The Complete Guide To Fasting

by Jason Fung

The underlying cause of obesity turns out to be a hormonal, rather than a caloric, imbalance.

Both the ketogenic diet (a low-carb, moderate-protein, high-fat diet) and intermittent fasting are excellent methods of reducing high insulin levels.

Excessive insulin causes obesity, and excessive insulin causes insulin resistance, which is the disease known as type 2 diabetes.

As my reader so succinctly put it, “fasting allows you to reclaim your hunger for what it is [so] it no longer dictates what you put in your mouth.”

And here’s the key to why keto is so great for fasting: being in ketosis teaches your body to burn fat for fuel rather than sugar, and since that’s what your body has to do during fasting, if you’re already in ketosis, your body is already using fuel the way it’s supposed to.

also included bouillon cubes to help with electrolyte imbalance. I’ve since learned that a better, healthier way to get these helpful effects is to drink kombucha and bone broth with sea salt.

Total cholesterol dropped 100 points in less than three weeks of fasting, without the need for any cholesterol-lowering medications like statin drugs. As patients, we’re often told that drugs are the only way to get our cholesterol down so we won’t have a heart attack, and yet here is a totally drug-free method for lowering cholesterol.

During the initial stages of fasting, insulin and blood glucose levels fall but remain in the normal range, maintained by the breakdown of glycogen as well as gluconeogenesis. After glycogen is used up, the body begins to switch over to burning fat for energy. Longer-duration fasts reduce insulin more dramatically. Regularly lowering insulin levels leads to improved insulin sensitivity—your body becomes more responsive to insulin. The opposite of insulin sensitivity, high insulin resistance, is the root problem in type 2 diabetes and has also been linked to a number of diseases, including: Heart disease Stroke Alzheimer’s disease High cholesterol High blood pressure Abdominal obesity Nonalcoholic steatohepatitis (fatty liver disease) Polycystic ovary syndrome Gout Atherosclerosis Gastroesophageal reflux disease Obstructive sleep apnea Cancer Figure 1.3. Extended fasting over 4 days leads to decreased insulin and blood sugar levels. Source: Zauner et al., “Resting Energy Expenditure in Short-Term Starvation Is Increased as a Result of an Increase in Serum Norepinephrine.” Lowering insulin also rids the body of excess salt and water because insulin is well known to cause salt and water retention in the kidneys. This is why low-carbohydrate diets often cause diuresis, the loss of excess water—in fact, much of the initial weight lost on a low-carb diet is water. This diuresis is beneficial in reducing bloating and helping you feel lighter. Some may also note a lower blood pres...

Adrenaline Increases and Metabolism Speeds Up 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: they feel energized and revitalized during fasting. Partly this is because the body is still being fueled—it’s just getting energy from burning fat rather than burning food. But it’s also because adrenaline is used to release stored glycogen and to facilitate fat-burning, even if blood sugar is high. The increased adrenaline levels invigorate us and stimulate the metabolism. In fact, studies show that after a four-day fast, resting energy expenditure increased by 12 percent. Rather than slowing the metabolism, fasting revs it up.

Excessively low growth hormone levels in adults leads to more body fat, less muscle mass, and decreased bone density (osteopenia). Growth hormone, along with cortisol and adrenaline, is a counterregulatory hormone. These hormones signal the body to increase the availability of glucose—countering the effect of insulin and producing higher blood sugar levels. Levels of counterregulatory hormones peak just before waking, at approximately 4:00 a.m. or so, increasing blood sugar levels, which fall during the night. The increase prepares the body for the upcoming day by making more glucose available for energy. Growth hormone also increases the availability of fats for fuel by raising levels of key enzymes, such as lipoprotein lipase and hepatic lipase. Since burning fat reduces the need for glucose, this helps maintain stable blood sugar.

However, exogenous growth hormone—that is, growth hormone that isn’t made by your own body—carries the risk of unwanted side effects.

The most potent natural stimulus to growth hormone secretion is fasting. In one study, over a five-day fasting period, growth hormone secretion more than doubled. During fasting, in addition to the usual early-morning spike of growth hormone (pulsatile), there is also regular secretion throughout the day (non-pulsatile). Both pulsatile and non-pulsatile release of growth hormone is increased during fasting. Interestingly, very low-calorie diets are not able to provoke the same growth hormone response.

Eat whole, unprocessed foods Humans evolved to eat a wide range of foods without detrimental health consequences.

Others, such as the Okinawans, ate a traditional diet based on root vegetables, which means it’s high in carbohydrates. But both populations traditionally did not suffer metabolic diseases.

There is nothing inherently unhealthy about carbohydrate-containing foods. The problem arises when we start changing these foods from their natural state and then consuming them in large amounts.

Foods should be recognizable in their natural state as something that was alive or has come out of the ground.

Just eat real food.

The basics of good nutrition can be summarized in these simple rules. Eat whole, unprocessed foods. Avoid sugar. Avoid refined grains. Eat a diet high in natural fats. Balance feeding with fasting.

We’ll explain in detail the “best practices” for fasting in Chapter 10, but in general, we encourage consuming plenty of noncaloric liquids (water, tea, coffee) and homemade bone broth, which is full of nutrients.

I started immediately on a water-only fasting regimen. I used mostly three- to five-day fasts because I wanted quick results that I couldn’t undo by eating something wrong. I wasn’t focused on weight loss but on beating diabetes—still, I lost twelve pounds the first month and then six pounds a month thereafter, for thirty pounds in four months—I went from 256 pounds to 226. Although alternate-day fasting doesn’t slow your metabolism, multiple-day fasts do seem to slow it a little. But the improvement in insulin sensitivity is well worth a long fast over a short one.

I underpin my foods with bulgur wheat, finely chopped and sautéed cauliflower, spaghetti squash, or just well-buttered and seasoned vegetables. I try to mix cheese and nuts into every meal because they fill me up.

Paracelsus (1493–1541), a Swiss German physician and the founder of toxicology, famously wrote, “The dose makes the poison.”

“Fasting is the greatest remedy—the physician within.”

Benjamin Franklin (1706–1790),

“The best of all medicines is resting and fasting.”

Mark Twain (1835–1910),

“A little starvation can really do more for the average sick man than can the best medicines and the best doctors.” Mark Twain wa...

Studies demonstrate this phenomenon clearly. In one, fasting every other day for twenty-two days resulted in no measurable decrease in BMR. There was no starvation mode. Fat oxidation—fat burning—increased 58 percent, from 64 g/day to 101 g/day. Carbohydrate oxidation decreased 53 percent, from 175 g/day to 81 g/day. This means that the body has started to switch over from burning sugar to burning fat, with no overall drop in energy.

In another study, four days of continuous fasting increased BMR by 12 percent. Levels of the neurotransmitter norepinephrine (also known as noradrenaline), which prepares the body for action, increased by 117 percent, keeping energy levels high. Fatty acids in the bloodstream increased over 370 percent as the body switched over from burning food to burning stored fats.

At baseline, eating normally, energy comes from a mix of carbohydrates, fat, and protein. As you start fasting, the body increases carbohydrate oxidation. This is just a fancy way of saying that it is burning sugar, in the form of glycogen, for the first twenty-four to forty-eight hours after you stop eating, until it runs out of glycogen. With no more sugar to burn, the body switches to burning fat. Fat oxidation increases as carbohydrate oxidation decreases toward zero. (See Figure 3.2.) At the same time, protein oxidation—that is, burning protein, such as muscle, for fuel—actually decreases. The normal protein breakdown of around seventy-five grams per day falls to fifteen to twenty grams per day during fasting. Rather than burning muscle during fasting, we start conserving muscle.

In fact, fasting is one of the most potent stimuli for growth hormone secretion, and increased growth hormone helps maintain lean body mass. In studies that used drugs to suppress growth hormone in fasted subjects, there was a 50 percent increase in protein oxidation.

you are worried about muscle loss, exercise more.

If you fast for longer than twenty-four to thirty-six hours, glycogen stores become depleted. The liver now can manufacture new glucose in a process called gluconeogenesis, using the glycerol that’s a by-product of the breakdown of fat. This means that we do not need to eat glucose for our blood glucose levels to remain normal.

Of the three major macronutrients, there are no essential carbohydrates that the body needs to function, so it is impossible to become carbohydrate deficient. However, there are certain proteins and fats that we have to get in our diet. These are called the essential amino acids (the building blocks of proteins) and essential fatty acids. These cannot be manufactured by the body and must be obtained in the diet. The body normally loses both essential amino acids and essential fatty acids in urine and stool. During fasting, it reduces these losses to hang onto much of the necessary nutrients. Bowel movements usually decrease during fasting—since no food is going into the stomach, there is less stool formation—and this helps to prevent loss of protein in the stool. Essential nutrients, particularly nitrogen, can be lost through the urine. Nitrogen in the urine is a sign of protein metabolism, and as protein metabolism decreases during fasting, nitrogen in the urine decreases significantly, to almost negligible levels. To further preserve proteins, the body breaks old proteins down into their component amino acids and recycles these into new proteins. By keeping essential nutrients in the body instead of excreting them, the body is able to recycle many of them during fasting. Of course, no matter how good the body is at compensating, fasting means we’re not consuming essential fatty acids and amino acids. Before and after fasting, it can be helpful to follow a low-carbohydrat...

Fasting’s most obvious benefit is weight loss. However, there are a myriad of benefits beyond this, many of which were widely known before the modern era.

They were more correct than they knew. Fasting: Improves mental clarity and concentration Induces weight and body fat loss Lowers blood sugar levels Improves insulin sensitivity Increases energy Improves fat-burning Lowers blood cholesterol Prevents Alzheimer’s disease Extends life Reverses the aging process Decreases inflammation We’ll talk more about these health benefits in later chapters.

Since refined carbohydrates are a prime contributor to high insulin levels, the natural place to start with my patients was a low-carbohydrate diet. Protein, especially animal proteins (dairy and meat), can also stimulate insulin production, and excessive intake of these foods can slow down progress.

So, the best diet would emphasize whole, unprocessed foods. It would be low in refined carbohydrates and high in natural fats with a moderate amount of protein.

Type 2 diabetes is a dietary disease, and it requires a dietary solution. Most importantly, it is a curable disease.

The Centers for Disease Control and Prevention in Atlanta closely tracks obesity trends in the United States, and according to its data, in 2015, no state had an obesity rate below 20 percent. Only twenty years earlier, in 1995, no state had an obesity rate above 20 percent.

Only 2 percent of dieters using a caloric-reduction strategy were able to maintain a twenty-pound weight loss for two years.

It is more accurate to use a two-compartment model, because there are two distinct ways energy is stored in the body: as glycogen in the liver and as body fat. When we eat, our body derives energy from three main sources: glucose (carbohydrates), fat, and protein. Only two of these are stored for later use, glucose and fat—the body can’t store protein, so excess protein that can’t be used right away is converted to glucose. Glucose is stored in the liver as glycogen, but the liver’s capacity for storing glycogen is limited. Once glycogen stores are full, excess calories must be stored as body fat. Dietary fat is absorbed directly into the bloodstream without passing through the liver, and what’s not used is stored as body fat. This was one of the reasons why low-fat diets were initially recommended, but the immediate destination of ingested calories is not the main determinant of weight gain.

Think of glycogen as a refrigerator. It’s designed for short-term storage of food; it’s very easy to move food in and out, but the storage space is limited. Body fat, on the other hand, is more like a basement freezer. It’s designed for long-term storage and is more difficult to access, but it has much greater capacity. Plus, you can always add more freezers to the basement if you need them.

Both body fat and glycogen are used for energy in the absence of food, but they aren’t used equally or at the same time. The body prefers to use glycogen for energy rather than body fat. This is logical because it is easier to burn glycogen—in terms of our analogy, it’s much easier to get food from the refrigerator in the kitchen than to trek all the way down to the freezer in the basement. And as long as there is food in the fridge, we won’t retrieve any from the freezer. In other words, if you need 200 calories of energy to go for a walk, the body will get that energy from glycogen as long as it’s available—it won’t go to the trouble of accessing body fat.

The two compartments, the fridge and the freezer, are not used simultaneously but sequentially. You need to (mostly) empty out the fridge before you can use what’s in the freezer—you need to burn most of the glycogen before you can burn fat. In essence, the body can burn either sugar or fat, but not both.

When we are not eating, insulin levels are low, allowing full access to the fat freezer—the body is able to easily get at the stored fat.

Similarly, with low insulin levels, the body can burn fat even if there is still some glucose around. That means that if you’re cutting calories and have low insulin levels, it’s easy for your body to compensate for the reduced food energy by getting some fat out of the freezer even if your glycogen fridge isn’t completely empty. But the emptier your glycogen fridge, the more likely you will be to use what’s in the fat freezer, and the easier it is to get to the freezer, the more likely it is that you will use it.

Insulin inhibits lipolysis—it stops the body from burning fat. High insulin levels, which are normal after meals, signal our body to store some of the incoming energy. Logically, therefore, we also stop burning stored fat (why bother when there’s energy from food?). This doesn’t just happen after meals, however—we also see this in diseases of too much insulin. For example, insulin injections, often used in the treatment of diabetes, commonly lead to increased fat accumulation because the body is unable to burn fat. (That’s great for type 1 diabetics, who have too little fat to begin with, but not so great for type 2 diabetics, who usually have too much.) Insulin resistance, sometimes called prediabetes or metabolic syndrome, is the most common situation where insulin levels are persistently kept abnormally high.

Insulin is the main driver of obesity and diabetes. A very low carb diet can reduce insulin by more than 50 percent, but you can go another 50 percent by fasting.

During fasting, unlike during caloric reduction, metabolism stabilizes or even goes up to maintain normal energy levels. Adrenaline and growth hormone increase to maintain energy and muscle mass. Blood sugar and insulin levels go down as the body changes from burning sugar to burning fat.