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

by Jason Fung

Our bodies do not shut down in response to short-term fasting.

In fact, metabolism revs up, not down, during fasting. This makes sense from a survival standpoint.

Humans have not evolved to require three meals a...

Instead, hormones allow the body to switch energy sources from food to body fat.

In one, fasting every other day for twenty-two days resulted in no measurable decrease in BMR. There was no starvation mode. Fat oxidation—fat burning—increased 58 percent, from 64 g/day to 101 g/day. Carbohydrate oxidation decreased 53 percent, from 175 g/day to 81 g/day.

In another study, four days of continuous fasting increased BMR by 12 percent.

An enormous amount of advertising money is spent every year in an effort to convince consumers that at any moment that they are not consuming food, they are at risk for underperforming.

Myth #2: Fasting Makes You Burn Muscle

Muscle, on the other hand, is preserved until body fat becomes so low that the body has no choice but to turn to muscle.

Real-world studies of fasting show that the concern over muscle loss is largely misplaced.

As you start fasting, the body increases carbohydrate oxidation.

fancy way of saying that it is burning sugar, in the form of glycogen, for the first twenty-four to forty-eight hours after you stop eating, until it runs out of glycogen.

Fat oxidation increases as carbohydrate oxidation decr...

At the same time, protein oxidation—that is, burning protein, such as muscle, for...

Rather than burning muscle during fasting, we start conserving muscle.

Much of the amino acids that are broken down during regular turnover of cells are reabsorbed into new proteins.

most Americans could walk from New York to Florida without technically needing a bite to eat.

Muscles and other proteins are functional tissues and have many purposes.

In fact, fasting is one of the most potent stimuli for growth hormone secretion, and increased growth hormone helps maintain lean body mass.

Muscle gain or loss is mostly a function of exercise.

Exercise is the only reliable way to build muscle.

If you are worried about muscle loss, exercise more. It’s not rocket science. Diet and exercise are two entirely separate issues.

On the other hand, if you are worried about weight loss and type 2 diabetes, then you need to worry about diet, not exercise. You can’t outrun a bad diet.

Body fat is, at its core, stored energy for us to “eat” when there is no food.

Myth #3: Fasting Causes Low Blood Sugar

During fasting, our body begins by breaking down glycogen (remember, that’s the glucose in short-term storage) in the liver to provide glucose.

They usually attribute this to achieving some kind of spiritual enlightenment, but the truth is much more down-to-earth and scientific than that: it’s the ketones!

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.

If you fast for longer than twenty-four to thirty-six hours, glycogen s...

The liver now can manufacture new glucose in a process called gluconeogenesis, using the glycerol that’s a by-...

Human brains, unique amongst animals, can also use ketone bodies—particles that are produced when fat is metabolized—as a fuel source.

When glucose is not available, the body begins to burn fat and produce ketone bodies, which are able to cross the blood-brain barrier to feed the brain cells.

I maintain muscle mass and have discovered that I function optimally on fewer calories than I thought necessary.

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 when no food is available.

Myth #4: Fasting Results in Overeating

Myth #5: Fasting Deprives the Body of Nutrients

There are two main types of nutrients, micronutrients and macronutrients.

Micronutrient deficiency is rare in the developed world.

Of the three major macronutrients, there are no essential carbohydrates that the body needs to function, so it is impossible to become carbohydrate deficient.

The body normally loses both essential amino acids and essential fatty acids in urine and stool.

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.

So, the best diet would emphasize whole, unprocessed foods. It would be low in refined carbohydrates and high in natural fats with a moderate amount of protein.

Type 2 diabetes is a dietary disease, and it requires a dietary solution. Most importantly, it is a curable disease.

The goal was to reduce insulin levels, and cutting carbohydrates was only one method of achieving that goal.

Fasting is the oldest dietary intervention in the world. It is profoundly different from all other dietary strategies. It is not the latest and greatest but the tried and true.

It is not something to do but something to not do.

Eat nothing. Drink water, tea, coffee, or bone broth. That’s it.

This means that a pound of fresh cherries may cost $6.99, while an entire loaf of bread will cost $1.99. An entire box of pasta may cost only $0.99 on sale. Feeding a family on a budget is a lot easier when you buy pasta and white bread.

Fasting, on the other hand, is the exact opposite. There is no time spent buying groceries, preparing ingredients, cooking, and cleaning up. It is a way to simplify your life.

It is, after all, the cycle of life. Feasts follow fasts. Fasts follow feasts. This is how we have always lived.