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

by Jason Fung

When glycogen stores in the liver are full, the liver starts converting excess carbohydrates into triglycerides instead. These triglycerides are then exported out of the liver as very low density lipoprotein (VLDL). VLDL is used to form LDL.

High levels of triglycerides in the blood are strongly associated with c...

Higher blood triglyceride levels, independent of LDL, increase the risk of heart disease by as much as 61 percent.

An estimated 31 percent of adult Americans are currently estimated to have elevated triglyceride levels, paralleling the rise in consumption of carbohydrates.

Fortunately, high triglycerides can be treated with a low-carbohydrate diet, which lowers the rate at which the liver creates triglycerides. But while triglyceride levels respond to diet, the same cannot be said for cholesterol.

The scientific community has long known that eating less cholesterol does not lower blood cholesterol. Our liver generates 80 percent of the cholesterol found in the blood, so eating less cholesterol makes little or no difference.

By the same token, eating more cholesterol does not raise blood cholesterol significantly. If we eat less dietary cholesterol, our liver simply compensates by creating more, so the net effect is negligible.

Millions of people follow a low-fat, low-cholesterol diet because they think that it’s good for their heart, without realizing that these measures were long ago proven ineffective.

The liver produces the vast majority of cholesterol found in the blood. Eating less cholesterol has almost no effect on the liver’s production. In fact, it may be counterproductive. As the liver senses less incoming cholesterol, it may simply increase its own production. So why does fasting affect the liver’s production of cholesterol? As dietary carbohydrates decline, the liver decreases its synthesis of triglycerides—since excess carbohydrates are converted to triglycerides, the absence of carbohydrates means fewer triglycerides. Remember that triglycerides are released from the liver as VLDL, which is the precursor of LDL. Therefore, reduced VLDL eventually results in lowered LDL.

The only reliable way to reduce LDL levels is to reduce the liver’s production of it.

In fact, studies prove that seventy days of alternate-day fasting could reduce LDL by 25 percent.

This is far in excess of what can be achieved with almost any other diet and about half of the effect achieved with a statin medication, one of the most powerful...

Intermittent fasting offers a unique solution. By randomly skipping meals and varying the intervals at which we eat, we can break our habit of eating three times a day, come hell or high water. We no longer have a conditioned response of hunger every three to five hours. We no longer become hungry simply because it’s noon or we’re at the movies. That’s not to say we don’t get hungry at all—we do, but not simply because we have a conditioned response to a certain time or occasion. Instead, we get hungry because we are hungry. We allow our body to tell us when it needs nourishment rather than eating by the clock.

For most people, the biggest obstacles to fasting are psychological, rather than physiological.

In order to fast successfully, try to divorce yourself from the notion that you are “supposed to” eat several times a day. It is okay—beneficial, in fact—to become reacquainted with feelings of hunger. In fact, it’s actually kind of nice to get reacquainted with the signals our bodies send us …

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.

Fasting can be a powerful tool to recalibrate your relationship with eating habits and perceived hunger. True hunger is generally experienced in the body and brain, not in the stomach. It may take some practice, but once you reconnect with the feeling of true hunger, you can follow your body’s lead and eat whenever the feeling strikes.

Traditional hunter-gatherer societies virtually never develop obesity or diabetes, even during times of plentiful food. In the pre-agricultural era, it is estimated that animal foods provided about two-thirds of the calories in the human diet. So, despite all the modern teeth gnashing about red meat and saturated fats, it seems that our ancestors had little problems eating them.

About ten thousand years ago, with the agricultural revolution and its greater reliability of food, we developed the habit of eating two or three times per day. But many early agricultural societies ate carbohydrate-based diets without problems with obesity. It seems to be a modern problem.

What matters most is our insulin response to food, since obesity is largely a problem of excessive insulin. And when it comes to insulin, as we’ve discussed in Chapters 5 and 6, the timing and frequency of meals is as important as the composition of the meal. That is, the question of when to eat is as important as what to eat. It is precisely here that intermittent fasting may help us the most.

The term intermittent fasting simply means that periods of fasting occur regularly between periods of normal eating.

Fasts can range from twelve hours to three months or more.

shorter regimens are generally used by those mostly interested in weight loss rather than in treating type 2 diabetes, fatty liver disease, or other metabolic diseases. However, shorter, more frequent fasts still often work well for these conditions.

During short-duration fasts, you are still eating daily, which minimizes the risk of malnutrition. Shorter fasts also fit into work and family-life schedules easily.

In years past, a daily twelve-hour fasting period was considered a normal eating pattern. You would eat three meals a day from, say, 7 a.m. to 7 p.m., and then fast from 7 p.m. to 7 a.m. At that point, you would “break your fast” with a small breakfast. This was pretty standard until the 1970s, and perhaps not coincidentally, there also was much less obesity back then. Two major dietary changes happened starting in 1977. With the publication of the USDA’s Dietary Guidelines for Americans that year, we changed to a higher-carbohydrate, lower-fat diet. Diets high in refined carbohydrates stimulate constant, high levels of insulin, which makes people gain weight and, eventually, become obese.

The daily sixteen-hour fast certainly has more power than the daily twelve-hour fast, but it should be combined with a low-carbohydrate diet for the best effect. Weight loss on this regimen tends to be slow and steady.

20-Hour Fasts: “The Warrior Diet” In his 2002 book The Warrior Diet, Ori Hofmekler stresses that the timing of meals matters almost as much as their composition—as I discussed earlier, both the “when to eat” and “what to eat” questions are important, but “when” is seriously underappreciated. Hofmekler, drawing upon inspiration from ancient warrior tribes such as the Spartans and Romans, devised a “warrior diet” in which all meals are eaten in the evening during a four-hour window. This results in a twenty-hour fasting period each day. Hofmekler’s diet also emphasizes natural, unprocessed foods and high-intensity interval training, both of which I believe to be healthy practices.

Weight gain is driven by insulin, and the higher insulin response in the evening was translating into more weight gain for the dinner group. This illustrates the very important point that obesity is a hormonal, not a caloric, imbalance, and it may help explain the well-known association of night shift work and obesity. (However, that could also have to do with the increased cortisol response due to disturbed sleep.)

Eating the largest meal in the evening seems to cause a much larger rise in insulin than eating earlier.

So the optimal strategy seems to be eating the largest meal in the midday, sometime between noon and 3:00 p.m., and only a small amount in the evening hours. Interestingly, this is the traditional Mediterranean eating pattern. They eat a large lunch, followed by a siesta in the afternoon, and then have a small, almost snack-sized dinner. While we often think of the Mediterranean diet as healthy due to the type of foods in it, the timing of meals in the Mediterranean may also play a role.

cannot stress enough that if you do not feel well at any point, you must stop fasting. You can be hungry, but you should not feel sick.

Nutrient deficiency is not a major concern with a twenty-four-hour fast. Since you are still eating daily, you just need to make sure that during that meal you consume adequate proteins, vitamins, and minerals by eating nutrient-dense, natural, unprocessed foods. You could follow this regimen daily, although most will get good results doing a twenty-four-hour fast two to three times per week. Brad Pilon, the author of Eat, Stop, Eat, recommends using a twenty-four-hour fast twice a week.

If someone had a serious amount of weight to lose and had spent the requisite time teaching their body to burn fat, I would have them try 24-, 36-, and 48-hour fasts. Probably not much longer than that, but probably with some frequency (2 days every week or every 2 weeks, for 6 weeks, and then rest). As long as they were exercising and performing substantial movement throughout the day, they would be burning fat, utilizing ketones, and possibly even building muscle.

Chronically malnourished and/or severely underweight people are at the highest risk of refeeding syndrome. This includes those with anorexia, chronic alcoholism, cancer, uncontrolled diabetes, or bowel disease—if any of these apply to you, fasting may not be for you; discuss your options with your doctor.

The truth is that hunger does not persist but instead comes in waves. If you’re experiencing hunger, it will pass. Staying busy during a fast day is often helpful.

As the body becomes accustomed to fasting, it starts to burn its stores of fat, which helps suppress hunger. Many people note that over several weeks, as they continue a fasting regimen, appetite not only doesn’t increase, it actually starts to decrease. During longer fasts, many people notice that their hunger completely disappears by the second or third day.

If you experience dizziness during your fast, most likely, you’re becoming dehydrated. Preventing this requires both salt and water. Be sure to drink plenty of fluids, and in case you’re low on salt, add extra sea salt to homemade bone broth or mineral water.

One theory is that fasting activates a form of cellular cleansing called autophagy that may help prevent age-associated memory loss

My stomach is always growling. What can I do? Try drinking some mineral water. The mechanism is unclear, but it is believed that some of the minerals help settle the stomach.

Does performance suffer? Not really. In one study, a three-and-a-half-day fast did not affect any measurements of athletic performance, including strength, anaerobic capacity, and aerobic endurance. However, during the period when you are adjusting to the change from burning sugar to burning fat, you may notice a decrease in your athletic performance. This lasts approximately two weeks. As you deplete the body of sugar, your muscles need time to adapt to using fat. Your energy, muscle strength, and overall exercise capacity will go down, but they will recover. This process is sometimes called keto-adaptation. Very low carb diets, ketogenic diets, and training in the fasted state may all have benefits in training your muscles to burn fat, but your muscles do need time to adapt. You can store far more energy in the form of fat than in glycogen, and for endurance athletes, the increase in available energy when they start burning fat is a significant advantage. If you are running ultra marathons, being able to utilize your almost unlimited fat stores instead of highly limited glycogen stores means that you won’t “hit the wall,” giving you a chance to win that race. Since your body is relying on your fat stores, there’s no shortage of energy during fasting, and you can and should do all your usual activities. There is no reason why exercise should stop during fasting. In fact, many elite athletes and endurance athletes train in the fasted state. The combination of low insulin a...

A constant calorie-reduced diet does not match the cycle of feast and fast and is thus doomed to failure.