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

by Jason Fung

When the body and brain are fueled primarily by fatty acids and ketones, respectively, the “brain fog,” mood swings, and emotional instability that are caused by wild fluctuations in blood sugar become a thing of the past and clear thinking is the new normal.

longer than twenty-four to thirty-six hours, glycogen stores become depleted.

gluconeog...

using the glycerol that’s a by-product of the b...

A related myth is that brain cells can only use glucose for energy.

Up to 75 percent of the brain’s energy requirements can be met by ketones. Of course, that means that glucose still provides 25 percent of the brain’s energy requirements.

Between the glucose we have already stored away in the form of body fat and what the liver produces in gluconeogenesis, we have plenty of fuel

Even prolonged fasting won’t send blood glucose levels dangerously low.

with hundreds of fasting patients shows that, over time, appetite tends to decrease as the fasting duration increases.

With shorter fasting periods (less than twenty-four hours), there is ample opportunity before and after the fast to eat nutrient-dense foods to make up for missed meals. For longer fasts, it is a good idea to take a general multivitamin. The longest fast recorded lasted 382 days, and a simple multivitamin prevented any vitamin deficiencies.

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

Before and after fasting, it can be helpful to follow a low-carbohydrate diet, which increases the percentage of fats and proteins consumed, so the body has more stored up for a rainy day.

Myth #6: “It’s Just Crazy”

Obesity, at its very core, involves some form of overeating. This is true whether you believe it is caused by consuming too many calories, carbohydrates, or fats. Fasting helps in all these cases. Its effectiveness is unquestioned. After all, if you don’t eat anything at all, don’t you think you would lose weight?

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

Protein, especially animal proteins (dairy and meat), can also stimulate insulin production,

The overall goal was not necessarily to reduce carbohydrate intake. The goal was to reduce insulin levels, and cutting carbohydrates was only one method of achieving that goal.

Yet all

foods, to varying degrees, stimulate the rele...

It is so simple that it can be explained in two sentences: Eat nothing. Drink water, tea, coffee, or bone broth. That’s it.

From a therapeutic standpoint, a key advantage of fasting is that it has no upper limit—there’s no maximum amount of time that you can fast.

So if fasting occasionally isn’t working, all you need to do is increase the

frequency or length of time you fast, until you ...

FASTING FOR WEIGHT LOSS

The dominant nutritional paradigm of the last half century has been “calories in, calories out.” The idea is that consuming fewer calories than are used will ultimately lead to lasting weight loss.

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.

reducing calories consumed and increasing calories burned—

1959 study estimated its failure rate at a sky-high 98 percent.

Only 2 percent of dieters using a caloric-reduction strategy were able to maintain a twenty-poun...

2015, researchers in the United Kingdom reviewed the weight-loss rates of more than 175,000 obese men and women over the previous nine years. The probability of achieving a normal weight by caloric reduction alone was 0.8 percent in women and 0.47 percent in men. So the best-case scenario using conventional calorie-counting methods is a 99.2 percent failure rate.

The Women’s Health Initiative, a gigantic randomized controlled trial, followed almost fifty thousand women over seven and a half years.

The actual difference in weight loss between the two groups was not even two pounds!

A study of Biggest Loser contestants showed that in thirty weeks of filming, average weight dropped from 329 pounds to 202 pounds.

Six years after their almost miraculous weight loss, thirteen of the fourteen contestants studied had regained the weight they’d lost. This is a failure rate of 93 percent.

Here’s the thing, though. Let’s say, for the sake of argument, that when followed to the letter, “eat less, move more” does work. In the end, it actually doesn’t matter. Whether it’s good advice that people don’t follow or bad advice that people do follow, the result is the same: it doesn’t produce weight loss. And if the end results are bad—and they are!—then the advice is bad. The proof is in the pudding.

How Our Bodies Really Use Calories

single-compartment model.

This model is like a bathroom sink.

Whether calories are stored as glucose, which is used for immediate energy; glycogen, which is used in the intermediate time frame; or fat, which is long-term energy storage, all calories are treated equally.

However, this model is known to be a complete fabrication. It does not exist except in our imaginations.

It is more accurate to use a two-compartment model, because there are two distinct ways energy is stored in the body: as gly...

excess protein that can’t be used right away is converted to glucose.

Dietary fat is absorbed directly into the bloodstream without passing through the liver, and what’s not used is stored as body fat.

the immediate destination of ingested calories is not the main determinant of weight gain.

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.

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.

With low insulin levels, you don’t even have to completely empty the glycogen refrigerator before opening the fat freezer,

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.

I am also a type 2 diabetic, and intermittent fasting has allowed me to keep my blood sugar levels in a normal range.