The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

by Jason Fung

Type 2 diabetes is far and away the leading cause of kidney disease, and I treat many hundreds of patients with this disease.

The underlying cause of obesity turns out to be a hormonal, rather than a caloric, imbalance. Insulin is a fat-storage hormone. When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use.

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

In type 1 diabetes, the body’s own immune system destroys the insulin-producing cells in the pancreas. The resulting low insulin level leads to high blood sugar. Therefore, since insulin levels are low to begin with, it makes sense to treat the problem with supplemental insulin.

In type 2 diabetes, however, insulin levels are not low but high. Blood sugar is elevated not because the body can’t make insulin but because it’s become resistant to insulin—it doesn’t let insulin do its job.

As it turns out, insulin causes insulin resistance.

The body responds to excessively high levels of any substance by developing resistance to it. If you drink excessive alcohol, your body will develop resistance, up to a point—we often call this “

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

Both carbohydrates and protein stimulate insulin. Fat triggers a far smaller insulin effect, but it’s rarely eaten alone.

Insulin allows glucose to enter directly into most cells of the body, which use it for energy.

Second, insulin helps store the excess energy. There are two ways to store the energy. Glucose molecules can be linked into long chains called glycogen and then stored in the liver. There is, however, a limit to the amount of glycogen that can be stored away. Once this limit is reached, the body starts to turn glucose into fat.

In times of low food availability, stored food is naturally released to fill the void. The body does not “burn muscle” in an effort to feed itself until all the fat stores are used up. (More on this myth in Chapter 3.)

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.

In a study of supervised fasts with only water and vitamins lasting up to 117 days, researchers confirmed that there

Where is the footnote?

Human growth hormone (HGH) is made by the pituitary gland.

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.

And the roots of metabolic syndrome lie in the Western diet, with its abundance of sugar, high fructose corn syrup, artificial flavors, artificial sweeteners, and overdependence on refined grains. Societies

If it has a nutrition label, it should be avoided. Real foods, whether broccoli or beef, have no labels. The true secret to healthy eating is this: 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.

Fasting does not make you tired. Fasting does not burn muscle. There is no starvation mode from fasting where you shrivel up into the fetal position on your couch.

From an evolutionary standpoint, eating three meals a day and snacking throughout the day is not a requirement for survival or good health.

One early fasting advocate was Hippocrates of Cos (c. 460–c. 370 BC), widely considered the father of modern medicine.

Think about the last time you had the flu, or even a cold. The last thing you probably wanted to do was eat. So fasting can be considered a universal human instinct for handling multiple kinds of illnesses.

The ancient Greeks also believed that fasting improved mental and cognitive abilities, and they recognized that they could solve problems and puzzles better during fasting.

Diet and exercise are two entirely separate issues. Don’t confuse the two. Don’t worry about what your diet (or lack of diet—that is, fasting) is doing to your muscle mass. Exercise builds muscle. Lack of exercise leads to atrophy of muscles.

You can’t outrun a bad diet.

During fasting, hormonal changes kick in to give us more energy (increased adrenaline) and preserve our lean muscles and bones (increased growth hormone). This is normal and natural and there is nothing here to be feared.

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.

Micronutrients are vitamin and minerals

The longest fast recorded lasted 382 days, and a simple multivitamin prevented any vitamin deficiencies.

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

It is by far the leading cause of blindness, amputation, and kidney failure in North America. Diabetes is also a leading contributor to heart attacks, stroke, and other cardiovascular diseases. Type 2 diabetes is a dietary disease, and it requires a dietary solution. Most importantly, it is a curable disease.

there is usually some underlying inflammatory or infectious issue. Fasting can provide instantaneous relief of symptoms and aids in supporting recovery from the underlying issue.

Grains enjoy substantial government subsidies, making them far cheaper than other foods.

If a diet is unaffordable, it does not truly matter if it is effective. The price makes it ineffective for those who cannot afford to follow it. This should not doom them to a lifetime of type 2 diabetes and disability.

And when you fast regularly, you do not need to feel guilty about enjoying one of life’s little pleasures, because you can make up for it.

Fasting can be done at any time. There is no set duration. You may fast for sixteen hours or sixteen days. You can mix and match time periods. You are never locked into a pattern. You may fast for one day this week, five days the next, and two days the week after. Life is unpredictable. Fasting fits wherever you need it to.

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.

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.

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.

Insulin levels are the prime determinant. 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.

Not only do low insulin levels allow access to the fat freezer, they actually trigger fat-burning for energy. If insulin levels are abnormally low, then fat is continually burned.

high insulin levels prevent the body from accessing the fat in the freezer. It is locked away behind steel bars. 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?).

The clue lies in its very name. Insulin resistance develops because cells need to resist the effects of too much insulin. The root cause of the problem is consistently high levels of insulin, which creates a vicious cycle: too much insulin creates resistance, insulin resistance triggers higher levels of insulin, and that in turn only serves to stimulate more resistance. The cycle reinforces itself each time it goes around. The way to successfully break the insulin resistance cycle is not to continually increase insulin levels but to drastically decrease insulin levels.

If we are unable to break the cycle of insulin resistance, then insulin levels remain high. This blocks our ability to burn the body fat we have so carefully stored away. Our body is constantly receiving the signal to store energy as fat and is never told to burn fat. Insulin plays a crucial role in the decision of which fuel to burn.

The body always wants to stay at a certain weight,

inexorably

To prevent the body from adapting to the new weight-loss strategy and maintain weight loss requires an intermittent strategy, not a constant one. This is a crucial distinction. Restricting some foods all the time differs from restricting all foods some of the time. This is the difference between failure and success.

Aim to drink two liters of water and other fluids daily.

In the pre-agricultural era, it is estimated that animal foods provided about two-thirds of the calories in the human diet. So, despite all the modern teeth gnashing about red meat and saturated fats, it seems that our ancestors had little problems eating them.