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

by Jason Fung

During long winters back in the Paleolithic era, there were many days where no food was available. After the first episode, you would be severely weakened as your metabolism falls. After several repeated episodes, you would be so weak that you would be unable to hunt or gather food, making you even weaker. This is a vicious cycle that the human species would not have survived. Our bodies do not shut down in response to short-term fasting.

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.

When food intake goes to zero (fasting), our body obviously cannot take BMR down to zero—we have to burn some calories just to stay alive. Instead, hormones allow the body to switch energy sources from food to body fat. After all, that is precisely why we carry body fat—to be used for food when no food is available.

fasting every other day for twenty-two days resulted in no measurable decrease in BMR. There was no starvation mode. Fat oxidation—fat burning—increased 58 percent, from 64 g/day to 101 g/day. Carbohydrate oxidation decreased 53 percent, from 175 g/day to 81 g/day. This means that the body has started to switch over from burning sugar to burning

fat, with no overall drop in energy.

In another study, four days of continuous fasting increased BMR by 12 percent. Levels of the neurotransmitter norepinephrine (also known as noradrenaline), which prepares the body for action, increased by 117 percent, keeping energy levels high. Fatty acids in the bloodstream increased over 370 percent as the body switched over from burning food to...

The “fasting is unhealthy” reflex is largely an outcome of the marketing push to encourage consumers to buy food.

just know that we’re designed to function well during periods of fasting—our bodies don’t start to shut down or go into “starvation mode.”

Myth #2: Fasting Makes You Burn Muscle One persistent myth of fasting is that it burns muscle, that our body, if we’re not eating, will immediately start using our muscles as an energy source. This does not actually happen.

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. Almo...

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.

Figure 3.2. During fasting, the body switches from burning sugar (carbohydrates) to fat for energy. Protein is spared.

At baseline, eating normally, energy comes from a mix of carbohydrates, fat, and protein. As you start fasting, the body increases carbohydrate oxidation. This is just a fancy way of saying that it is burning sugar, in the form of glycogen, for the first twenty-four to forty-eight hours after you stop eating, until it runs out of glycogen. With no more sugar to burn, the body switches to burning fat. Fat oxidation increases as carbohydrate oxidation decreases toward zero. (See Figure 3.2.) At the same time, protein oxidation—that is, burning protein, such as muscle, for fuel—actually decreases. The normal protein breakdown of around seventy-five grams per day falls to fifteen to twenty grams per day during fasting. Rather than burning muscle during fasting, we start conserving muscle. Much of the amino acids that are broken down during regular turnover of cells are reabsorbed into new proteins.

After all, why would your body store excess energy as fat if it meant to burn protein as soon as the chips were down? Muscles and other proteins are functional tissues and have many purposes. They are not designed to store energy. Glycogen and fat are.

In fact, fasting is one of the most potent stimuli for growth hormone secretion, and increased growth hormone helps maintain lean body mass.

Muscle gain or loss is mostly a function of exercise. You can’t eat your way to more muscle. Supplement companies, of course, try to convince you otherwise. Taking creatine and drinking whey protein shakes do not build muscle. That’s wishful thinking. Exercise is the only reliable way to build muscle.

If you are worried about muscle loss, exercise more.

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.

if you are worried about weight loss and type 2 diabetes, then you need to worry about diet, not exercise.

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

when we fast, we “eat” our own fat. This is natural. This is normal. This is the way we were designed. Otherwise,

Sometimes people worry that blood sugar will fall very low during fasting and they will become shaky and sweaty. Luckily, this does not actually happen. Blood sugar level is tightly monitored by the body, and there are multiple mechanisms to keep it in the proper range.

People who engage in fasting for religious or spiritual purposes often report feelings of extreme clear-headedness and physical and emotional well-being. Some even feel a sense of euphoria. They usually attribute this to achieving some kind of spiritual enlightenment, but the truth is much more down-to-earth and scientific than that: it’s the ketones! Ketones are a “superfood” for the

brain.

Many authorities warn against missing even a single meal because it could make you extra hungry and unable to avoid temptations, leading to overeating and ultimately no weight loss. Studies of caloric intake do, in fact, show a slight increase on the first day after fasting. On the day after a one-day fast, average caloric intake increases from 2,436 to 2,914. But if you factor in what would have normally been consumed during that two-day period, 4,872 calories, there is still a net deficit of 1,958 calories. The increased calories don’t come close to making up for the lack of calories on the fasting day.

Interestingly, with repeated fasting, you may see the opposite effect. 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.

Myth #5: Fasting Deprives the Body of Nutrients

With shorter fasting periods (less than twenty-four hours), there is ample opportunity before and after the fast to eat nutrient-dense foods to make up for missed meals. For longer fasts, it is a good idea to take a general multivitamin.

The body normally loses both essential amino acids and essential fatty acids in urine and stool.

Bowel movements usually decrease during fasting—since no food is going into the stomach, there is less stool formation—and this helps to prevent loss of protein in the stool.

To further preserve proteins, the body breaks old proteins down into their component amino acids and recycles these into new proteins. By keeping essential nutrients in the body instead of excreting them, the body is able to recycle many of them during fasting.

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.

THE ADVANTAGES OF FASTING Fasting’s most obvious benefit is weight loss.

Fasting: Improves mental clarity and concentration Induces weight and body fat loss Lowers blood sugar levels Improves insulin sensitivity Increases energy Improves fat-burning Lowers blood cholesterol Prevents Alzheimer’s disease Extends life Reverses the aging process Decreases inflammation

Obesity and type 2 diabetes are both problems of excessive insulin. Since refined carbohydrates are a prime contributor to high insulin levels,

Protein, especially animal proteins (dairy and meat), can also stimulate insulin production,

And processed foods also play a key role in disease. So, the best diet would emphasize whole, unprocessed foods. It would be low in refined carbohydrates and high in n...

What I needed was a new strategy. The overall goal was not necessarily to reduce carbohydrate intake. The goal was to reduce insulin levels, and cutting carbohydrates was only one method of achieving that goal. Yet all foods, to varying degrees, stimulate the release of insulin. So the most efficient method of lowering insulin would be to eat nothing at all. In other words: to fast.

Advantage #1: It’s Simple

Eat nothing. Drink water, tea, coffee, or bone broth. That’s it.

Diets can fail because they are ineffective. But they just as surely fail if people are not able to follow them. The most obvious benefit to fasting is that its simplicity makes it effective. When it comes to dietary rules, the simpler, the better.

Fasting is free. In fact, not only is it free, it actually saves money because you do not need to buy any food at all!

Advantage #3: It’s Convenient

There is no time spent buying groceries, preparing ingredients, cooking, and cleaning up. It is a way to simplify your life.

There is nothing easier than fasting, because fasting involves doing nothing. Most diets tell you what to do. Fasting tells you not to do anything. It doesn’t get easier than that.

Advantage #4: You Can Enjoy Life’s Little Pleasures Some diets advise people to never, ever again eat ice cream or desserts. Good advice for weight loss, obviously. But I don’t think it is actually very practical advice. Sure, you might be able to swear off desserts for six months or a year, but for life? And would you really want to?

Do we need to deny ourselves that little bit of pleasure forever? Enjoy a birthday salad instead of birthday cake? Thanksgiving kale chips! All-you-can-eat Brussels sprouts!

Forever is a long time.