Intermittent fasting is the type of eating patterns that alternates between periods of eating and fasting.
Although, there are so many forms of intermittent fasting exist, most involve escaping from food for periods of 16–24 hours at a time.
When implementing fasting, your body goes through the fed-fast cycle, which is characterized by changes in your metabolism and hormone levels.
This dietary cycle improves metabolic health along with providing some other health benefits.
This article deeply describes the different stages of Intermittent fasting.
1. Fed state
The fed state starts within the initial few hours after eating as your body digests and absorbs nutrients from food.
During this period, your blood sugar levels rise and larger amounts of insulin are secreted. Insulin is pancreatic hormone which for transports glucose from your bloodstream into your cells (1).
The amount of insulin released depends on the composition of your diet, the amount of carbs consumed, and sensitivity of your body for insulin (2).
Extra glucose (sugar) is stored in the liver and muscles as glycogen. Glycogen is your main form of stored carbs in your body and it can be converted back into sugar as a source of energy when required (3).
During this time, levels of other hormones, including leptin and ghrelin, also changes.
Note that the fed-fast cycle readjusts back to the fed state immediately after food is consumed during a fast.
Also, the amount and composition of your food affect how long your body resides in the fed state.
2. Early fasting state
Around 3–4 hours after eating, your body changes into the early fasting state, which lasts until around 18 hours after eating.
During this phase, your blood glucosa and insulin levels start reducing which leads to initiation of glycogen conversion into glucose (sugar) to use as energy (1).
Toward the end of this phase, your body will gradually consumes all of liver glycogen stores and begins searching for another energy source.
Your body also use amino acids as a source of energy, which are the constituents of proteins.
Many common types of intermittent fasting, such as the 16/8 method, cycle between the non fasting or eating phase and early fasting phase.
3. Fasting state
The fasting phase lasts from about 18 hours to 2 days of the Intermittent fasting.
By this point, your glycogen stores in the liver have been consumed, and your body starts breaking down protein and fat stores for energy source.
This leads to the production of ketone bodies, a type of molecule produced during breakdown of fats for energy (8).
However, the transition into ketosis may not occurs immediately as you enter the fasting state, but probably later on (10).
As with the fasting state in general, the quantity and composition of your normal diet and last meal, along with individual differences, affect how fast you enter ketosis.
Some of the most common signs of ketosis are:
reduced appetite, weight loss, fatigue, bad or fruity-smelling breath, and raised levels of ketone bodies in the blood, breath, or urine (11).
Ketosis can also be attained through other methods, such as, by following the ketogenic diet, which involves remarkably lowering your carbohydrate consumption. (12).
Ketoacidosis generally occurs as a result of illness, infection, or uncontrolled diabetes, and unlike ketosis, it is very serious and requires immediate medical attention (13).
Additionally, note that forms of intermittent fasting that have shorter durations ranging from 12–18 hours in a day may not attain this state, because ketosis may not be attained with less than 24 hours of fast unless you also follow a very low carb diet.
4. Long-term fasting state (starvation state)
During enlarged periods of fasting, your body enters the long-term fasting state, which usually occurs around 48 hours after taking of food. Some people called this state as the starvation state.
Your kidneys also continue to produce glucose by a process called as gluconeogenesis, which serves as the main source of energy for the brain. Ketone bodies provide energy for the brain as well at this point (1, 15).
The breakdown of branched-chain amino acids (BCAAs), which are three of the essential amino acids, is also reduced to help in retaining muscle tissue in the body (1).
It should be noted that long-term fasts are not suggested for most people and should only be followed under the medical supervision.
While implementing intermittent fasting, your body goes through many phases of the fed-fast cycle, depending on the duration of your fast.
The four phases of intermittent fasting include the fed state, early fasting state, fasting state, and long-term fasting state or starvation state.
Each phase differs on the basis of primary source of energy used for the body, as well as how it affects your metabolism and the levels of specific hormones.
If you have any fundamental health conditions or are taking any types of medications, make sure to advice with your doctor or healthcare provider before starting intermittent fasting.
Additionally, keep in mind that prolonged fasting should only be conducted under medical supervision.