Made by fasters, for fasters.

Key points about exercise:

• The calories we burn by taking more exercise do not generally help us lose a significant amount of weight

• But exercise is good for our general health – and also affects our metabolism in healthy ways – in fact, its benefits are similar to those of fasting

• And combining exercise with intermittent fasting appears to be an excellent idea – early evidence suggests it increases the benefit of both.

Let’s look at each of these points in turn.

The calories we burn by taking more exercise do not generally help us lose a significant amount of weight

A person who exercises is clearly going to burn more calories than someone who sits at home all day. So, surely exercise will be a good way to lose weight? Unfortunately, no. Studies have found that people following the standard health advice of moderate exercise of around 30 minutes on most days of the week lose hardly any weight.¹ There are two main reasons for this:

1) This kind of exercise can make us hungry so we indulge in a treat following our trip to the gym. Usually the treat contains more calories than we were able to burn during our exercise.

2) This kind of exercise can make us more tired and we tend to move less to compensate for the exercise calories expended.

In fact, it seems that the most efficient way of encouraging fat burning, weight loss and the other health benefits is not to spend hours in the gym but to use high intensity interval training (HIIT).

High intensity interval training

If you don’t have the time or inclination to spend hours in the gym or out running, or simply wish to maximize the health benefits of your exercise, you should consider including high intensity interval training (HIIT) into your life.

Research has found that as few as 3 bouts of 20 seconds of high intensity exercise done three times a week can have as many beneficial effects on the body as an hour of running on the treadmill.

Indeed, the author of the Fast Diet  book, Dr Michael Mosley was so impressed with the results of his experience of HIIT that he wrote another book: Fast Exercise which explains the science behind HIIT and guides you through how to do it.

Exercise is good for our general health: in fact, its benefits are similar to those of fasting

Two important metabolic improvements from exercise are also benefits of fasting. These are:

• Increased metabolic flexibility. Exercise encourages the body to use its fat-burning processes, rather than relying on glucose from food. As a result, exercise can reduce metabolic inflexibility – a key factor  preventing some people from losing weight.  Inside our cells, what happens after exercise is that the mitochondria, which are the parts of our cells that actually release energy from fuel, multiply and any worn out mitochondria are repaired.² The size of our glycogen stores is increased³ and the machinery that allows fat burning is up-graded so we can use fat as a fuel at higher intensities than previously.⁴

• Exercise can reduce insulin resistance⁵ which is a key element in the development of diabetes, heart disease and other diseases. This is because your muscles burn glucose and during exercise and your body uses more glucose to replenish its glycogen stores.  When blood glucose lowers, so do insulin levels and this helps to reduce insulin resistance.⁵

As well as these positive metabolic changes, exercise has further health benefits unrelated to your metabolism. For example:  the heart is also a muscle and, to enable enough blood circulation to provide the oxygen needed for exercise and to remove the carbon dioxide generated, it needs to beat faster and more strongly.  Normally, the heart gets 90% of its energy from fats but when beating fast it too will use glucose. The heart will also benefit from its workout by increasing mitochondria, undergoing repair and improving its ability to utilize fuel.⁶

Curiously, perhaps, your brain can also benefit from exercise. Exactly how this works is unclear, but it seems that exercise triggers release of various brain chemicals that stimulate nerve repair.⁷

Combining exercise with intermittent fasting appears to increase the benefit of both

The benefits of exercise for the way our bodies function are many, and exercise can compliment our weight loss efforts, but for maximum health we need to attend to both exercise and food (or, rather, absence of food). Many of the metabolic changes that occur with exercise are also seen with fasting. Like fasting, exercise reduces blood glucose and so reduces the risk of diabetes. Like fasting, exercise encourages fat burning and so aids weight loss. Like fasting, exercise aids brain repair.

Learn more about exercise and fasting

There are many explanations for why we eat more than we need, and gain weight.

These explanations include:

• Social and cultural reasons: we live in societies that are full of temptation, and we are constantly surrounded by food advertising, huge portion sizes and other prompts to eat too much. The food industry wants us to eat more, and they manage to persuade us.

• Psychological reasons: some people are ‘emotional eaters’; others have fallen into bad habits and eat when they are bored or need something to do. Depression is also a common cause of overeating.

• Evolutionary reasons: living in a land of plenty is very new for humankind in evolutionary terms.  We have been programmed by evolution to ‘stock up’ on food while it is plentiful because our ancestors would have experienced balancing periods without food. In the modern world, we are always stocking up, and never burning our reserves – hence we gain weight.

You may be interested to learn, however, that it is not all in our heads, and neither is it all about social factors, indeed there are many reasons. One of the most important is that our bodies also appear to get ‘hooked’ on overeating.

There are several theories about the factors driving the obesity epidemic.  Some of these (the first two) are generally accepted by scientists; others are important and interesting but controversial.

1. We have circuits in our brain that control ‘hedonic hunger’ (eating when not hungry).¹ These circuits exist for evolutionary reasons, telling us to stock up on food while it is plentiful. These pathways, unfortunately, appear to be more active after a period of weight loss and drive us to put weight back on. When food is always available and we never experience periods of famine, these hedonic hunger circuits are always turned on: we are hooked on overeating. Our hormones, including thyroid hormones, leptin and ghrelin do try to minimize the weight gained, but for most people a gradual increase in weight over the years is normal.

2. Our bodies develop insulin resistance. Insulin is released whenever blood glucose rises ( i.e. after a meal) and acts to keep blood glucose within safe limits. When we overeat, particularly on sugary, high carbohydrate foods, our insulin levels are continually high. The constant influx of glucose may prompt the body’s cells, which do not want to absorb any more glucose, to start to resist the effects of insulin. This means glucose levels in the blood remain high, and the body makes even more insulin to try to combat it. A vicious cycle is created. Insulin and glucose levels are both high and we become locked into a cycle of putting on fat but never burning it off.² Insulin resistance is a highly dangerous situation and is the first step on the road to type 2 diabetes, heart disease, and many other diseases. Eating a diet high in sugary foods creates a kind of ‘one way street’ in which the body can only store fat but never release it and this excessive storage leads to increased hunger.³ As we age, we typically become more and more insulin resistant, again leading to a gradual increase in weight over the years.

3. A more controversial theory⁴ is that the prevalence of fructose in our diets plays a unique and causative role in the increasing rates of cardiovascular disease, hypertension, diabetes, cancer, and nonalcoholic fatty liver disease. The theory is that fructose behaves in the liver and brain in a similar way to alcohol. In the liver, fructose stimulates fat production and also induces hepatic (liver) inflammation. In the brain, fructose may stimulate the “hedonic pathway”, creating an addiction similar to alcohol addiction.

4. Some recent scientific studies have suggested that, as with sugar, a similar addictive-like state may emerge with foods that combine sugar and fat.⁵ Rats and mice fed a diet high in fat and sugar but then returned to their old healthy diet show signs of stress on withdrawal of the fat and sugar diet and are willing to withstand discomfort to gain access to the unhealthy diet once again. This does not happen with diets high in fat alone and to a lesser degree with high sugar diets. The combination of fat and sugar seems to be a particularly powerful one. It is notable that almost all ‘junk foods’ are high in both fat and carbohydrate (ice cream, chocolate, pastries, chips, french fries). It is also interesting that there are almost no foods in nature that combine significant amounts of sugar and fat.

5. Changes in the types of bacteria that inhabit our intestines or even infections with certain viruses may be a factor. Scientists have identified a variety of viruses and bacteria that can increase fat stores in animals and, conversely, some kinds of bacteria that can promote fat burning.⁶

6. Inflammation has long been known to be associated with obesity, but it is uncertain whether the inflammation is a cause or the result of obesity.⁶

7. If our parents were overweight, we are probably not surprised if we develop the same problems, but the potential for one’s children to become obese is not fixed. For example, scientists have discovered that children born to women who suffered a period of famine are more likely to develop obesity than those whose mothers did not,⁷ while for mothers who have had weight loss surgery, any children born before the surgery have a higher risk of developing obesity than those born after the surgery.⁶ Many other environmental factors seem to be able to make changes that can alter the risk of developing obesity, in particular a woman’s age and health when she has her children.⁶

8. Sleep, or the lack of it, has been shown in many studies to be linked with obesity.⁶ The levels of several hormones that affect appetite and metabolism are affected by poor sleep. Even mild sleep deprivation (5–6 hours of sleep per night compared with 7–9 hours of sleep) was associated with higher rates of obesity.

9. Living in a warm, not too hot, not too cold, environment is very comfortable and desirable but even that seems to be detrimental when it comes to keeping slim. Like with fasting, the mild stress of being exposed to heat or cold appears to be beneficial, not only because we expend more calories when too hot or too cold in trying to keep our body temperature stable but also because of effects on our whole metabolism.⁶

10. Environmental toxins and prescription drugs influence many aspects of our bodies’ functioning resulting in an increased likelihood of gaining weight.⁶ In particular, the build up of the effects of exposure to environmental factors over time might contribute to the tendency to gain weight as we age.

Intermittent fasting can reverse the trend

Luckily for us, though, Intermittent fasting seems to have a beneficial effect on many aspects of the hormonal and brain systems that influence body weight, making it easier to lose weight, and easier to keep it off.

At FastDay we believe that a really effective Way of Eating needs to be sustainable in our minds and in our social environments….but it also needs to help our bodies be free from being ‘hooked’ on bad habits.

How does the body balance calories eaten with calories used?

Our metabolisms are very complex, and although it’s a pretty good rule of thumb that calories in need to balance with calories out (if we eat more than we burn we will put on weight; if we eat less than we burn, we lose weight), knowing how many calories our bodies actually use is quite hard to know for sure. A good starting point is to use the FastDay progress tracker which will give you an estimate based on your age, height, weight and activity level.

Some definitions:

• Basal metabolic rate (BMR): how much energy your body needs just to exist before you do anything else

• Resting energy expenditure (REE): the basal metabolic rate plus the energy needed for digestion

• Total daily energy expenditure (TDEE): the amount of energy your body needs to supply you with the fuel to do your daily activities and to maintain your body without losing or gaining weight. TDEE, therefore, is the sum of your REE and the energy you use to walk around, go to the gym, eat your dinner etc.

Learn more about how the body stores and uses energy

What determines how much energy we burn?

The thing that has the greatest influence on your energy expenditure is how big your body is. The bigger the body, the more energy required to keep it going and also the more energy it takes to move it around. You might be surprised to learn that most of your energy intake is used to keep your body operating at the ideal temperature and to keep your body in good repair (your basal metabolic rate).

Taking exercise and being active, for example by taking 10,000 steps every day, are undoubtedly good for your health, and you should do it if you can. But don’t expect to lose weight as a result; it helps, but only a little bit. Exercise burns energy (calories) – but typically makes only a small difference to the amount of energy we burn in a day.  Just think about how long you have to spend on a treadmill to burn 500 calories!

Can exercise have a second effect, by increasing our metabolic rate during the periods when we are not exercising? Many people believe this to be true, but there is little evidence for it. As our metabolic rate is influenced by the amount of muscle mass we have, exercise that builds muscle can increase metabolic rate to a small degree. High intensity interval training may also raise metabolic rate by a very small amount. Unfortunately, the small changes in metabolic rate seen in laboratory studies of exercise are insufficient to make any practical difference in our day to day lives. In fact, there is good evidence that some kinds of exercise, particularly prolonged ‘cardio’ type exercise, can result in a compensatory reduction in our non-exercise activity levels in an apparent attempt by the body to limit the amount of calories used overall, and at the same time our appetites increase making it easier to overeat following the exercise session. There is a lot of research going on in this area at the moment and we look forward to the results.

Learn more about how exercise affects metabolism

Our hormones, brain and ‘brown fat’ also affect our metabolic rate. Complex hormonal mechanisms exist to counterbalance the effects of overeating, for example. And it seems likely that some of us are luckier than others – having bodies that are more or less good at adjusting our metabolic rate to burn excess energy.

• The most important hormones affecting metabolic rate are the thyroid hormones and adrenaline. The thyroid influences energy expenditure by its effects on heart rate, temperature, and muscle efficiency. An under-active thyroid usually results in weight gain.¹

• Periods of overeating tend to increase thyroid function, while periods of under-eating tend to decrease thyroid function.² So, the thyroid adjusts our metabolism to cope with over- or under-eating.

• Nerves from the hypothalamus in the brain also influence your heart rate, thyroid function and muscle tone and so can affect your metabolic rate.

• Finally, we have your brown adipose tissue (BAT). This ‘brown fat’ as it’s called is very different to the white adipose tissue which makes up the body’s main fat stores and that hardly use up any energy. By contrast, BAT is a big user of energy and generates a lot of heat by burning fat. It was once thought that, although born with large amounts of BAT, we lost it all in adulthood. But recent advances in scanning technology revealed that we do have BAT and that there is enough to contribute significantly to our metabolic rate.³ The activity of BAT increases in the presence of cold and during fasting.^4,5

Learn more about metabolism and weight loss

What controls how many calories we eat?

When you start fasting, you become much more aware of hunger and appetite: some fast days pass easily with few hunger pangs, others seem to be hard work, and you may ask yourself “what controls my appetite?”

Hunger does not happen just because your stomach is not full. There are a surprising number of hormones and other factors that control our appetite. Here are just some of the most important:

• Leptin is a hormone made in our body fat that, essentially, tells the body when the fat stores are full. When your leptin levels increase, your appetite decreases.⁶ When you lose weight, therefore, leptin levels decrease making you more hungry.

• Ghrelin is a substance released by the stomach and intestines. It works on the nervous system to create feelings of hunger. Ghrelin levels rise before a meal and drop after eating.

• Insulin levels are also thought to affect hunger. When insulin levels rise after a carbohydrate-containing meal, hunger decreases. Later, as insulin levels fall, hunger returns.

• Blood glucose levels are, of course, affected by insulin levels. A drop in blood glucose seems to trigger hunger.⁷ There are specialised neurons in the brain that detect blood glucose concentrations, which have been shown to release a substance called neuropeptide Y that is known to stimulate hunger.⁸

Learn more about the metabolism and weight loss

Learn more about the what happens when we fast

Our bodies use hormones to control all the processes of metabolism. The most important hormones involved in controlling how glucose is used are insulin and glucagon.

Insulin is the key hormone acting after a meal. Its job is to ensure that most of the glucose we get from our food is stored away (as fat or glycogen) and does not remain in our blood for too long – this is why our bodies make insulin when our blood glucose levels rise.  Controlling the amount of glucose in our blood is important for our health because glucose is toxic and damaging to the body. If insulin fails to keep blood glucose down to safe levels, damage occurs to many different tissues (our blood vessels, nerves, kidneys, eyes can all be damaged by high blood glucose) and we may eventually develop symptoms of diabetes. Because insulin’s job is to fill up the fat stores, when insulin levels are high we cannot release fat for energy. To allow fat to be released and used for energy we need other hormones to oppose the action of insulin.

Glucagon’s role is to stimulate the body to restore glucose levels if they are getting low and to ensure the body has a good energy supply. Glucagon stimulates the release of the stored glycogen, manufacture of glucose from fat and amino acids and release of fat from fat stores.

Learn more about fasting and diabetes

Diagram of insulin and glucagon in action

Is glucose or fat the best energy source for the body?

Fat is an excellent source of energy, and most of us have a large store of fat! For our normal activities (low intensity activity), fat can supply the energy we need and our muscles can be fuelled by fat burning.  This is what normally happens when we are in the fasting state.

Our bodies will be fuelled by glucose instead of fat in one of two main situations:

• When we need a short burst of intensive activity (running away from a marauding lion, for example) our muscles have to use glucose from the blood or from glycogen stored in our muscles. The chemical reactions for turning fat into energy are too slow in this situation. (Because our store of glycogen is fairly small, we can only operate at high intensities for short periods; this is why sprinters can only run at top speed for short distances.)

• If you eat a normal Western diet that is high in carbohydrate, glucose is available pretty much all of the time and your body burns this glucose instead of fat. Many people’s bodies become dependent on a constant supply of glucose (from sugar or carbohydrate) in their diet, and the body becomes resistant to burning fat – a phenomenon called metabolic inflexibility.

Once someone becomes metabolically inflexible, it is difficult for them to lose weight. Their bodies are stuck in a situation where they are forever increasing their fat stores and never releasing the fat to be used for energy. This situation means they are often hungry despite having many thousands of calories-worth of fat stores because their bodies cannot access this store.¹  Obese people tend to have metabolic inflexibility, particularly if the fat is carried in the abdomen.²

Adopting a new Way of Eating can free people from metabolic inflexibility – we recommend Intermittent fasting, but diets that incorporate a low-carb element, such as the Paleo diet can also be effective. Our bodies need a Way of Eating that re-educates them into using fat for energy, hence making weight loss easier.

Learn more about metabolism in weight loss

Learn more about how exercise affects metabolism

Scientists think of our bodies as being in either the absorptive (fed) or postabsorptive (or fasting) state. Once all the food you have eaten has been used or stored, your body enters the fasting state.

In the fasting state, your body still needs energy. It can either use stored glucose (glycogen) or stored fat to meet this need.

Key points:

• It takes around 12 hours of fasting before any significant amount of fat is being used for fuel.

• The longer the fasting state progresses, the more of the body’s energy supply comes from fats and the less from glucose.

• Being active and taking exercise can increase the rate at which the body burns fat while you are fasting.

• Some people’s bodies are able to move into a fat-burning mode more quickly and effectively than others. This is called ‘metabolic flexibility’. The good news is that with a few weeks’ practice at Intermittent Fasting, your body will become more metabolically flexible.

Learn about insulin and how your body controls storage and burning of glucose and fat

To understand the science of fasting, we first need to understand how our bodies uses and stores the food we eat – aka metabolism.

You probably already know that most of our food is composed of a mixture of ‘macronutrients’: carbohydrates, proteins and fats. Some foods are higher in one type of nutrient than others. For example sugar, honey, potatoes, bread, pasta, rice are all or almost all carbohydrates; while meat, fish, poultry are mainly protein; butter and cooking oils are fats. Foods like cakes, biscuits and fried food are a mixture of carbohydrate and fat.

After we have eaten some food, the process of metabolism starts. Metabolism begins with the process of these macronutrients being broken down into their component parts:

• Carbohydrates are converted into sugars (mainly glucose).

• Proteins are broken into amino acids.

• Fats become fatty acids.

The body then uses these components for energy (glucose and fatty acids) or repairing itslef (amino acids). Any surplus energy is stored for the future – mostly in our fat stores.

Let’s look at how the body deals with glucose, amino acids and fatty acids in more detail.

• Glucose levels in your blood rise quickly after a meal and this triggers release of the hormone, insulin. Insulin’s job is to move the glucose out of your blood and into your muscles and other body tissues where it can be used for energy. Any glucose not immediately needed for energy is stored in your muscles and liver. The stored form of glucose is called glycogen. However, your liver and muscles only have space for a limited amount of glycogen, so the rest is turned into fat and stored as body fat (in the fat cells or adipose tissue).

• Amino acids from the protein in your food are used by the body to repair your tissues and make new cells. Most adults need quite small amounts of protein for repair.  The body cannot store amino acids for later use, so any excess is converted by the liver into glucose.

• Fatty acids from the fats in your food are likely not needed right away for energy (unless you have eaten no carbohydrates at all), so most are stored for use later. A small amount of fat is needed to repair your body cells along with the amino acids.

After eating, then, our bodies are busy storing away the food we have eaten, mostly by adding to our fat stores. This process usually continues for several hours after our most recent meal.

Once this process of storage is complete, our bodies are said to the enter a ‘fasting state’ when they are no longer adding to our fuel stores but instead are using that stored energy for fuel.

Now learn about fasting and burning fat

Losing weight is hard, but keeping it off is even harder. If you are doing intermittent fasting in order to lose weight, it is probably not the first time you have tried to lose weight.

The sad truth is that very few people who lose weight manage to keep the weight off. Usually they blame their lack of willpower for not being able to stick to the diet or maintenance plan. If this sounds like you, stop blaming yourself right now: you don’t lack willpower (if you have already managed to stick to a diet for long enough to lose some weight, it proves you have willpower).

What you didn’t have on these other diets (but do now have with the combination of intermittent fasting and the support from FastDay) is the tools to overcome the incredibly powerful changes that occur in your body when you lose weight which work to encourage weight regain.

What happens with a ‘standard’ weight-loss diet?

If we are overweight, the benefits to our bodies from losing some weight are many: reduced risk of heart disease, diabetes, cancer, high blood pressure, back problems to name a few. Every year, millions of people attempt to lose weight, but if they follow a ‘standard diet’ their bodies will conspire against them:

Daily energy requirements reduce: it is a well accepted fact that your body needs less energy as you lose weight. However, this is not the fabled ‘starvation mode’ in which your body supposedly ‘holds onto fat’. The reduction in energy used by your body is made up of several changes:

• Your lower weight means it takes less energy to move your body

• Your smaller body needs less energy to keep warm and functioning

• Your muscles become more efficient so that they need less energy¹

• You tend to move around less (this does not mean formal exercise, but how much you fidget, move around the house – what scientists call NEAT)¹

• Your metabolic rate (principally controlled by your thyroid gland) slows.¹

If you lose 10% of your body weight, the amount of calories you need to eat to maintain your new weight might be up to 20–25% less than you needed at your old weight.¹ Scientific studies of people who have successfully kept to a new lower body weight have found that this extra reduction in energy needs persists for years into the maintenance phase.¹

What does this mean in practice? A 165cm, 40 year old, sedentary woman starting her diet weighing 100kg is predicted to be burning 2000 calories per day. If she managed to reduce her weight, by a fairly modest amount, to 90kg, the reduction in her energy requirements might be as much as 500 calories per day. No wonder she regains the lost weight!

The actual amount by which energy requirements are reduced following weight loss is a matter of some debate by scientists, but they all agree that there is some reduction. However, it is usual for people who ‘go on a diet’ to return to their previous eating habits once they reach target.

If you want to lose weight and keep it off you need to find a Way of Eating that is sustainable for many years. This is where intermittent fasting wins.

Learn more about balancing calories eaten with calories burned

Hunger levels and cravings for food increase: as well as reducing the daily energy required, your new smaller body encourages you to eat more.² It does this in subtle ways that you may not notice unless you are paying attention.

• The amount of the fullness hormone, leptin, is reduced in the body, while levels of the hunger hormone, ghrelin, are higher, resulting in increased hunger levels.^3,4

• Activity in the hypothalamus (the part of the brain that controls appetite) increases the desire and palatability of food, increasing hedonic hunger, particularly for high calorie foods.⁵

• You tend to under-estimate the size of the portions you are eating.^6,7

• The increased hunger tends to persist even if you overeat.⁸

It is hardly surprising, then, that keeping to a lower body weight is extremely hard.

Learn more about how the body controls appetite

Intermittent fasting makes weight loss and weight maintenance easier

Scientific research into the metabolism in fasting has found that losing weight through fasting is different from standard diets:

• Leptin levels may not fall so dramatically

• Hunger is reduced

• Hedonic hunger (cravings) are reduced

Of course your smaller body will still need fewer calories than it did when you were overweight, but using fasting will enable you to keep your calorie intake down to match your new needs.

Thousands of people in the FastDay community can confirm that intermittent fasting is a very different (and much easier) way of losing weight and then maintaining that loss. This is not to say that it is always easy: everyone has times when hunger or cravings seem to be particularly strong. That’s when a visit to the FastDay forum can help you to overcome these difficult moments.

Learn more about what happens when we fast