body finds it harder to put on weight. Have you noticed how most people who go on one of the many different diets that are based on restricting calories actually put on more weight after they quit? That’s because when they diet, they lose muscle and when they stop dieting their reduced muscle mass burns fewer calories. Let me make it very clear that constantly going on diets is counterproductive, and it is detrimental to our health too. As I stated in the introduction, living primally is not a diet but a lifestyle. Because we don’t have to count calories and instead work on understanding how different food types behave inside our body, it’s simple to stick to and most importantly we get to eat lots of fantastic wholesome and tasty foods. Let’s now look at the different ways of fasting. I recommend you read up on these and then just try whichever one you think fits in best with your daily routine.

The 5:2 Diet

The 5:2 Diet by Michael Mosley and Mimi Spencer is a great read for those who are sceptical about intermittent fasting. I fully recommend you purchase the 5:2 book before you start as it gives you both lots of tips on how to integrate fasting into your life, and more importantly will provide the motivation to keep you on the right track once you have started.

On the 5:2 diet, we eat normally for five days a week, and we get to choose which
two days we restrict our calorie intake to just 600 calories for males and 500 calories for females. As long as we don’t overeat on the other five days, then we have reduced our calorie intake by approximately 3,000 to 4,000 calories per week, and therefore mathematically should lose approximately a pound in weight each week. However, the authors are also believers in food with a low glycaemic index (GI) and high intensity training, so the likelihood is that if you follow the book closely, you will lose weight even faster. Remember I promised in the introduction of the book where I said that you wouldn’t need to count calories? When it comes to fasting, for a short while you’re going to need to. It’s not that I lied in the beginning or that I forgot to mention this, it’s just that it is going to take a few weeks into any of the above fasting programs for you to know what 500 or 600 calories looks like.

The 1:1 Diet or Alternate Day Fasting

The 1:1, or alternate day fasting, is similar to the 5:2 diet, but we alternate our fasting day with a normal day. This doesn’t work for me because of my lifestyle, but thousands of people swear by it. Of course, what is great about this technique is that if we restrict our food intake to around 500 calories a day on our fasting days and managed to maintain it for a year (which believe it or not is easier than you might think once you get into it), 180 days of 500 calories a day sees us consuming around 360,000 calories less in a year, which equates to around 45kg or 100lb (more than seven stone). If you haven’t got seven stone to lose, then once you have your body fat where you want it to be you can either slightly overeat on your non-fasting days, or move to the 5:2 method or even just fast once a week. Dr Krista Varady and Bill Gottlieb have written a great book promoting the merits of this approach called The Every Other Day Diet.

The 18-Hour Fast Diet

The 18-hour fast diet is where you commit to only eating in a six-hour timeframe each day, so that you are regularly fasting for 18 hours every day. This is very similar to the approach I use and it has now become a lifestyle that I find extremely easy to adhere too. I never feel hungry, I never feel like I am missing out, I always feel energised and my brain seems to be able to focus on things with much better clarity. 

Fasting plate

The Three- Or Four-Day Fast

With so many benefits of fasting, we might want to occasionally think about doing an extended fast. While there are many articles written and research done on fasting periods in excess of several weeks and in some cases even months, I haven’t yet researched them sufficiently to entertain trying it for myself. And I would never recommend anything to others that I hadn’t experimented with on myself.

I have tried and had wonderful experiences with three- and four-day fasts. While
they may sound difficult to do, they actually get easier and easier the further into the fast we get. What’s quite amazing is how the body reacts on a three- or four-day total fast. By day three, rather than feeling tired and sluggish like we might expect, we feel amazing and the body reaches peak performance. Research by the American Society for Clinical Nutrition carried out a study on people participating in elongated fasts and discovered that their basal metabolic rate (BMR) was at its highest on day three. Isn’t that incredible? While most people believe our metabolism slows down if we even skip just one meal, it’s actually at a peak on the third day of a fast! In fact, the same research showed that day three was also the day when those on the trial were at their maximum exercise capacity. So much for needing a sports gel and a bottle of sugar-loaded energy drink to be at our physical best.

Imagine the repair the body can do in three or four days without food. We really do starve all of those free radicals, especially those that like to kick-start cancer. We also stop or at least dramatically slow down any inflammation, and we give our guts time to complete a full spring clean of our entire digestive tract. I personally try to do a three- or four-day fast at least once every month. I recommend that once you have kicked the CARBS and experimented with either the 5:2, the alternate day fast or the 18-hour fast, that you pick a three or four day period where you know you have no dining commitments and give it a go. If in the unlikely, and I mean really unlikely event that it makes you feel ill, then you can always stop.

So as not to deplete the body’s store of vitamins and minerals, one thing you might want to consider during a three or four day fast, is to increase the number of supplements that you take.

Author Patrick Holford 

Author Patrick Holford

When you fast, and you start to burn your body fat, your liver turns the fat into something called ketones. Ketones are a sort of new fuel. I got particularly interested when looking at brain cells, which is kind of my area of speciality, and if you feed a brain either glucose; sugar fuel, or ketones; fat fuel, the brain actually prefers ketones. Babies for their first six months of life are mainly running on ketones, and they are building up to 1 million connections in their brain per second. So, the brain loves ketones.


Starting Out

Let’s say we have lots of weight to lose. We might start on the alternate day fasting method, and as we near our desired goal switch to the 5:2 diet. Then when we are happy with where we have arrived we move to the 18-hour fast. Here we get all the health benefits of fasting and our body gets to enter its repair mode every single day, but we are able to maintain our desired weight.

While fasting, we should of course continue to consume plenty of nutrients and minerals. And don’t do what I did when I first started to fast and allow some of your calories to come from wine! During fast days we ideally shouldn’t consume any alcohol, as we really want the body to fully maximise the benefit of its repairing mode.

let's get started

Now, if on your first attempt you feel giddy, experience a headache or you really dislike the rumble in the stomach, then you have two options:

  1. Just push on. Remember that this is how our body is designed to eat and you will soon get used to it. Personally, I have come to love the hungry feeling, because it’s my body’s way of sending me a message to say, “All is well boss, I am in repair mode and things are taking shape down here”. But I confess it can take a bit of getting used too.
  1. Don’t give up, but instead for a few weeks don’t drop straight to 500 calories but slowly start decreasing them on your fast day. Maybe try 1,200 calories for a few attempts, then when you have got used to it try 900 or 800.

But, whatever you do, no matter how hard you find it at first you must not give up. If you are on medication of any kind, then do check with your doctor before you try it. But try it you should, as it’s crucial to our body to occasionally enter its natural repairing mode.

try your best

Professor Tim Noakes 

Tim Noakes

Stephen Phinney talks about Europeans that went to live with the Inuits in the Arctic and ate a completely high-fat diet. For the first three or four days, they felt terrible and spent most of the time lying down. But after three weeks they had completely adapted, and they could do all of the activity that the Inuits could do. It just takes time to adapt.

Dr Jason Fung 

Dr Jason Fung
Intermittent fasting tips:

1. Drink water: Start each morning with a full eight-ounce glass of water.
2. Stay busy: It’ll keep your mind off food. It often helps to choose a busy day at work for a fast day.
3. Drink coffee: Coffee is a mild appetite suppressant. Green tea, black tea and bone broth may also help.
4. Ride the waves: Hunger comes in waves; it is not continuous. When it hits, slowly drink a glass of water or a hot cup of coffee. Often by the time you’ve finished, your hunger will have passed.
5. Don’t tell everybody you are fasting: Most people will try to discourage you, as they do not understand the benefits. A close-knit support group is beneficial but telling everybody you know is not a good idea.

6. Give yourself one month: It takes time for your body to get used to fasting. The first few times you fast may be difficult, so be prepared. Don’t be discouraged. It will get easier.

7. Follow a nutritious diet on non-fast days: Intermittent fasting is not an excuse to eat whatever you like. During non-fasting days, stick to a nutritious diet low in sugars and refined carbohydrates.

8. Don’t binge: After fasting, pretend it never happened. Eat normally, as if you had never fasted.

9. The last and most important tip is to fit fasting into your own life! Do not limit yourself socially because you’re fasting. Arrange your fasting schedule so that it fits in with your lifestyle.

Fasting Conclusion

It’s important to experiment a little and find an intermittent fasting regime that works for you. But don’t even think of trying to fast until you are on a low CARB diet as it will be too difficult and unpleasant. Once you have moved over to the colourful side of life and ditched all the starchy, sugary white and beige boring stuff, you will find intermittently fasting a breeze. One of the key things to try is to fit fasting into your life rather than your life around fasting. You don’t need to do it in any type of rhythm. Because of social events and family holidays, you might find it harder in the summer months or at Christmas, but that’s absolutely fine. When the time is right, just return to a method of fasting that suits your needs. I personally tend to mix it all up. Most weekdays ,if I am working, I don’t eat anything all day and then just have an evening meal. At the weekends or on holiday, I might sit down and have a little breakfast with the kids. Then, once a month or so I will try to do a three- or four-day fast. I love them, I really do. By day three I feel on top of the world – full of energy, totally liberated and buzzing from consuming nothing but my own body fat. Regardless of which approach you finally settle into, let me summarise some of the potential benefits you might experience by becoming part of the intermittent fasting generation:

  • Improved memory
  • Slowing the ageing process
  • Better concentration
  • Reduced inflammation
  • Lower heart rate
  • Less fatty liver
  • Reduced blood pressure
  • Increase fat burning
  • Decreased leptin
  • Increased insulin sensitivity
  • Decreased risk of cancer 

Dr James DiNicolantonio 

Dr James DiNicolantonio

I like to take the concept of ‘do what works best for you’. For me, skipping breakfast is so easy. I will say, though, for me if I worked out in the gym very hard say the evening before, I might not skip breakfast, I might eat three or four pastured eggs. When you want to build muscle, eat more protein, eat more food. When you want to start breaking down your body’s old cells through autophagy, then fast. Fasting isn’t like some weird voodoo thing. If our primal ancestors couldn’t catch an animal or find any plants that day, then there were times where they would just fast. If you are a healthy person, you could fast for weeks, the body can do that, but you have to be careful because a lot of people aren’t healthy. And you have got to be careful about the vitamins and minerals you are losing when fasting. So I like to think about it as doing periods of longer fasting, say a two day fast every couple of months, seems to provide additional benefit. Especially in people who have cancer, if you fast two or three days prior to chemotherapy, there are numerous studies showing that you get less radiation damage, less side effects and even improved outcomes.


When you restrict particularly protein but also sugar in the diet while fasting, you start activating longevity enzymes such as AMPK, FGF21 and these signals are not only improving insulin sensitivity, so when you get your next meal your body is going to absorb the nutrients better, but you are breaking down all the old and damaged cells.