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

by Jason Fung

And a 1992 study showed a fivefold increase in growth hormone in response to a two-day fast.

There is nothing inherently unhealthy about carbohydrate-containing foods. The problem arises when we start changing these foods from their natural state and then consuming them in large amounts. The same also applies to processed fats. Processing transforms relatively innocuous vegetable oils into fats that contain trans fats, toxins whose dangers have now been well recognized.

The true secret to healthy eating is this: Just eat real food.

recent data suggests that these oils are very high in inflammatory omega-6 fats. While some omega-6 fats are necessary, we are likely eating ten to twenty times more than we did in the past, and when our consumption of omega-6 fats is out of balance with our consumption of omega-3 fats (found in fatty cold-water fish, nuts, and seeds), the result is systemic inflammation, which is a factor in heart disease, type 2 diabetes, inflammatory bowel disease, and other chronic illnesses.

I started supplementing with 500 to 1000 mcg of chromium per day starting in month 3.

Fasting is

not so much a treatment for illness but a treatment for wellness.

most of the population of Crete followed the Greek Orthodox tradition of fasting. This may have contributed to the healthy longevity of this population.

A brilliant and transformative scientist, he also wrote, “Fasting is the greatest remedy—the physician within.”

wrote, “The best of all medicines is resting and fasting.”

Franz Kafka based his short story “A Hunger Artist” on the practice.

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.

Studies demonstrate this phenomenon clearly. In one, 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.

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.

Much of the amino acids that are broken down during regular turnover of cells are reabsorbed into new proteins.

Running just on the fat stored in their bodies, most Americans could walk from New York to Florida without technically needing a bite to eat.

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. To burn muscle for energy would be like storing firewood and then, as soon as cold weather hits, chopping up your sofa and throwing it into the fire.

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. 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.

On the other hand, if you are worried about weight loss and type 2 diabetes, then you need to worry about diet, not exercise. You can’t outrun a bad diet.

Blood sugar level is tightly monitored by the body, and there are multiple mechanisms to keep it in the proper range. During fasting, our body begins by breaking down glycogen (remember, that’s the glucose in short-term storage) in the liver to provide glucose. This happens every night as you sleep to keep blood sugars normal as you fast overnight.

If you fast for longer than twenty-four to thirty-six hours, glycogen stores become depleted. The liver now can manufacture new glucose in a process called gluconeogenesis, using the glycerol that’s a by-product of the breakdown of fat. This means that we do not need to eat glucose for our blood glucose levels to remain normal.

So, the best diet would emphasize whole, unprocessed foods. It would be low in refined carbohydrates and high in natural fats with a moderate amount of protein.

This work is a derivative of “Effective Simplicity”

Feasts follow fasts. Fasts follow feasts.

It adds simplicity where other diets add complexity.

began with three days of fasting a week.

I now weigh 68 kg (150 pounds) and have lost a total of 20.64 kg (46 pounds) of body fat (12.4%) and a total of 77.5 cm (31 inches) from my waist.

Source: Wells and Buzby, “Dietary Assessment of Major Trends in U.S. Food Consumption, 1970–2005.”

When we eat, our body derives energy from three main sources: glucose (carbohydrates), fat, and protein. Only two of these are stored for later use, glucose and fat—the body can’t store protein, so excess protein that can’t be used right away is converted to glucose. Glucose is stored in the liver as glycogen, but the liver’s capacity for storing glycogen is limited. Once glycogen stores are full, excess calories must be stored as body fat. Dietary fat is absorbed directly into the bloodstream without passing through the liver, and what’s not used is stored as body fat. This was one of the reasons why low-fat diets were initially recommended, but the immediate destination of ingested calories is not the main determinant of weight gain.

Think of glycogen as a refrigerator. It’s designed for short-term storage of food; it’s very easy to move food in and out, but the storage space is limited. Body fat, on the other hand, is more like a basement freezer. It’s designed for long-term storage and is more difficult to access, but it has much greater capacity. Plus, you can always add more freezers to the basement if you need them. When we buy groceries, we store food in the refrigerator first, and then when the fridge is full, we store the excess food in the freezer—that is, we store food energy first

as glycogen and then, when the space for glycogen is full, as body fat. Both body fat and glycogen are used for

energy in the absence of food, but they aren’t used equally o...

The two compartments, the fridge and the freezer, are not used simultaneously but sequentially. You need to (mostly) empty out the fridge before you can use what’s in the freezer—you need to burn most of the glycogen before you can burn fat. In essence, the body can burn either sugar or fat, but not both.

When we are not eating, insulin levels are low, allowing full access to the fat freezer—the body is able to easily get at the stored fat. With low insulin levels, you don’t even have to completely empty the glycogen refrigerator before opening the fat freezer, since it’s so easily accessible. Think about your fridge at home. Does it have to be completely empty of everything, including that half-empty bottle of ketchup and tub of yogurt, before you open that pack of burgers from the freezer? Of course not! Similarly, with low insulin levels, the body can burn fat even if there is still some glucose around. That means that if you’re cutting calories and have low insulin levels, it’s easy for your body to compensate for the reduced food energy by getting some fat out of the freezer even if your glycogen fridge isn’t completely empty. But the emptier your glycogen fridge, the more likely you will be to use what’s in the fat freezer, and the easier it is to get to the freezer, the more likely it is that you will use it.

Insulin resistance, sometimes called prediabetes or metabolic syndrome, is the most common situation where insulin levels are persistently kept abnormally high.

The same logic applies to insulin resistance. When our cells become less sensitive to insulin, the body’s knee-jerk reaction is to increase insulin production.

The answer is really the exact opposite: since insulin resistance develops in response to persistently high insulin levels, we must create recurrent

periods of very low insulin levels.

To burn fat, two things must happen: you must burn through most of your stored glycogen, and insulin levels must drop low enough to release the fat stores.

The reason the body has to resort to decreasing metabolism and increasing hunger is because insulin remains high, so it doesn’t have access to the energy stored as fat. Your body has no other option but to slow metabolism—it’s trying to conserve energy because it can’t get at the fat freezer. This is why insulin resistance plays such a crucial role in obesity: the high insulin levels tell your body to hang onto body fat and simultaneously trigger the body to lower your metabolism. This inexorably erodes weight-loss efforts. Body weight plateaus and then ruthlessly climbs back up, even as the proper diet is maintained. For some of us, changing what we eat is clearly not enough. Let’s take an example. Suppose you eat 2,000 calories per day. Your weight is stable, so you are burning 2,000 calories per day. There are 3,500 calories in a pound of fat, so if you carry 100 pounds of fat, there are 350,000 calories in fat stores.

The high insulin is instructing the body to store energy, not burn it. The body is used to burning 2,000 calories, but now, only 1,200 are available, so it is forced to reduce its energy expenditure to match. Basal metabolic rate decreases to 1,200 calories.

This holds true even when the number of calories consumed is identical. In other words, the glucose-lowering benefits of carb restriction are not simply due to calorie restriction.

The very low carb diet does remarkably well, providing you 71 percent of the benefits of fasting, without actual fasting. But sometimes low-carb just isn’t enough. I’ve had many patients who limited their carbohydrates but still had elevated blood sugars. How do you get more power? Fasting. Insulin is the main driver of obesity and diabetes. A very low carb diet can reduce insulin by more than 50 percent, but you can go another 50 percent by fasting. That’s power.

The short answer is that when you’re eating regularly, even if you’re eating fewer calories, you’re not getting the beneficial hormonal changes of fasting. During fasting, unlike during caloric reduction, metabolism stabilizes or even goes up to maintain normal energy levels. Adrenaline and growth hormone increase to maintain energy and muscle mass. Blood sugar and insulin levels go down as the body changes from burning sugar to burning fat. All this begins to address the long-term problem of insulin resistance.

After six months, both groups had similar levels of weight loss and fat loss. But the 5:2 fasting group showed a clear, substantial improvement in insulin levels and insulin resistance, whereas the caloric-reduction group did not.

The inability of most diets to reduce insulin resistance is exactly why they eventually result in weight regain. Fasting, on the other hand, introduces prolonged periods of low insulin levels, which breaks the cycle of high insulin and insulin resistance.

Another way to look at this is that most diets ignore the biological principle of homeostasis. The body adapts to changing environments. For example, if you are in a dark room and suddenly go into the bright sunlight, you will momentarily be blinded, but within moments your eyes will adapt.

The same applies to weight loss. If you maintain a constant reduced-calorie diet, the body will quickly adapt to it. Energy expen...

the reduced intake. Weight plateaus, then is regained. This is not because you have stopped your diet but because your body has now adapted to it. To prevent the body from adapting to the new weight-loss strategy and maintain weight loss requires an intermittent strategy, not a constant one. This is a crucial distinction. Restricting some foods all the time differs ...