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.

Most type 2 diabetics also suffer from obesity.

Despite what you might think, nutrition is not a topic covered extensively in medical school.

Doctors spend almost no time thinking about this question. Why? Because we think we already know the answer. We think that excessive caloric intake causes obesity. And if eating too many calories is the problem, then the solution is eating fewer calories and burning more through an increase in activity.

The underlying cause of obesity turns out to be a hormonal

Insulin is a fat-storage hormone.

Type 1 diabetes is an entirely different problem than type 2.

type 1 diabetes, the body’s own immune system destroys the insulin-producing cells in the pancreas.

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.

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

Type 2 diabetes, like obesity, is a disease of too much insulin.

The treatment is to lower insulin, not raise it.

Fasting is completely different from starvation in one crucial way: control. Starvation is the involuntary abstention from eating.

Starving people have no idea when and where their next meal will come from.

When we eat, we ingest more food energy than we can immediately use.

Some of this energy needs to be stored away for later.

The key hormone involved in both the storage and use of food energy is insulin, w...

Both carbohydrates and protein stimulate insulin. Fat triggers a far smaller insulin effect, ...

Protein does not raise blood glucose, but it can raise insulin levels. The effect is variable, and it surprises many people to learn that some proteins can stimulate insulin as much as some carbohydrate-containing foods. Fats are directly absorbed as fat and have minimal effect on insulin.

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. This process is called de novo lipogenesis ...

The process of using and storing food energy that occurs when we eat goes in reverse when we fast.

Insulin levels drop, signaling the body to start burning stored energy. Glycogen (the glucose that’s stored in the liver) is the most easily accessible energy source, and the liver stores enough to provide energy for twenty-four hours or so. After that, the body starts to break down stored body fat for energy.

So you see, the body really only exists in two states—the fed (high-insulin) state and the fasted (low-insulin) state. Either we are storing food energy or we are burning food energy. If eating and f...

If, however, we spend the majority of the day storing food energy (because we’re in the fed state), then over time, we will gain weight. What is needed then is to restore balance by increasing the amount of ti...

In essence, what we are describing here is the process of switching from burning glucose to burning fat. Fat is simply the body’s stored food energy. 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.)

Periods of low food availability have always been a natural part of human history, and our body evolved mechanisms to adapt to this fact of Paleolithic life.

There are no adverse health consequences to activating these protocols, except in the case of malnourishment (you should not fast if you’re malnourished, of course, and extreme fasting can cause malnourishment, too).

The body is not “shutting down”; it’s merely changing fuel sources, from food to our own fat. It does this with the help of seve...

A decreased insulin level is one of the most consistent hormonal effects of fasting.

All foods raise insulin to some degree. Refined carbohydrates tend to raise insulin the most and fatty foods the least, but insulin still goes up in both cases.

Therefore, the most effective method of reducing insulin is to avoid ...

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.

Prolonged studies of fasting have found no evidence of electrolyte imbalances—the body has mechanisms in place to keep electrolytes stable during fasting.

In a study of supervised fasts with only water and vitamins lasting up to 117 days, researchers confirmed that there were no changes in serum electrolytes, lipids, proteins, or amino acids. Moreover, they found that hunger was virtually absent during these prolonged fasts.

Most people expect that a period of fasting will leave them feeling tired and drained of energy.

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.

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

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.

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 ...

overeating suppresses growth hormone levels by as much as 80 percent.

The most potent natural stimulus to growth hormone secretion is fasting.

Modern medicine’s greatest challenges are metabolic diseases: obesity, type 2 diabetes, high blood pressure, high blood cholesterol, and fatty liver, collectively known as metabolic syndrome.

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.

Because consistently high insulin levels are the root cause of all the diseases of metabolic syndrome, it’s especially important for those with metabolic syndrome to consider how foods stimulate the release of insulin.

Fasting, of course, is the ultimate weapon in your arsenal when it comes to lowering insulin levels—since all foods stimulate insulin to some degree, the best way to lower insulin is to eat nothing at all. However, we cannot fast indefinitely, so there are some simple rules to follow to lower insulin levels.

Our body has evolved to handle natural foods, and when we feed it unnatural ones, the result is illness.

There is nothing inherently unhealthy about carbohydrate-containing foods.