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

by Jason Fung

nutrition is not a topic covered extensively in medical school. Most schools, including the University of Toronto, spend a bare minimum of time teaching nutrition. There were perhaps a handful of lectures on nutrition in my first year of medical school and virtually no teaching on nutrition throughout the rest of medical school, internship, residency, and fellowship. Out of the nine years spent in formal medical education, I would estimate I had four hours of lectures on nutrition.

“Eat less, move more” has been done to death over the past fifty years, and it doesn’t work. For all practical purposes, it doesn’t really matter why it doesn’t work (although we’ll look into that in Chapter 5); the bottom line is that we’ve all done it, and it doesn’t work.

The underlying cause of obesity turns out to be a hormonal, rather than a caloric, imbalance. Insulin is a fat-storage hormone.

As it turns out, insulin causes insulin resistance. The body responds to excessively high levels of any substance by developing resistance to it.

Since refined carbohydrates highly stimulate insulin,

First, I was still drinking diet sodas during the fast, and that stoked hunger and cravings

The way you experience yourself physically when you are fasting is practically identical to the way you experience yourself physically when you are eating. The reason this is so important is that when you think you experience hunger while eating normally, that same experience of hunger is present when you are fasting. In other words, the hunger sensations in fasting are the same as while eating normally. You then have to ask yourself how you can be hungry when you have eaten three hours ago when it is the same hunger sensation when you have not eaten anything at all for a week. What we think is hunger is not really hunger. That impulse to eat cannot be taken seriously.

You have at least 40,000 calories’ worth of fat on your body right now, but only 2,000 calories’ worth of sugar. If you’re a fat-burner, when you start fasting, your body simply continues to use fat as its primary fuel. 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.

As a sugar-burner, you’ll feel the effects of hunger during a fast much earlier and more intensely.

(Yes, I know, Dr. Fung advises against this—see here—and they’re probably a big reason why my first attempt at fasting was so tough.

He said the vitamins I took and other things I added to my fast may have sent “mixed signals to the body,” making the fast more difficult. Dr. Seyfried noted that a cancer-preventing fast should probably be done using distilled water only and nothing

else.

Chicken and beef bouillon contain some calories and salts, which might prevent glucose from reaching the lowest levels needed to put maximum metabolic pressure on tumor cells.

If your energy is just gone and not coming back, or you’re not feeling like yourself, or you’re insanely hungry, don’t push through. Fasting shouldn’t be physically painful. End it, have something to eat, and try again in a week or so.

true hunger is much different than what we have come to expect. The sad fact is, most people don’t listen to what their body is telling them. Instead, they eat more out of habit, comfort, and boredom than anything else.

your body’s internal clock trying to keep you on your normal, familiar routine of eating.

The notion that if you don’t eat, you’ll experience an increasing feeling of hunger the longer it lasts, is simply not true. In fact, I daresay that after fasting for a few days, you will actually feel more normal than perhaps you ever have before. And when you don’t think so much about what you’ll eat, when you’ll eat, where you’ll eat, and all the other social customs surrounding food, it frees you up to do other things. You’ll realize the urge and desire to eat is more mental than physical.

the evening of day 17, my stomach was growling for forty-five minutes straight! Because it was close to bedtime, I decided to wait until the next morning to see if the hunger would dissipate. It did not, and so I broke the fast just a few days short of my original goal.

I had the idea to fast for the entire month.

The scan showed I lost ten pounds of body fat and another ten pounds of what it identified as “lean tissue,” generally interpreted as muscle. All of this supposed “muscle loss” happened in the trunk area; I actually gained muscle in my arms and legs. When I discussed this result with Dr. Fung, he noted that the DXA scan can mistake

fat loss in organ tissues for muscle loss. In other words, most likely I lost not muscle but rather fat around my internal organs—a very good thing!

What happened to that lean tissue it showed I lost during my fast? Every single pound of that so-called muscle loss was completely gone and I was back to the level I was at prior to the fast.

The fact is, I didn’t lose any muscle during my twenty-eight days of fasting, and that’s pretty remarkable! It goes against everything commonly believed about the side effects of fasting. (Dr. Fung discusses the myth that fasting causes muscle loss in more detail on here.)

Fasting is completely different from starvation in one crucial way: control. Starvation is the involuntary abstention from eating. It is neither deliberate nor controlled.

the fact that fasting is voluntary is a critical distinction.

It is the difference between recreational running and running because a lion is chasing you.

anytime that you are not eating, you are technically fasting.

So what’s the problem? Nobody makes money when you fast. Not Big Food. Not Big Pharma. Nobody wants you to find out the ancient secret of weight loss.

What Happens When We Eat?

Insulin has two major functions.

Protein does not raise blood glucose, but it can raise insulin levels.

Fats are directly absorbed as fat and have minimal effect on insulin.

Once this limit is reached, the body starts to turn glucose into fat. This process is called de novo lipogenesis

Glycogen

the liver stores enough to provide energy for twenty-four hours or so.

Either we are storing food energy or we are burning food energy.

What is needed then is to restore balance by increasing the amount of time we burn food energy (by going into the fasted state).

George Cahill, one of the leading experts in fasting physiology:

Low insulin levels stimulate lipolysis, the breakdown of fat 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 over seventyfold during fasting.

The protein conservation phase (five days after be...

Increased norepinephrine (adrenaline) levels prevent any decrease in metabolic rate.

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.)

The body is

not “shutting down”; it’s merely changing fuel sources, from food to our own fat.

All foods raise insulin to some degree.

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

Regularly lowering insulin levels leads to improved insulin sensitivity

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.