Intermittent fasting is defined as a dietary approach that alternates between designated periods of eating and abstaining from food. Various methods of intermittent fasting are available, with the majority requiring individuals to refrain from food intake for durations ranging from 16 to 24 hours.
During the fasting phase, the body undergoes a cycle of fed and fasted states, which leads to significant alterations in metabolism and hormonal levels.
This cycle plays an integral role not only in the metabolic transformations associated with intermittent fasting but also contributes to its numerous health advantages. This article provides a comprehensive examination of the distinct phases of fasting.
Different Stages of Fasting
1. Fed state
The fed state manifests shortly after eating, typically within the first few hours, as the body processes and assimilates nutrients from the ingested food. During this timeframe, there is a rise in blood sugar levels, which leads to an increased release of insulin.
This hormone helps move glucose from the bloodstream into the cells. The amount of insulin secreted depends on the meal’s nutritional makeup, the quantity of carbohydrates consumed, and the individual’s insulin sensitivity.
The liver and muscles store any surplus glucose as glycogen, which serves as the primary storage form of carbohydrates in the body.
The body can convert this glycogen back into glucose to meet energy needs when necessary.
During this period, the levels of various hormones, such as leptin and ghrelin, undergo changes. Ghrelin, a hormone that promotes hunger, experiences a decline in its levels following food intake.
Conversely, leptin, which serves to suppress appetite, sees an increase after eating. It is important to note that the fed-fast cycle reverts to the fed state immediately upon the consumption of food during a fasting phase.
Additionally, the quantity and nutritional composition of the meal influence the duration of the body’s fed state. Given the length of the fasting period, individual metabolic rates, and other factors, it is crucial to be aware of the potential risk of refeeding syndrome.
Overeating after an extended fasting period can pose serious health risks, including the possibility of fatal outcomes.
2. Early fasting state
Approximately 3 to 4 hours post-consumption, the body enters the initial fasting phase, which persists until roughly 18 hours after eating.
During this interval, there is a noticeable decrease in blood sugar and insulin levels, prompting the body to begin the conversion of glycogen into glucose for energy utilization. As this phase progresses, the liver’s glycogen reserves gradually diminish, leading the body to seek alternative energy sources.
This shift enhances lipolysis, a metabolic process that decomposes triglycerides stored in fat cells into smaller molecules, which serve as an alternative fuel source.
Additionally, the body utilizes amino acids, the fundamental components of proteins, to generate energy. Various popular intermittent fasting strategies, such as the 16/8 method, alternate between the fed state and the early fasting state.
3. Fasting state
The fasting period typically ranges from approximately 18 hours to 2 days.
During this duration, the glycogen reserves in the liver become exhausted, prompting the body to utilize protein and fat reserves for energy. This process results in the formation of ketone bodies, compounds produced during the conversion of fat into energy.
Consequently, the body enters a state known as ketosis, wherein fat becomes the primary energy source. It is important to note that the shift into ketosis may not occur immediately upon entering the fasting state but rather at a later stage.
Several factors, including the amount and type of food in your typical diet, your most recent meal, and personal differences, affect the rate at which you enter ketosis.
Common indicators of ketosis include a reduced appetite, weight loss, fatigue, a distinctive bad or fruity odor in the breath, and elevated levels of ketones detectable in the blood, breath, or urine.
To achieve ketosis, one can follow various approaches, particularly by adopting a ketogenic diet, which requires significantly reducing carbohydrate intake.
It is crucial to understand that ketosis is distinct from ketoacidosis, a potentially life-threatening condition resulting from excessive acidity in the blood. Ketoacidosis usually occurs due to illness, infection, or poorly controlled diabetes and demands immediate medical care.
Moreover, one should consider that fasting periods of 12 to 18 hours within the framework of intermittent fasting may not trigger ketosis. You typically do not reach this metabolic state with fasting durations of less than 24 hours, unless you also follow a strict low-carbohydrate diet.
4. Long-term fasting state
Following extended fasting periods, the body transitions into a long-term fasting state, generally occurring approximately 48 hours after the last meal. Some individuals often refer to this condition as the starvation state.
In this long-term fasting state, insulin levels continue to decline while the concentration of beta-hydroxybutyrate (BHB), a specific type of ketone body, progressively increases.
Additionally, the liver persists in producing glucose through a process known as gluconeogenesis, which becomes the primary energy source for the brain. At this stage, ketone bodies also contribute to the brain’s energy requirements.
The body minimizes the degradation of branched-chain amino acids (BCAAs), which are among the essential amino acids, to aid in the preservation of muscle tissue.
It is important to note that muscle breakdown will commence during starvation mode. Extended periods of starvation can lead to the deterioration of the body’s organs, ultimately resulting in their failure.
It is crucial to understand that most individuals should generally avoid long-term fasting, and they should only undertake it with medical oversight.
The bottom line
When practicing intermittent fasting, the body transitions through several phases of the fed-fast cycle, which vary according to the duration of the fasting interval.
The four phases include the fed state, early fasting state, fasting state, and the long-term fasting state, often referred to as the starvation state. Each phase is characterized by the main energy source that the body employs, influencing metabolic processes and the concentrations of different hormones.
Acknowledge that fasting has medical ramifications and approach it seriously. It is advisable to discuss any plans for intermittent fasting with a healthcare professional beforehand. Moreover, you should only conduct prolonged fasting with medical supervision.
