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Fasting: Further Discussed by Dr. Tremblay

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    Hello our awesome readers. As you all know, we at Action Medicine are extremely interested in helping you obtain our optimal health. It is for this reason that I would like to discuss the recent book I read on Fasting.  It is a little lengthy, but if you are interested in fasting or alternative lifestyle interventions for health, please take the time to finish our blog.

So, let us begin our further discussion on fasting. Fasting has been part of our DNA since the beginning of our tree dwelling existence. We have two major states of metabolism, feeding and fasting. So how does it work?

Dr. Jason Fung explains this quite well in his book, “The Complete Guide to Fasting”. The process of using and storing food energy that occurs when we eat goes in reverse when we fast. Our insulin levels drop, and we burn stored energy. Glycogen (stored blood sugar in the liver) is depleted after 24 hours and then our body breaks down body fat for energy.

The body only exists in two states, the fed, high insulin state and the fasted, low insulin state. Either we are storing food energy, or we are burning food energy. If eating and fasting are balanced, then there is no weight gain. If instead we spend most of our day storing food energy (fed state), we gain weight. In order to restore balance, we need to increase the amount of time we burn food energy by going into the fasted state.

The transition from the fed state to the fasted state occurs in several stages as described by George Cahill:


  1. 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. 2. The postabsorptive phase (6 to 24 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 24 to 36 hours.
  3. Gluconeogenesis (24 hours to two days after beginning fasting): at this point, glycogen stores have run out. The liver manufactures new glucose from amino acids in a process called gluconeogenesis (literally, “making new glucose”). In non-diabetic persons, glucose levels fall but stay within the normal range.
  4. Ketosis (2 to 3 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 back bone and three fatty acid chains. The glycerol is used for gluconeogenesis, so the amino acids formally used, can be reserved for protein synthesis. The fatty acids are used directly for energy by the 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% of the energy used by the brain is provided by ketones. The two major types of ketones produced our beta-hydroxybutyrate and acetoacetate, which can increase over 70-fold during fasting.
  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.

 Fat is simply the body’s stored food energy. In times of famine, we burn this, instead of food calories, for energy. The body does not burn muscle for energy until all the fat stores depleted. Periods of low food availability have always been a natural part of human history and we have evolved mechanisms to adapt to this fact. (Otherwise we would not have survived as a species!) Just about anyone can fast without adverse health consequences, but there are some exceptions. You should not fast if you’re malnourished or struggle with eating disorders.

Fasting lowers insulin levels, reducing insulin resistance. Insulin resistance has been linked to heart disease, stroke, Alzheimer’s, high cholesterol, hypertension, abdominal obesity, fatty liver disease, polycystic ovarian syndrome, gout, hardening of the arteries, gastroesophageal reflux, obstructive sleep apnea, cancer and other maladies.


Fasting of some sort should be part of everyone’s routine. If you have additional questions, I suggest you read the complete guide to fasting by Dr. Jason Fung or make an appointment with your provider.

-Brooke Rieth NP

-Dr. Tremblay

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