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

by Jason Fung

Now, I am not saying that you should eat dessert ...

Feasts follow fasts. Fasts follow feasts. This is how we have always lived.

when you fast regularly, you do not need to feel guilty about enjoying one of life’s little pleasures, because you can make up for it.

Advantage #5: It’s Powerful

The quickest and most efficient way to lower insulin and insulin resistance is fasting. It has unrivaled power to break through weight-loss plateaus and reduce the need for insulin.

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. The world record for fasting was 382 days, during which the patient suffered no ill effects. So if fasting occasionally isn’t working, all you need to do is increase the frequency or length of time you fast, until you reach your goal.

every drug has a maximum dose. If you take penicillin for an infection, for example, there is a maximum dose above which there is little extra ben...

At that point, if you still have the infection, you need to change medications. The same applies to low-carbohydrate or low-fat diets. Once you hit zero carbs or fat, you can go no...

you can keep fasting until the desired effects are seen. The dose can go up indefinitely. Ask yourself this question: if you don’t eat, will you lose weight? Of course. Even a child understands that you must lose weight.

So fasting’s efficacy is unquestioned. It is the most powerful method of weight loss available. It is only a question of safety and compliance. For more complicated or serious cases of obesity, we can simply increase the dose.

Advantage #6: It’s Flexible

finding or packing something to eat six or seven or eight times a day is very intrusive. I could not imagine interrupting my life every two and a half hours to snack. It’s too disruptive on an already hectic schedule. And it’s simply not necessary. Fasting can be done at any time. There is no set duration. You may fast for sixteen hours or sixteen days. You can mix and match time periods. You are never locked into a pattern. You may fast for one day this week, five days the next, and two days the week after. Life is unpredictable. Fasting fits wherever you need it to. Fasting can be done anywhere. It does not matter if you live in the United States, the United Kingdom, or the United Arab Emirates. You may live in the polar desert of the Arctic or the sandy desert of Saudi Arabia. It matters not at all. Once again, because fasting is about not doing something, it simplifies our life. It adds simplicity where other diets add complexity. If you do not feel well for any reason, you simply stop fasting.

you want to indulge during the Christmas holidays or during a summer cruise, you can do that as well. Simply get back on the program once you are finished.

Advantage #7: It Works with Any Diet

You don’t eat meat? You can still fast.

You don’t have time? You can still fast.

You don’t have money? You can still fast.

You are traveling all the time? You c...

You don’t cook? You can s...

You have problems chewing? You can still fast.

All diets

diet—seem to produce weight loss in the short term. But after some initial success, weight loss plateaus, and the dreaded weight regain follows.

all diets fail. The

to really judge whether something is successful or not, you must look at the end results. Just because you think something will work does not necessarily mean it will.

When we eat, our body derives energy from three main sources: glucose (carbohydrates), fat, and protein. Only two of these are stored for later use, glucose and fat—the body can’t store protein, so excess protein that can’t be used right away is converted to glucose. Glucose is stored in the liver as glycogen, but the liver’s capacity for storing glycogen is limited. Once glycogen stores are full, excess calories must be stored as body fat.

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

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.

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.

if you’re cutting calories and have low insulin levels, it’s easy for your body to compensate for the reduced food energy by getting some fat out of the freezer even if your glycogen fridge isn’t completely empty.

high insulin levels prevent the body from accessing the fat in the freezer.

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.

When stored glycogen gets low, your body senses it and starts to get antsy. It triggers hunger signals, so you want to eat more. If you don’t eat enough to fill up the glycogen stores but your insulin remains high, body fat can’t be released.

When either food or glycogen is available, we do not use our less-accessible fat stores. This ensures that body fat is only used in times of need.

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,

The reason the body has to resort to decreasing metabolism and increasing hunger is because insulin remains high, so it doesn’t have access to the energy stored as fat.

When we eat, insulin rises and blocks fat-burning, and the body instead burns glucose,

But of the three macronutrients—carbohydrates, fat, and protein—carbohydrates stimulate the production of insulin the most. Refined carbs and sugar in particular have the greatest effect on insulin, so a diet low in these is most certainly a great start for breaking the insulin resistance cycle and losing weight.

Yet for some people, this is not enough. Since all foods raise insulin levels, the best answer is to ...

Insulin is the main driver of obesity and diabetes. A very low carb diet can reduce insulin by more than 50 percent, but you can go another 50 percent by fasting. That’s power.

Fasting is simply the most efficient and effective way to lower insulin levels. Notice, however, I did not say it was the easiest. But do you want a method that is easy, or a method that works?

During fasting, unlike during caloric reduction, metabolism stabilizes or even goes up to maintain normal energy levels. Adrenaline and growth hormone increase to maintain energy and muscle mass. Blood sugar and insulin levels go down as the body changes from burning sugar to burning fat.

The study compared the effectiveness of daily caloric reduction and intermittent fasting among 107 women. One group reduced their daily caloric intake from 2,000 calories to 1,500 calories. The other group was allowed normal caloric intake (2,000 calories) five days a week but only 25 percent of that (500 calories) on the remaining two days—this is referred to as a 5:2 fast. This means that over the course of a week, average caloric intake for the two groups was very similar: 10,500 calories per week for the reduced-calorie group and 11,000 calories per week for the fasting group. Both groups consumed similar Mediterranean-style diets with 30 percent fat. After six months, both groups had similar levels of weight loss and fat loss. But the 5:2 fasting group showed a clear, substantial improvement in insulin levels and insulin resistance, whereas the caloric-reduction group did not. Figure 5.11. A study of type 2 diabetics showed that a very low carb diet does reduce insulin compared to a standard diet, but fasting reduces it even more.

fasting reduces insulin levels more effectively than calorie restriction.

a ketogenic diet and fasting have several features in common.

ketone bodies. These are substances the body produces during fat-burning; they’re what fuels the brain when glucose is scarce. A ketogenic diet helps shift the body from burning glucose to burning fat, which results in the creation of ketones. Of course, fasting causes the body to burn fat, too—and that means it also results in the creation of ketones.

the brain still requires glucose to function normally during fasting, but we do not need to eat glucose. We can manufacture enough glucose to power the entire body simply from body fat.

Adrenaline and growth hormone increase, helping to maintain lean muscle mass and keep metabolism high. Insulin and blood glucose levels fall. The “eat less, move more” strategy for daily caloric reduction doesn’t provide these hormonal benefits. However, fasting does.

fasting becomes easier the more often you do it.

During both World War I and World War II, the mortality rate from type 2 diabetes dropped precipitously. This was due to wartime food rationing, which resulted in a sustained, severe reduction of calories.