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

by Jason Fung

One of the more interesting and memorable nuggets from that half-hour interview came at the very end of our conversation, when Dr. Seyfried made the bold assertion that an annual seven- to ten-day water fast could be a useful tool for preventing cancer.

didn’t measure my blood sugar every single day, but the few times I did I saw readings in the 60s. Sure, this was below 80, the level I generally saw on my healthy low-carb lifestyle, but that’s what happens when you don’t eat any food at all. Controlling blood sugar and giving your pancreas a week to rest from producing insulin is an excellent reason for attempting a fast like this.

learned later that the weight loss on a one-week fast like this is mostly water weight because the fasting process depletes glycogen stores.

Insulin has two major functions. First, it allows the body to immediately start using food energy. Carbohydrates are absorbed and rapidly turned into glucose, raising blood sugar levels. Insulin allows glucose to enter directly into most cells of the body, which use it for energy. Proteins are broken down into amino acids and absorbed, and excess amino acids may also be turned into glucose. Protein does not raise blood glucose, but it can raise insulin levels. The effect is variable, and it surprises many people to learn that some proteins can stimulate insulin as much as some carbohydrate-containing foods. Fats are directly absorbed as fat and have minimal effect on insulin.

The transition from the fed state to the fasted state occurs in several stages, as classically described by George Cahill, one of the leading experts in fasting physiology: 1. Feeding: Blood sugar levels rise as we absorb the incoming food, and insulin levels rise in response to move glucose into cells, which use it for energy. Excess glucose is stored as glycogen in the liver or converted to fat. 2. The postabsorptive phase (six to twenty-four hours after beginning fasting): At this point, blood sugar and insulin levels begin to fall. To supply energy, the liver starts to break down glycogen, releasing glucose. Glycogen stores last for approximately twenty-four to thirty-six hours. 3. Gluconeogenesis (twenty-four hours to two days after beginning fasting): At this point, glycogen stores have run out.

Ketosis (two to three days after beginning fasting): Low insulin levels stimulate lipolysis, the breakdown of fat for energy. Triglycerides, the form of fat used for storage, are broken into the glycerol backbone and three fatty acid chains. The glycerol is used for gluconeogenesis, so the amino acids formerly used can be reserved for protein synthesis. The fatty acids are used directly for energy by most tissues of the body, though not the brain. The body uses fatty acids to produce ketone bodies, which are capable of crossing the blood-brain barrier and are used by the brain for energy. After four days of fasting, approximately 75 percent of the energy used by the brain is provided by ketones.

5. The protein conservation phase (five days after beginning fasting): High levels of growth hormone maintain muscle mass and lean tissues. The energy for basic metabolism is almost entirely supplied by fatty acids and ketones. Blood glucose is maintained by gluconeogenesis using glycerol. Increased norepinephrine (adrenaline) levels prevent any decrease in metabolic rate. There is a normal amount of protein turnover, but it is not being used for energy.

Regularly lowering insulin levels leads to improved insulin sensitivity—your body becomes more responsive to insulin. The opposite of insulin sensitivity, high insulin resistance, is the root problem in type 2 diabetes and

has also been linked to a number of diseases, including: Heart disease Stroke Alzheimer’s disease High cholesterol High blood pressure Abdominal obesity Nonalcoholic steatohepatitis (fatty liver disease) Polycystic ovary syndrome Gout Atherosclerosis Gastroesophageal reflux disease Obstructive sleep apnea Cancer

Lowering insulin also rids the body of excess salt and water because insulin is well known to cause salt and water retention in the kidneys.

Meals very effectively suppress the secretion of growth hormone, so if we’re eating three meals per day, we get effectively no growth hormone during the day. Worse, overeating suppresses growth hormone levels by as much as 80 percent.

The most potent natural stimulus to growth hormone secretion is fasting.

Reduce sugars and refined grains Avoiding all processed foods is preferable but not always 100 percent possible, for many reasons. It is therefore important to recognize which foods are the biggest offenders, so that we may avoid them.