Your body may move through fed and fasting states while practicing intermittent fasting. Learn more about the different stages and what you can expect during each one.
Intermittent fasting refers to eating patterns that cycle between periods of eating and fasting.
Although several forms of intermittent fasting exist, most involve abstaining from food for periods of 16–24 hours at a time.
When practicing fasting, your body moves through the
This cycle is not only responsible for the metabolic changes that occur during intermittent fasting but also credited with providing some of its health benefits.
It’s important to keep in mind that prolonged fasting should only be conducted under medical supervision.
Learn more about the different stages of fasting.
The fed state occurs within the
During this period, blood sugar levels increase, and higher amounts of insulin are secreted. Insulin is the hormone responsible for transporting sugar from the bloodstream into the cells.
The amount of insulin released depends on your meal, including the amount of carbs consumed, and how sensitive your body is to insulin.
Extra glucose (sugar) is stored in the liver and muscles as glycogen. Glycogen is your body’s primary form of stored carbs. It can be converted back into sugar as a source of energy as needed.
During this time, levels of other hormones, including leptin and ghrelin, also shift.
Ghrelin is a type of hormone that stimulates hunger, and its levels decrease after you eat. Meanwhile, leptin, which has an appetite-suppressing effect, increases after eating.
Note that the fed-fast cycle resets back to the fed state as soon as food is consumed during a fast.
Around 3–4 hours after eating, your body transitions into the early fasting state, which lasts until around 18 hours after eating.
During this phase, your blood sugar and insulin levels start to decline, causing your body to start converting glycogen into glucose (sugar) to use as energy.
Toward the end of this phase, your body will slowly run out of liver glycogen stores and start searching for another energy source.
This intensifies lipolysis, a process in which fat cells are broken down into smaller molecules that can be used as an alternative source of fuel.
Many common forms of intermittent fasting, such as the 16/8 method, cycle between the fed state and early fasting state.
The fasting state lasts from about 18 hours to two days of fasting.
By this point, your glycogen stores in the liver have been depleted, and your body begins breaking down protein and fat stores for energy instead.
This results in the production of ketone bodies, a type of compound produced when your body converts fat into fuel. This also causes your body to transition into ketosis, a metabolic state in which your body uses fat as its primary source of energy.
However, the transition into ketosis may not happen immediately as you enter the fasting state but likely later on.
As with the fasting state in general, the size and composition of your usual diet and last meal, along with individual differences, affect how quickly you enter ketosis.
Some of the most common signs of ketosis include:
- decreased appetite
- weight loss
- fatigue
- fruity-smelling breath
- increased levels of ketone bodies in the blood, breath, or urine
Keep in mind that ketosis is different from ketoacidosis, which is a dangerous condition that occurs when your blood becomes too acidic.
Additionally, note that forms of intermittent fasting that have shorter windows of fasting ranging from 12–18 hours per day may not reach this state, as ketosis may not be achieved with fasts lasting less than 24 hours unless you also follow a very low carb diet.
During extended periods of fasting, your body enters the long-term fasting state, which typically occurs around 48 hours after food intake. Some people refer to this state as the starvation state.
In the long-term fasting state, insulin levels will continue to decrease and levels of beta-hydroxybutyrate (BHB), a type of ketone body, will steadily rise.
Your liver also continues to generate sugar via a process called gluconeogenesis, which serves as the main source of fuel for the brain. Ketone bodies provide energy for the brain as well at this point.
Be aware that in starvation mode, muscle breakdown may start to occur. Prolonged starvation means the body’s organs will eventually begin to break down and ultimately stop working.
Keep in mind that long-term fasts are not recommended for most people and should only be performed under medical supervision.
While practicing intermittent fasting, your body moves through several phases of the fed-fast cycle, depending on the amount of time that you fast.
The four phases include the fed state, early fasting state, fasting state, and long-term fasting state (starvation state).
Each phase varies based on the primary source of energy used for the body, as well as how it affects your metabolism and levels of specific hormones.
Fasting always has medical impacts and should not be taken lightly. Be sure to talk with a healthcare professional before trying intermittent fasting.
Additionally, keep in mind that prolonged fasting should only be conducted under medical supervision.