Fasting Physiology

By Jason Fung, MD

  • The body uses two main fuel sources – glucose (a type of carbohydrate) and triglycerides (fat)
  • Glucose can be stored in the liver as glycogen
  • Fasting allows you to use stored energy in the body fat

By Jason Fung, M.D., Co-founder of The Fasting Method


In order to learn about what happens in the body while we’re fasting, it’s important to understand how the body fuels itself.

The body uses two main sources of energy:

  1. Glucose
  2. Fat

If glucose is available, it is used first. If it is not available, then the body will adjust by using fat, without any detrimental health effects. The body also stores food energy (calories) in two main ways:

  1. Glucose
  2. Body Fat

Glucose is stored in the form of glycogen in the liver. The individual glucose molecules are strung together in long chains and stored in the liver. Protein, the other one of the three main macronutrients is not primarily a source of energy, but used for building functional tissues.

If you are fasting, your body still requires energy. But since there is no food, your body must get the calories it needs from its storage systems (glycogen and body fat). Protein, although not primarily an energy storage system, can be used. The transition from the fed state to the fasted state occurs in several stages.

  1. Feeding – During meals, insulin levels are raised. This allows uptake of glucose into tissues such as the muscle or brain to be used directly for energy. Excess glucose is stored as glycogen in the liver.
  2. The post-absorptive phase – 6-24 hours after beginning fasting. Insulin levels start to fall. Breakdown of liver glycogen releases glucose for energy. Glycogen stores last for roughly 24 hours.
  3. Gluconeogenesis – 24 hours to 2 days – The liver manufactures new glucose from amino acids in a process called “gluconeogenesis”. Literally, this is translated as “making new glucose”. In non-diabetic persons, glucose levels fall but stay within the normal range.
  4. Ketosis – 2-3 days after beginning fasting – The low levels of insulin reached during fasting stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used for gluconeogenesis. Fatty acids may be used directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. 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 are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting.
  5. Protein conservation phase – >5 days – High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.

The human body has well developed mechanisms for dealing with periods of low food availability. In essence, what we are describing here is the process of switching from burning glucose (short term) to burning fat (long term). Fat is simply the body’s stored food energy. In times of low food availability, stored food is naturally released to fill the void. There are also many changes in hormonal levels during fasting.


Fasting is the most efficient and consistent strategy to decrease insulin levels. Almost all foods raise insulin to some degree, so the most effective method of reducing insulin is to avoid all foods. Blood glucose levels remain normal, as the body begins to switch over to burning fat for energy. This effect is seen with fasting periods as short as 24-36 hours. Longer duration fasts reduce insulin even more dramatically. More recently, alternate daily fasting has been studied as an acceptable technique of reducing insulin. Regular fasting also improves insulin sensitivity significantly.

Lower insulin levels allow the body to excrete excess salt and water, because high insulin causes salt and water retention in the kidney. This water weight often accounts for some of the early rapid weight loss in low carbohydrate diets and fasting. This diuresis reduces bloating, and makes you feel ‘lighter’. It may also contribute to a slightly lower blood pressure. For the first five days, weight loss averages 0.9 kg/ day, far exceeding the caloric restriction and likely due to a diuresis of salt and water.

Growth Hormone

Growth hormone is known to increase the availability and utility of fats for fuel. It also helps to preserve muscle mass and bone density. Secretion is known to be pulsatile, making accurate measurement difficult. Growth hormone secretion decreases steadily with age. One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion more than doubled. The net physiologic effect is to maintain muscle and bone tissue mass over the fasting period.


Concerns about malnutrition during fasting are generally misplaced. Insufficient calories are not a major worry, since fat stores are quite ample in most cases where people are trying to lose weight. The main concern is the development of micronutrient deficiency. However, even prolonged studies of fasting have found no evidence of malnutrition. Potassium levels may decrease slightly, but even two months of continuous fasting does not decrease levels below 3.0 mEq/L, even without the use of supplements. Magnesium, calcium and phosphorus levels during fasting are stable. Presumably, this is due to the large stores of these minerals in the bones. Ninety nine percent of the calcium and phosphorus in the body is stored in the bones. The use of a multi-vitamin supplement will provide the recommended daily allowance of micronutrients. A therapeutic fast of 382 days was maintained with only a multivitamin with no harmful effect on health. Actually, this man maintained that he had felt terrific during this entire period. The only concern may be a slight elevation in uric acid that has been described in fasting.


Adrenalin levels are increased so that we have plenty of energy to go get more food. For example, 48 hours of fasting produces a 3.6% increase in metabolic rate, not the dreaded metabolic ‘shut-down’. In response to a 4 day fast, resting energy expenditure increased up to 14%. Rather than slowing the metabolism, instead the body revs it up. Presumably, this is done so that we have energy to go out and find more food.

Fasting, but not low calorie diets results in numerous hormonal adaptations that all appear to be highly beneficial on many levels. In essence, fasting transitions the body from burning sugar to burning fat. Resting metabolism is NOT decreased as we are, effectively, feeding our bodies through our own fat. We are ‘eating’ our own fat. This makes total sense since body fat is nothing more or less than stored food. In fact, studies show that the epinephrine (adrenalin) induced fat burning does not depend upon lowering blood sugar.

Recall our previous discussion of How Insulin Works. Fat is food stored away in the long term, like money in the bank. Short term food is stored as glycogen, like money in the wallet. The problem we have is how to access the money in the bank. As our wallet depletes, we become nervous and go out to fill it again. This prevents us from getting access to money in the bank.

Fat is stored away in the ‘bank’. As our glycogen ‘wallet’ depletes, we get hungry and want to eat. That makes us hungry, despite the fact that there is more than enough ‘food’ stored as fat in the ‘bank’. How do we get to that fat to burn it? Fasting provides an easy way.

For more, see The Obesity Code.

Learn more about Pique Fasting Tea.

For fasting education, support and community, go to The Fasting Method.


Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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