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

by Jason Fung

The problem arises when we start changing these foods from their natural state and then consuming them in large amounts.

it’s most important to avoid sugars and refined grains, such as flour and corn products. These are more fattening than other foods, even when they contain the same number of calories, which is why low-carbohydrate diets are effective for weight loss.

Trans fats raise LDL (“bad”) cholesterol and lower HDL (“good”) cholesterol, increasing the risk of heart disease and stroke.

The basics of good nutrition can be summarized in these simple rules. Eat whole, unprocessed foods. Avoid sugar. Avoid refined grains. Eat a diet high in natural fats. Balance feeding with fasting.

Absolute fasts are not generally recommended for health purposes. The accompanying dehydration confers no additional health benefits that compensate for the increase in difficulty. The risk of medical complications is also much higher with absolute fasts.

From an evolutionary standpoint, eating three meals a day and snacking throughout the day is not a requirement for survival or good health.

For millions of people across the world, regular fasting is commonplace and has been part of spiritual practice for thousands of years. But before that, fasting was simply a way of life. With no storable grains, and few other foods that stayed fresh for very long, most of our ancestors experienced both feast and famine on a regular basis. When game was scarce, seasons changed, or the pickings were slim, hunter-gatherers did without. Eating all the time is not normal.

After a large meal, blood is shunted to your digestive system to cope with the huge influx of food, leaving less blood to go to the brain.

“A little starvation can really do more for the average sick man than can the best medicines and the best doctors.

Mark Twain

For most of human history, large amounts of food were not readily accessible all throughout the day. Intermittent fasting was likely a regular part of human evolution, and it’s possible our bodies—and brains—have come to expect periods of food scarcity.

Because we are blessed with abundant food all year round in the twenty-first century, we now have to make a special effort to impose food scarcity upon ourselves for therapeutic purposes.

If metabolic diseases such as type 2 diabetes are caused by eating too much, then logically, the solution is to eat little to balance it out.

Reduced metabolism makes us generally cold, tired, hungry, and less energetic—our bodies are essentially conserving energy by not burning calories to keep us warm and moving.

From a weight standpoint, reduced metabolism is a double curse. First, we feel lousy while dieting. Even worse, because we’re burning fewer calories per day, it’s both harder to lose weight and much easier to gain weight back after we’ve lost it. This is the main problem with most caloric-reduction diets.

The “fasting is unhealthy” reflex is largely an outcome of the marketing push to encourage consumers to buy food. 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. The Snickers campaign launched in 2015 offers a perfect example: candy bars labeled with words—“sleepy,” “grouchy,” “impatient,” etc. —hinted that these undesirable conditions were best overcome by consuming food, essentially claiming psychoactive effects for the candy bar.

Dr. Bert Herring

Real-world studies of fasting show that the concern over muscle loss is largely misplaced. Alternate-day fasting over seventy days decreased body weight by 6 percent, but fat mass decreased by 11.4 percent and lean mass (muscle and bone) did not change at all.

To burn muscle for energy would be like storing firewood and then, as soon as cold weather hits, chopping up your sofa and throwing it into the fire.

Muscle gain or loss is mostly a function of exercise.

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

So, when we fast, we “eat” our own fat.

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 that we have to get in our diet. These are called the essential amino acids (the building blocks of proteins) and essential fatty acids. These cannot be man...

The body normally loses both essential amino acids and essential fatty ac...

During fasting, it reduces these losses to hang onto much of the necessary nutrients. Bowel movements usually decrease during fasting—since no food is going into the stomach, there is less stool formation—...

To further preserve proteins, the body breaks old proteins down into their component amino acids and recycles these into new proteins.

Type 2 diabetes is a terrible disease. It is by far the leading cause of blindness, amputation, and kidney failure in North America.

Diabetes is also a leading contributor to heart attacks, stroke, and other cardiovascular diseases.

Type 2 diabetes is a dietary disease, and it requires a...

Diets can fail because they are ineffective. But they just as surely fail if people are not able to follow them.

When it comes to dietary rules, the simpler, the better.

Grains enjoy substantial government subsidies, making them far cheaper than other foods. 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.

Feasts follow fasts. Fasts follow feasts.

But those feasts should be followed by fasts.

You can indulge yourself, as long you balance that with some abstinence.

Fasting is really about balance. It is the flip side, the B-side, of eating. Balance the time that you are eating and the time you are not eating to remain healthy. When those two fall out of balance, that’s when we get into trouble.

The reason “eat less, move more” doesn’t work for weight loss is that it’s based on a false idea about how our bodies use calories: the single-compartment model.

Think of glycogen as a refrigerator. It’s designed for short-term storage of food; it’s very easy to move food in and out, but the storage space is limited.

Body fat, on the other hand, is more like a basement freezer. It’s designed for long-term storage and is more difficult to access, but it has much greater capacity.

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.

In essence, the body can burn either sugar or fat, but not both.

When we are not eating, insulin levels are low, allowing full access to the fat freezer—the body is able to easily get at the stored fat.

With low insulin levels, you don’t even have to completely empty the glycogen refrigerator before opening the fat freezer, since it’s so easily accessible.

Not only do low insulin levels allow access to the fat freezer, they actually trigger fat-burning for energy. If insulin levels are abnormally low, then fat is continually burned.

On the other hand, high insulin levels prevent the body from accessing the fat in the freezer.

Insulin inhibits lipolysis—it stops the body from burning fat.

Insulin resistance, sometimes called prediabetes or metabolic syndrome, is the most common situation where insulin levels are persistently kept abnormally high.

Insulin resistance develops because cells need to resist the effects of too much insulin.

The root cause of the problem is consistently high levels of insulin, which creates a vicious cycle: too much insulin creates resistance, insulin resistance triggers higher levels of insulin, and that in turn only serves to stimulate more resistance. The cycle reinforces itself each time it goes around.

The way to successfully break the insulin resistance cycle is not to continually increase insulin levels but to dra...