What Really Happens to Our Body When We Fast?

The process of using and storing food energy that occurs when we eat goes in reverse when we fast.

Insulin levels drop, signalling the body to start burning stored energy.

Glycogen (the glucose that’s stored in the liver) is the most easily accessible energy source, and the liver stores enough to provide energy for twenty-four hours or so.

After that, the body starts to break down stored body fat for energy.

So, you see, 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 net weight gain.

If, however, we spend the majority of the day storing food energy (because we’re in the fed state), then over time, we will gain weight.

What is needed then is to restore balance by increasing 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:

Feeding:

Blood sugar levels rise as we absorb the incoming food, and insulin levels rise in response to moving glucose into cells, which use it for energy.
Excess glucose is stored as glycogen in the liver or converted to fat.

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.

Gluconeogenesis (twenty-four 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 nondiabetic persons, glucose levels fall but stay within the normal range.

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.
The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, which can increase over seventyfold during fasting.

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.

In essence, what we are describing here is the process of switching from burning glucose to burning fat.

Fat is simply the body’s stored food energy.

In times of low food availability, stored food is naturally released to fill the void.

The body does not “burn muscle” to feed itself until all the fat stores are used up.

One critical point to underscore is that these mechanisms are entirely natural and entirely normal.

Periods of low food availability have always been a natural part of human history.

Otherwise, we would not have survived as a species.

There are no adverse health consequences to activating these protocols, except in the case of malnourishment (you should not fast if you’re malnourished, of course, and extreme fasting can cause malnourishment, too).

The body is not “shutting down”; it’s merely changing fuel sources, from food to fat.

It does this with the help of several hormonal adaptations to fasting.