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

by Jason Fung

Insulin plays a crucial role in the decision of which fuel to burn.

To burn fat, two things must happen: you must burn through most of your stored glycogen, and insulin levels must drop low enough to release the fat stores.

The body always wants to stay at a certain weight, and any deviation above or below that weight triggers adaptive mechanisms to get us to return to that weight.

That’s why, after weight loss, we become hungrier and our metabolism relentlessly slows, so that we have to eat even less just to maintain our lower weight.

That’s the body trying to get us to gain weight to get us bac...

The reason the body has to resort to decreasing metabolism and increasing hunger is because insulin remains high, so it doesn’t hav...

Let’s take an example. Suppose you eat 2,000 calories per day. Your weight is stable, so you are burning 2,000 calories per day. There are 3,500 calories in a pound of fat, so if you carry 100 pounds of fat, there are 350,000 calories in fat stores. Now, let’s say you want to lose weight, so you reduce your daily calories to 1,200. Initially, fat will be lost to make up for the reduced calories. However, if you have insulin resistance, then persistently high insulin levels will make it difficult to access fat stores. The high insulin is instructing the body to store energy, not burn it. The body is used to burning 2,000 calories, but now, only 1,200 are available, so it is forced to reduce its energy expenditure to match. Basal metabolic rate decreases to 1,200 calories.

The major problem, as you can see, is not that there aren’t enough calories available. There are 350,000 calories stored away in the fat freezer. The problem is that these calories are not available for the body to use. The main issue is how to get access to the energy locked away in the fat. Insulin is the crucial factor to consider here, not the number of calories you eat.

The inability of most diets to reduce insulin resistance is exactly why they eventually result in weight regain.

Fasting, on the other hand, introduces prolonged periods of low insulin levels, which breaks the cycle of high insulin and insulin resistance.

To prevent the body from adapting to the new weight-loss strategy and maintain weight loss requires an intermittent strategy, not a constant one. This is a crucial distinction. Restricting some foods all the time differs from restricting all foods some of the time. This is the difference between failure and success.

Body fat is composed mostly of triglycerides, which are molecules made of one glycerol backbone to which three fatty acids of varying lengths are attached.

In this situation, insulin levels should be high to handle the blood sugar, but with the insulin-producing pancreatic beta cells destroyed, the body doesn’t produce enough insulin. (This is why type 1 diabetics need to take insulin—their bodies simply don’t make enough.) Due to the lack of insulin, the body produces lots of ketones. But since there’s plenty of glucose in the bloodstream and the brain prefers to use glucose, the ketones are not burned for fuel. Instead they pile up outside cells, like unused logs of firewood. This creates a dangerous, even life-threatening situation.

Keto acidosis

If you don’t have type 1 diabetes, don’t worry: you won’t develop ketoacidosis!

Cortisol is a hormone that’s released during times of stress, whether physical or psychological.

However, cortisol is also one of the major drivers of obesity.

In fact, synthetic cortisol, a medication called prednisone, consistently causes weight gain, particularly in the trunk.

But studies of intermittent fasting show that cortisol levels are generally unaffected.

While levels may vary among individuals, on the whole, elevated cortisol levels are not a major concern during fasting.

The amount of weight lost on a fasting regimen varies tremendously from person to person.

The longer that you have struggled with obesity, the more difficult you’ll find it to lose weight.

the key is simply to change the fasting protocol.

Type 1 diabetes is an autoimmune disease. For unknown reasons, the body’s own immune system attacks and destroys the insulin-producing cells in the pancreas, leading to a severe insulin deficiency.

Type 2 diabetes, on the other hand, is a dietary and lifestyle disease. In response to frequent high blood sugar, the body produces excessive insulin, which leads to insulin resistance—just as we stop being able to smell a particular odor in a room after a while, the body stops being able to respond to insulin’s signals after prolonged exposure to excess insulin. There is a clear association between type 2 diabetes and obesity, and weight loss often reverses this type of diabetes.

Type 2 diabetes was quite uncommon until the twentieth century, for two reasons:

first, it is typically diagnosed after age fifty (in fact, it used to be called adult-onset diabetes) and average life expectancy was lower than it is today, and second, food was not nearly as available and plentiful.

Interestingly, visceral fat, fat that’s stored in and around the organs, likely plays a large role in type 2 diabetes.

It’s more harmful to health and, unfortunately, more common than subcutaneous fat.

One of insulin’s main jobs is to move glucose from the blood into the tissues, which use it as energy.

When insulin resistance develops, the normal level of insulin is not able to move glucose into tissue cells.

But the main cause of the insulin resistance is that the cell was already overflowing with glucose.

There are really only two methods of getting the toxic glucose overload out of the body.

First, you need to stop putting glucose into the body. You can achieve this with very low carbohydrate diets or ketogenic diets. Indeed, many people have reversed their diabetes by following such diets. Fasting also eliminates carbohydrates—and all other foods, for that matter.

Second, your body needs to burn off the excess glucose. Fasting is agai...

Your body requires energy just to keep all the vital organs, such as the heart, lungs, liver, and kidneys, working. Your brain in particular requires substantial en...

During fasting, no new glucose is coming in, so your body has no choice but to us...

At its core, type 2 diabetes is a disease of excessive glucose, in our blood but also in our bodies. If you don’t eat, your blood sugar level will come down. Once your blood sugars are consistently in the normal range, you will no longer be ...

I consider the optimal blood sugar range while fasting to be 8.0 to 10.0 mmol/L, if you are taking medication.

HbA1c

So what actually happens during caloric deprivation is that the brain maintains or even boosts its abilities.

Humans, like all mammals, have an increase in mental activity when hungry and a decrease when satiated.

Despite popular belief, it’s not the tryptophan in the turkey causing that postprandial drowsiness—in fact, turkey has about the same amount of tryptophan as other poultry. It’s just the sheer amount of food. As the amount of blood going to the digestive system is increased to handle all that turkey and pie, less blood is available to go to the brain.

There appears to be significant research indicating a dramatic drop in inflammation, improvements in insulin signaling, and a near total “reset” of immune function with fasts of 3–5 days. Abnormal and or pre-cancerous cells appear to be pushed towards apoptosis, which essentially selects for healthy cell types. In total this describes a process which should (in theory) reverse many of the signs and symptoms of aging while reducing the processes that appear to be at play in autoimmunity and cancer.

Increased levels of glucose, insulin, and proteins all turn off autophagy.

The mammalian target of rapamycin (mTOR) pathway is an important sensor of nutrient availability. When we eat carbohydrates or protein, insulin is secreted, and the increased insulin levels, or even just the amino acids from the breakdown of ingested protein, activate the mTOR pathway. The body senses that food is available and decides that since there’s plenty of energy to go around, there’s no need to eliminate the old subcellular machinery. The end result is the suppression of autophagy. In other words, the constant intake of food, such as snacking throughout the day, suppresses autophagy.

fasting cleanses the body of unhealthy or unnecessary cellular debris. This is the reason longer fasts were often called cleanses or detoxifications.

fasting also stimulates growth hormone, which signals the production of some new snazzy cell parts, giving our bodies a complete renovation. Since it triggers both the breakdown of old cellular parts and the creation of new ones, fasting may be considered one of the most potent anti-aging methods in existence.

Fasting can limit growth of glucose-dependent tumors. Fasting can also target inflammation that contributes to the initiation and progression of tumors. We showed that fasting or calorie restriction could significantly reduce distal tumor invasion in our preclinical models of brain cancer.

What we call “high cholesterol” refers to LDL cholesterol, and large epidemiological studies have associated increased levels of LDL with increased risk of cardiovascular disease.

Another risk factor for heart disease is a type of fat called triglycerides.