Notes on

The Complete Guide to Fasting

by Dr. Jason Fung


  • Obesity is a hormonal imbalance; not caloric.
  • "When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use."
  • If more insulin leads to obesity, then the solution to obesity lies in reducing insulin. There are two methods for achieving this:
    • Ketogenic diet (low-carb, moderate-protein, high-fat diet)
    • Intermittent fasting
  • The secret to healthy eating: eat real food.
    • "If it comes prepackaged in a bag or a box, it should be avoided."
  • Avoid sugars and refined grains (such as flour & corn).
  • Eat a diet high in natural fats (from sources such as meat and dairy).
  • Balance feeding with fasting.
  • When you fast, body fat will be preferred to be burned over muscle — unless you have a low enough body weight (sub 4%).
  • Guide to fasting: Eat nothing. Drink water, tea, coffee, or bone broth.
    • Don't add sugar or milk (or anything else) to it.
  • If you reach a weight-loss plateau when fasting, try changing up your fasting protocol.
  • Fasting for fat loss will, on average, lead to a loss of 0.5 pound a day. Anything above that is probably water weight.
  • Persistence and patience is key.
  • Hunger is not determined by not eating for a certain period of time. It's a hormonal signal. It doesn't happen just because your stomach is empty. Avoiding stimuli that would usually trigger hunger can help you avoid it. Hunger is a state of mind.
  • Artificial sweeteners are not recommended to use during fasting. They stimulate hunger and insulin production.

Highlights & Notes

Introduction, by Jason Fung, MD

The underlying cause of obesity turns out to be a hormonal, rather than a caloric, imbalance. Insulin is a fat-storage hormone. When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use. It’s a natural and essential process that has helped humans survive famine for thousands of years, but excessively and persistently high insulin levels result inexorably in obesity. Understanding this leads naturally to a solution: if excessive insulin is causing obesity, then clearly the answer lies in reducing insulin. Both the ketogenic diet (a low-carb, moderate-protein, high-fat diet) and intermittent fasting are excellent methods of reducing high insulin levels.

Chapter 1: What Is Fasting?

Foods should be recognizable in their natural state as something that was alive or has come out of the ground. Boxes of Cheerios do not grow in the ground. If it comes prepackaged in a bag or a box, it should be avoided. If it has a nutrition label, it should be avoided. Real foods, whether broccoli or beef, have no labels.
> The true secret to healthy eating is this: Just eat real food.

We will cover many different fasting schedules later in the book. Intermittent fasting can be successfully implemented with either short fasts (less than twenty-four hours) or longer fasts (more than twenty-four hours). Extended fasting (more than three days) can also be safely used for weight loss and other health benefits.
We’ll explain in detail the “best practices” for fasting in Chapter 10, but in general, we encourage consuming plenty of noncaloric liquids (water, tea, coffee) and homemade bone broth, which is full of nutrients.

A therapeutic fast of 382 days was maintained with only a multivitamin with no harmful effects on health. In fact, this man felt terrific during this entire period.

Relying on macronutrient-based guidelines or calorie limits makes eating far more complicated than it should be. We do not eat a specific percentage of fats, protein, and carbohydrates. We eat foods. Certain foods are more fattening than others. Therefore, the best advice focuses on eating or not eating specific foods, not specific nutrients.

Eat whole, unprocessed foods

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.

Eat more natural fats

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.

Eat less artificial fats

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.

Chapter 3: Busting the Myths of Fasting

The human body evolved to survive periods of fasting. We store food energy as body fat and use this as fuel when food is not available. Muscle, on the other hand, is preserved until body fat becomes so low that the body has no choice but to turn to muscle. This will only happen when body fat is at less than 4 percent. (For comparison, elite male marathon runners carry approximately 8 percent body fat and female marathoners slightly more.) If we did not preserve muscle and burn fat instead when no food is available, we would not have survived very long as a species. Almost all mammals have this same ability.
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. (Page 0)

In fact, metabolism revs up, not down, during fasting. This makes sense from a survival standpoint. If we do not eat, our bodies use our stored energy as fuel so that we can find more food. Humans have not evolved to require three meals a day, every day.

If you are worried about muscle loss, exercise more. It’s not rocket science. Diet and exercise are two entirely separate issues. Don’t confuse the two. Don’t worry about what your diet (or lack of diet—that is, fasting) is doing to your muscle mass. Exercise builds muscle. Lack of exercise leads to atrophy of muscles.

So let me lay it out as simply as I can. Body fat is, at its core, stored energy for us to “eat” when there is no food. It’s not there for looks, right? So, when we fast, we “eat” our own fat. This is natural. This is normal. This is the way we were designed. Otherwise, famine cycles in Paleolithic times would have eventually left us as a ball of 100 percent fat! During fasting, hormonal changes kick in to give us more energy (increased adrenaline) and preserve our lean muscles and bones (increased growth hormone). This is normal and natural and there is nothing here to be feared.

Chapter 4: The Advantages of Fasting

Fasting, by taking a completely different approach, is much easier to understand. It is so simple that it can be explained in two sentences: Eat nothing. Drink water, tea, coffee, or bone broth. That’s it.

And 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.

What’s more, low-fat diets, low-carbohydrate diets, Paleo diets, and indeed almost any diet work for some people but not others, and when a diet doesn’t work for you, there is little you can do to make it more effective. However, with fasting, all you need to do is increase the amount of time spent fasting. The longer you go, the more likely you are to lose weight—and it will happen eventually.

Chapter 5: Fasting for Weight Loss

You’ll probably eventually experience a weight-loss plateau as the weight lost during fasting begins to match the amount regained during eating. (The only way not to plateau at all is to continually fast for weeks or months at a time; otherwise, reaching an equilibrium is inevitable.) Changing either your fasting regimen or diet, or both, may help. Some patients increase fasting from twenty-four-hour periods to thirty-six-hour periods, or try a forty-eight-hour fast. Some try eating only once a day, every day. Others try a continuous fast for an entire week. Any of these can be effective; the key is simply to change the fasting protocol.
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.

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. Certain medications, such as insulin, may make it hard to lose weight. You must simply persist and be patient.

Chapter 9: What You Need to Know About Hunger

How can people fast for days without being hungry? It comes down to the fact that hunger is not determined by not eating for a certain period of time. Rather, it is a hormonal signal. It does not come about simply because the stomach is empty. When you avoid natural stimuli to hunger, such as the sight and smell of food, as well as the conditioned stimuli to hunger—specific mealtimes, movies, ball games, any event where you normally eat and have learned to expect food—you help avoid that hormonal signal.
Fasting helps to break all the conditioned stimuli and thus helps to reduce, not enhance, hunger. Hunger is a state of mind, not a state of stomach.

It’s also helpful to avoid artificial sweeteners. Even though they contain no calories, they may still kick off the cephalic phase response, stimulating hunger as well as insulin production. I don’t recommend artificial sweeteners during fasting for this very reason. Recent studies confirm that diet sodas are generally not helpful for weight loss, probably because they trigger hunger but don’t satisfy it.

What’s the best way to endure the wave of hunger during fasting? Drinking green tea or coffee is often enough. By the time you’re finished, the hunger has passed and you’ve gone onto the next thing to do that day. Hunger is not a continuously growing phenomenon. It will build up, peak, and then dissipate, and all you have to do is ignore it. It will certainly return, but knowing that it will pass once again gives you the power and confidence to handle it.

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