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

by Jason Fung

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.

What Is Intermittent Fasting? The term intermittent fasting simply means that periods of fasting occur regularly between periods of normal eating. How long each period of fasting lasts, and how long the period of normal eating lasts, can vary widely. There are many different fasting regimens, and there’s no “best” one. They all work to different degrees for different people. One regimen may work for one person but be utterly ineffective for the next. One person may prefer shorter fasts while another prefers longer ones. Neither is correct or incorrect. It is all personal preference. Fasts can range from twelve hours to three months or more. You can fast once a week or once a month or once a year. Shorter fasts are generally done more frequently, even daily, while longer fasts—twenty-four to thirty-six hours is the most common duration—are usually done two to three times per week. Prolonged fasting may range from one week to one month. I categorize fasting periods as short (less than twenty-four hours) or long (more than twenty-four hours), but this is somewhat arbitrary. In the IDM Program, 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.

Longer-duration fasts give quicker results but are usually done less frequently. Fasting for more than twenty-four hours may sound difficult, but I’ve found that a surprising number of patients prefer to fast longer and less frequently.

Remember, you can always switch from one fasting regimen to the other. You are never locked in. But keep in mind that the first several fasting periods are always difficult,

fasting becomes easier the more you do it.

12-Hour Fasts

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.

By 2003, that figure was closer to six per day. People were eating three meals and three snacks daily, and insulin levels were beginning to be kept perpetually high. Over time, the constant stimulation of insulin leads to the development of insulin resistance, which leads back to high insulin levels, which, in turn, leads to obesity.

Daily twelve-hour fasting introduces a period of very low insulin levels during the day. This prevents the development of insulin resistance, making the twelve-hour fast a powerful preventative weapon against obesity. In fact, the combination of whole foods, lower-carbohydrate diets, less added sugars, and a daily twelve-hour fast was enough to prevent most Americans in the 1950s and 1960s from developing obesity—even though they still ate plenty of white bread and jam, and whole-wheat bread was rare and whole-wheat pasta

unheard-of.

But while a daily twelve-hour fast may be a great preventative strategy, it may not be powerful enough to reverse weight gain. Slightly longer fas...

16-Hour Fasts This regimen incorporates a sixteen-hour period of fasting into your daily meal schedule. For example, you might fast from 7 p.m. to 11 a.m. daily. You could also say that you have an eight-hour eating window every day. For that reason, it is sometimes called time-restricted eating. On this schedule, most people skip the morning meal every day. But how many meals you eat within that eight-hour window is your choice. Some people choose to eat two meals during that window and others eat three.

ABEL JAMES

For most people, I recommend 16:8 intermittent fasting (a compressed eating window) over longer-term fasting. The fact that you sleep through the majority of your fast makes it relatively painless.

major advantage of the sixteen-hour fast is that it is fairly simple to incorporate into everyday life. For most people, it only means skipping breakfast and eating lunch and dinner within eight hours of each other. Many people do not feel hungry in the morning despite skipping breakfast and find this method extremely easy to implement.

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.

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.

It is believed that food was relatively scarce in Paleolithic times and was predominantly available during daylight hours. Humans hunted and ate by day, and once the sun went down, well, you just couldn’t see the food in front of your face.

In a 2013 study, two groups of overweight women were randomly assigned to eat a large breakfast or a large dinner. Both ate 1400 calories per day; only the timing of the largest meal was changed. The breakfast group lost far more weight than the dinner group. Why? Despite following similar diets and eating about the same amount, the dinner group had a much larger overall rise in insulin. An earlier 1992 study showed similar results. In response to the same meal given either early or late in the day, the insulin response was 25 to 50 percent greater in the evening.

Fasting is a powerful tool, but like all tools, you really need to consider why you are using it and what the specific context is.

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,

Eating the largest meal in the evening seems to cause a much larger rise in insulin than eating earlier. Folk wisdom, of course, also advises to avoid eating large meals in the evening. The reason offered usually is something along the lines of, “If you eat just before bed, you don’t get a chance to burn it off and it will turn to fat.” Maybe that’s not technically true, but perhaps there is something here. Eating late at night seems to be especially problematic for weight gain. This response may have evolved to help us gain fat, which may have had a survival advantage in times past.

Hunger, too, has a natural circadian rhythm. If hunger were simply due to lack of food, we would consistently be hungry in the morning after the long overnight fast. But personal experience and studies show that hunger is lowest in the morning, and breakfast is typically the smallest, not the largest, meal of the day. Hunger follows a natural, circadian rhythm that is independent of the eat/fast cycle. Ghrelin, the hunger hormone, rises and falls in a natural circadian rhythm, with a low at 8:00 a.m. and a high at 8:00 p.m.

These are natural rhythms inherent in our genetic makeup. Hunger is not so simple as “the longer you don’t eat, the hungrier you’ll be.” Hormonal regulation of hunger plays a key role.

Interestingly, during extended fasting, ghrelin peaks during the first two days and then steadily falls. This aligns perfectly with what we see clinically: hunger is the worst problem on the first two days, but many people on lon...

Timing the Largest Meal of the Day So, what’s the practical implication of these hormonal rhythms for daily eating? At 8 a.m., our hunger is actively suppressed. It is counterproductive to force-feed ourselves then. What’s the point? Eating does not produce weight loss. Forcing ourselves to eat at a time when we are not hungry is not a winning strategy. Eating late at night is also a poor strategy. Hunger is maximally stimulated at approximately 7:50 p.m. At this time, insulin is maximally stimulated by food, which means that the same amount of food results in higher insulin levels. This higher insulin level will naturally drive weight gain. Unfortunately, that timing coincides with the largest meal of the day in North America. Making dinner the biggest meal of the day is mostly driven not by health concerns but by the hours of the workday and school. Shift workers are at a particular disadvantage. They tend to eat larger meals even later in the evening, leading to higher insulin. 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. Referenc...