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

by Jason Fung

When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use.

Both the ketogenic diet (a low-carb, moderate-protein, high-fat diet) and intermittent fasting are excellent methods of reducing high insulin levels.

In type 1 diabetes, the body’s own immune system destroys the insulin-producing cells in the pancreas. The resulting low insulin level leads to high blood sugar. Therefore, since insulin levels are low to begin with, it makes sense to treat the problem with supplemental insulin. And sure enough, it works.

In type 2 diabetes, however, insulin levels are not low but high. Blood sugar is elevated not because the body can’t make insulin but because it’s become resistant to insulin—it doesn’t let insulin do its job. By prescribing more insulin to treat type 2 diabetes, we were not treating the underlying cause of high blood sugar: insulin resistance. That’s why, over time, patients saw their type 2 diabetes get worse and required higher and higher doses of medications.

Excessive insulin causes obesity, and excessive insulin causes insulin resistance, which is the disease known as type 2 diabetes.

With that understanding, the problem with how doctors treat type 2 diabetes became clear: we were prescribing insulin to treat it, when excessive insulin was the problem in the first place.

We doctors had been treating type 2 diabetes exactly wrong. With the proper treatment, it is a curable disease. Type 2 diabetes, like obesity, is a disease of too much insulin. The treatment is to lower insulin, not raise it. We were making things worse. We were fighting the fire with gasoline.

I needed to help my obesity and type 2 diabetes patients lower their insulin levels, but what was the best approach? Certainly, there are no medications that do this. There are surgical options that help, such as bariatric surgery (commonly called “stomach stapling”), but they are highly invasive and have many irreversible side effects. The only feasible treatment left was dietary: reducing insulin levels by changing eating habits.

And here’s the key to why keto is so great for fasting: being in ketosis teaches your body to burn fat for fuel rather than sugar, and since that’s what your body has to do during fasting, if you’re already in ketosis, your body is already using fuel the way it’s supposed to.

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.

podcast in more than eight countries, Fat-Burning Man,

Nutritional authorities now allege that even skipping one single meal will have dire health consequences. You must always eat breakfast. You must snack constantly, all day long. You should eat a bedtime snack. You must never, ever miss a meal. These messages are everywhere—on television, in the newspaper, in books. Hearing them over and over again creates the illusion that they are absolutely true and scientifically proven beyond a doubt. The truth is exactly the opposite. There is no correlation whatsoever between constant eating and good health.

Insulin has two major functions. First, it allows the body to immediately start using food energy. Carbohydrates are

absorbed and rapidly turned into glucose, raising blood sugar levels. Insulin allows glucose to enter directly into most cells of the body, which use it for energy. Proteins are broken down into amino acids and absorbed, and excess amino acids may also be turned into glucose. Protein does not raise blood glucose, but it can raise insulin levels. The effect is variable, and it surprises many people to learn that some proteins can stimulate insulin as much as some carbohydrate-containing foods. Fats are directly absorbed as fat and have minimal effect on insulin.

The body does not “burn muscle” in an effort to feed itself until all the fat stores are used up.

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. This is why low-carbohydrate diets often cause diuresis, the loss of excess water—in fact, much of the initial weight lost on a low-carb diet is water.

Foods high in monounsaturated fats, such as olive oil, nuts, and avocados, which were previously shunned, are now considered “superfoods” because they are so healthy. Consuming fatty fish, such as wild salmon, has been proven to reduce the risk of heart disease. And more and more evidence is showing that naturally occurring saturated fats, such as those found in meat and dairy, are also not harmful to our health.

Think about the last time you ate a huge Thanksgiving meal. Did you feel more energetic and mentally alert afterwards? Or did you feel sleepy and a little dopey? Most of us feel sleepy. 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. The result? A food coma. Maybe a little nap. Now think about a time when you had not eaten for many hours. Do you remember being lethargic and mentally sluggish? Not likely. It is more likely you felt mentally sharp and completely attuned to your environment. This is not by accident.

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.

In the Intensive Dietary Management clinic, my personal experience with hundreds of fasting patients shows that, over time, appetite tends to decrease as the fasting duration increases.

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.

Obesity and type 2 diabetes are both problems of excessive insulin.

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

because fasting is about not doing something, it simplifies our life. It adds simplicity where other diets add complexity.

Think of glycogen as a refrigerator.

Excellent analogy

The body prefers to use glycogen for energy rather than body fat. This is logical because it is easier to burn glycogen—in terms of our analogy, it’s much easier to get food from the refrigerator in the kitchen than to trek all the way down to the freezer in the basement. And as long as there is food in the fridge, we won’t retrieve any from the freezer. In other words, if you need 200 calories of energy to go for a walk, the body will get that energy from glycogen as long as it’s available—it won’t go to the trouble of accessing body fat.

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 short answer is that when you’re eating regularly, even if you’re eating fewer calories, you’re not getting the beneficial hormonal changes of fasting.

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.

bariatric surgery is surgically enforced fasting. Bariatric surgery requires patients to dramatically reduce

People say this to me all the time. “I can’t fast for twenty-four hours.” So I ask them, “How do you know? Have you tried?” To which they answer, “No, I just know that I can’t.”

Fasting has also been noted to trigger an early period of rapid weight loss, often averaging one to two pounds per day for the first few days. This isn’t, unfortunately, due to loss of body fat. Fat loss during fasting averages approximately ½ pound per day. If you are losing 1 pound or more a day, the excess above ½ pound is water weight and will rapidly be regained upon eating. This is not abnormal or unusual. Don’t be disappointed when the water weight returns or decide that fasting doesn’t work.

I wasn’t even hungry by the end of the twenty-two hours. I didn’t need to eat—I just wanted to. That was when I truly knew that fasting is mind over matter.

In other words, the constant intake of food, such as snacking throughout the day, suppresses autophagy.

They often report that despite their own expectations, they are eating less than half their usual amount of food on a daily basis, yet they feel completely full. For most people, this is the most pleasant surprise of fasting.

We become hungry merely because of the time, not any true intrinsic hunger. This is learned only through decades of association. Children, on the other hand, often refuse food early in the morning because they are simply not hungry. Similarly, by consistently pairing movies with delicious popcorn and sugary drinks, the mere thought of a movie may make us hungry. Food companies, of course, spend billions of dollars trying to condition us to make these associations. Food at the ball game! Food at the movies! Food while watching TV! Food in between halves at our kids’ soccer games! Food while listening to a lecture! Food at a concert! Conditioned responses, every one. The possibilities are endless.

Arguing that fasting causes anorexia is like arguing that washing your hands will lead to obsessive-compulsive disorder. Excessive hand-washing is a symptom of the disease, not its cause.

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