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

by Jason Fung

it wasn’t just sugar that was rationed—almost all foods were restricted,

Let’s consider an analogy. Imagine the cell to be a subway train. Glucose molecules are the passengers waiting to get inside the train. Insulin gives the signal to open the train doors, and the passengers—the glucose molecules—march in a nice orderly manner into the empty subway train. Normally, it doesn’t really require much of a push to get this glucose into the cell.

when a cell is overfilled with glucose, it appears resistant to insulin’s signals to allow more glucose to enter.

But what happens if the train is not empty? What if it’s already jam-packed full of passengers? Insulin gives the signal to open the door, but the passengers waiting on the platform cannot enter. From the outside, it appears that this train (cell) is now resistant to insulin’s signals. So, what can you do to pack more people into the train? One solution is to hire subway pushers to shove people into the trains.

Insulin is the body’s subway pusher, shoving glucose into the cell no matter the consequences. If the normal amount of insulin can’t get the glucose in, then the body calls for reinforcements: even more insulin. But the main cause of the insulin resistance is that the cell was already overflowing with glucose. Because the cell is overfilled with glucose, glucose spills out of the cell, leading to increased blood glucose levels. This leads to the diagnosis of type 2 diabetes. If you now give more insulin, or drugs that stimulate the production of insulin, then yes, temporarily, more glucose can be shoved into the cell. However, there is a natural limit. At that point, even extra insulin will not be able to move more glucose into the cell.

Finally, insulin itself is prescribed in higher and higher doses. The progression of treatments is clearly not helping the underlying problem: the type 2 diabetes is just getting worse. The medications only help control blood sugar; they don’t address what’s causing type 2 diabetes.

If the core issue is that glucose is overfilling the cells, then the solution seems rather obvious: get all that glucose out of the cells!

So how do you get rid of excess glucose in the body?

There are really only two methods of getting the toxic glucose overload out of the body. First, you need to stop putting glucose into the body. You can achieve this with very low carbohydrate diets or ketogenic diets. Indeed, many people have reversed their diabetes by following such diets. Fasting also eliminates carbohydrates—and all other foods, for that matter. Second, your body needs to burn off the excess glucose. Fasting is again an obvious solution. Your body requires energy just to keep all the vital organs, such as the heart, lungs, liver, and kidneys, working. Your brain in particular requires substantial energy to function properly, even during sleep. During fasting, no new glucose is coming in, so your body has no choice but to use up the stored glucose. At its core, type 2 diabetes is a disease of excessive glucose, in our blood but also in our bodies. If you don’t eat, your blood sugar level will come down.

The most obvious benefits of fasting are that it helps with weight loss and type 2 diabetes, but there are many other benefits, including autophagy (a cellular cleansing process), lipolysis (fat-burning), anti-aging effects, and neurological benefits. In other words, fasting can benefit your brain and help your body stay younger.

when I began fasting through the morning, I immediately noticed a significant increase in mental focus, energy, and productivity.

Humans, like all mammals, have an increase in mental activity when hungry and a decrease when satiated. We have all experienced a “food coma”—think about how you feel after a big Thanksgiving meal, complete with turkey and pumpkin pie. Are you mentally sharp as a tack? Or dull as a concrete block?

As the amount of blood going to the digestive system is increased to handle all that turkey and pie, less blood is available to go to the brain.

How about the opposite? Think about a time that you were really, really hungry. Were you tired and sluggish? I doubt it. You were probably hyperalert, your senses sharp as a needle. Animals that are cognitively sharp and physically agile during times of food scarcity have a clear advantage when it comes to survival. If missing a single meal reduced our energy and mental acuity, we would have even more trouble finding food, making it more likely that we would go hungry again, leading to a vicious cycle ending in death.

Our ancient ancestors grew more alert and active when hungry so that they could find their next meal—and the same thing still happens to us.

Fasting and hunger energize us and activate us to advance towards our goal, despite popular misconceptions to the contrary.

Another study of two days of almost total caloric deprivation found no detrimental effect on cognitive performance, activity, sleep, and mood.

Animal studies show that fasting has remarkable promise as a therapeutic tool. Aging rats started on intermittent fasting regimens markedly improved their motor coordination, cognition, learning, and memory. Interestingly, there was even increased brain connectivity and new neuron growth from stem cells. A protein called brain-derived neurotrophic factor (BDNF), which supports the growth of neurons and is important for long-term memory, is believed to be responsible for some of these benefits. In animals, both fasting and exercise significantly increase the beneficial BDNF effects in several parts of the brain. Compared to normal mice, mice on an intermittent fasting regimen showed less age-related deterioration of neurons and fewer symptoms of Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. Human studies on caloric reduction find similar neurologic benefits—and since fasting certainly restricts calories, this is one area where fasting and caloric reduction provide similar benefits. With a 30 percent reduction in calories consumed, memory significantly improved and the synaptic and electrical activity in the brain increased. In addition, insulin levels have an inverse correlation to memory—that is, the lower the insulin level, the more memory improves. On the flip side, a higher body mass index is linked to decline in mental abilities and decreased blood flow to those areas of the brain involved in attention, focus, reasoning, and more complex, abstract...

There appears to be significant research indicating a dramatic drop in inflammation, improvements in insulin signaling, and a near total “reset” of immune function with fasts of 3–5 days. Abnormal and or pre-cancerous cells appear to be pushed towards apoptosis, which essentially selects for healthy cell types. In total this describes a process which should (in theory) reverse many of the signs and symptoms of aging while reducing the processes that appear to be at play in autoimmunity and cancer. In this sense, the cells of the body are like cars.

As they age, subcellular parts need to be removed and replaced, and eventually, a cell gets too old to repair and needs to be destroyed to make way for a healthy new cell.

In a process called apoptosis, also known as programmed cell death, cells that reach a certain age are programmed to commit suicide. While this may sound kind of macabre at first, the process constantly renews cell populations, making it essential for good health. But when just some c...

The word autophagy, coined by Nobel Prize–winning scientist Christian de Duve, derives from the Greek auto (“self”) and phagein (“to eat”). So the word literally means “to eat oneself.” Autophagy is a form of cellular cleansing: it is a regulated, orderly process of breaking down and recycling cellular components when there’s no longer enough energy to sustain them. Once all the diseased or broken-down cellular parts have been cleansed, the body can start the process of renewal. New tissues and cells are bui...

While we often focus on new cell growth, we sometimes forget that the first step in renewal is destroying the old, broken-down cellular machinery. But apoptosis and autophagy are both necessary to keep our bodies running well. When these processes are hijacked, diseases such as cancer occur, and the accumulation of older cellular components may be responsible for many of the effects of aging. These...

Increased levels of glucose, insulin, and proteins all t...

Even as little as 3 grams of the amino acid leucine can stop autophagy. Here’s how it works: The mammalian target of rapamycin (mTOR) pathway is an important sensor of nutrient availability. When we eat carbohydrates or protein, insulin is secreted, and the increased insulin levels, or even just the amino acids from the breakdown of ingested protein, activate the mTOR pathway. The body senses that food is available and decides that since there’s plenty of energy to go around, there’s no need to eliminate the old subcellular machinery. The end r...

dormant—when it’s not being triggered by increased insulin levels or amino acids from ingested food—autophagy is promoted. As the body senses the temporary absence of nutrients,...

The oldest and most worn-out cellular parts get discarded, and amino acids from the broken-down cell parts are delivered to the liver, which uses them...

By eating constantly, from the time we wake up to the time we sleep, we prevent the activation of autophagy’s cleansing pathways. Simply put, fasting cleanses the body of unhealthy or unnecessary cellular debris. This is the reason longer fasts were often called cleanses or detoxifications.

At the same time, fasting also stimulates growth hormone, which signals the production of some new snazzy cell parts, giving our bodies a complete renovation. Since it triggers both the breakdown of old cellular parts and the creation of new ones, fasting may be considered one of the most potent anti-aging methods in existence.

Autophagy also plays an important role in the prevention of A...

Cancer is yet another disease that may be a result of disordered autophagy.

Fasting can limit growth of glucose-dependent tumors. Fasting can also target inflammation that contributes to the initiation and progression of tumors. We showed that fasting or calorie restriction could significantly reduce distal tumor invasion in our preclinical models of brain cancer.

little starvation can really do more for the average sick man than can the best medicines and the best doctors. —Mark Twain

I do IF every other day, about 20 hours. I have absolutely no problems finishing my workouts or other activities. I have found the biggest thing it has helped with is my digestion. I am way more regular now, and I also can also go longer between meals. In one year I also improved my triglycerides from 135 to 100 and raised my HDL a couple points to 60.

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.

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 cholesterol-reducing medications available.

Triglyceride levels drop by 30 percent, similar to what can be achieved with a very low carbohydrate diet or medication. Not bad for an all-natural, cost-free dietary intervention.

Plus, whereas statins carry the risk of diabetes and Alzheimer’s disease, fasting reduces body weight, preserves fat-free mass, and decreases waist circumference. In addition, fasting preserves HDL, unlike low-fat diets, which tend to decrease both LDL and HDL. Overall, fasting produces significant improvements in multiple cardiac risk factors. For people worried about he...

while they’re on an intermittent fasting regimen, they most often see their hunger diminish, not increase.

they are eating less than half their usual amount of food on a daily basis, yet they feel completely full.

We start to feel hunger pangs approximately four hours after our last meal. So we imagine that fasting for a full twenty-four hours creates hunger sensations six times stronger—and that would be intolerable. But this does not happen.

hunger is partly a learned phenomenon. Even when we don’t think we’re hungry, smelling a steak and hearing it sizzle may make us quite ravenous.

Hunger, in this case, starts with the eyes. But

there are infinite other possible associations with foods that make us hungry.

If we consistently eat every morning at 7:00 a.m., then we develop a conditioned response to that time and become hungry at 7:00 a.m., even if we ate a huge meal at dinner the night before. The same applies to lunch and dinnertime. We become hungry merely because of the time, not any true intrinsic hunger. This is learned only through decades of association. Children, on the other hand, often refuse food early in the morning because they are simply not hungry. Similarly, by consistently pairing movies with delicious popcorn and sugary drinks, the mere thought of a movie may make us hungry. Food companies, of course, spend billions of dollars trying to condition us to make these associations. Food at the ball game! Food at the movies! F...

There is a coffee shop or fast food restaurant on every corner.

we find it increasingly difficult to resist McDonald’s. We are bombarded daily with images of and references to food. The combination of their convenience and our ingrained Pavlovian response is deadly and fattening. How can we fight this?

Defeating Hunger Conditioning 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.

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.