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

by Jason Fung

These studies all came to the same startling conclusion: the low-carb diet was significantly better for weight loss than the low-fat diet.

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

Insulin is a fat-storage hormone.

Understanding this leads naturally to a solution: if excessive insulin is causing obesity, then clearly the answer lies in reducing insulin.

As it turns out, insulin causes insulin resistance.

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.

Since refined carbohydrates highly stimulate insulin, reducing these carbohydrates should be an effective method of lowering insulin.

First, I was still drinking diet sodas during the fast, and that stoked hunger and cravings that I wouldn’t have had otherwise.

Second, I wasn’t getting enough salt during the fast, which led to fatigue and energy drain.

Better than those diet sodas would have been bone broth with sea salt, which provides much-needed e...

he was adamant that a one-week fast would be beneficial as an annual cancer-prevention strategy.

What we think is hunger is not really hunger. That impulse to eat cannot be taken seriously.

The second thing that propelled me to do a one-week fast was that I’d been learning a lot about all the benefits of being in a state of nutritional ketosis—and fasting and ketosis go together perfectly,

The carbohydrate restriction and protein moderation help to keep your blood sugar and insulin levels under control, and consuming adequate amounts of healthy saturated and monounsaturated fats keeps your hunger at bay.

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.

You have at least 40,000 calories’ worth of fat on your body right now, but only 2,000...

If you’re a sugar-burner, though, your body burns those 2,000 calories of sugar until it’s all gone, and then it triggers hunger until it’s adapted to using fat.

This is why going keto (covered in detail in my book Keto Clarity) is a great first step for fasting, both IF and extended fasting.

Controlling blood sugar and giving your pancreas a week to rest from producing insulin is an excellent reason for attempting a fast like this.

I learned later that the weight loss on a one-week fast like this is mostly water weight because the fasting process depletes glycogen stores.

I did not stop taking my regular supplements for the duration of the fast. I continued with my multivitamin, vitamin D3, magnesium, probiotics, and other vitamins that have been a part of my healthy low-carb lifestyle for years.

I’ve since learned that a better, healthier way to get these helpful effects is to drink kombucha and bone broth with sea salt.

He also lamented that I didn’t measure the level of ketones—byproducts of the body burning fat—in my blood.

which might prevent glucose from reaching the lowest levels needed to put maximum metabolic pressure on tumor cells.

especially when my blood ketone levels exceeded 1.0 millimole.

Once my body made the switch from burning glucose to burning fat, the idea of eating breakfast, snack, lunch, snack, dinner, snack, midnight snack just seemed silly.

Fasting shouldn’t be physically painful. End it, have something to eat, and try again in a week or so.

Instead, they eat more out of habit, comfort, and boredom than anything else. This is critical to understand if you’re going to attempt to engage in fasting.

Fasting is completely different from starvation in one crucial way: control.

The truth is exactly the opposite. There is no correlation whatsoever between constant eating and good health.

Fasting is effective, simple, flexible, practical, and virtually guaranteed to work.

The key hormone involved in both the storage and use of food energy is insulin, which rises during meals.

Fat triggers a far smaller insulin effect, but it’s rarely eaten alone.

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.

1. Feeding: Blood sugar levels rise as we absorb the incoming food, and insulin levels rise in response to move glucose into cells,

2. The postabsorptive phase (six to twenty-four hours after beginning fasting): At this point, blood sugar and insulin levels begin to fall.

3. Gluconeogenesis (twenty-four hours to two days after beginning fasting): At this point, glycogen stores have run out.

4. Ketosis (two to three days after beginning fasting): Low insulin levels stimulate lipolysis, the breakdown of fat for energy.

The body uses fatty acids to produce ketone bodies, which are capable of crossing the blood-brain barrier and are used by the brain for energy.

After four days of fasting, approximately 75 percent of the energy used by the brain is provided by ketones.

The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, which can increase ov...

5. The protein conservation phase (five days after beginning fasting): High levels of growth hormone maintai...

In essence, what we are describing here is the process of switching from burning glucose to burning fat.

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

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

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.

Growth hormone, along with cortisol and adrenaline, is a counterregulatory hormone. These hormones signal the body to increase the availability of glucose—

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.

Growth hormone also increases the availability of fats for fuel by raising levels of key enzymes, such as lipoprotein lipase and hepatic lipase.