Stages and Timelines of Fasting (Benefits of Each Stage)
Fasting Explained
Fasting is the deliberate act of restricting food intake for a certain period. There are several physiological changes that occur in the body in response to fasting; to mobilize energy and help the body adapt to lack of energy supplied from the food we take.
The body uses energy supplied from the food we consume to execute physiological functions such as breathing, movement, reproduction, development, coordination, and metabolism.
Food is consumed, digested, absorbed, and either converted to glucose for energy supply or the surplus stored for later use. The body is intelligent, storing what is not needed for times of reduced supply. Below is the body’s response to fasting, and this can vary from one person to the other depending on many factors.
1. The Fed State (few hours after eating)
Once food is ingested, processing (digestion and absorption) starts. Carbohydrates, proteins, and fats are broken down into glucose, amino acids, and fatty acids which are further metabolized into energy or stored in cells and tissues for later use. Few hours (between 3-4hours) after eating, blood glucose/sugar levels increase.
The level of blood glucose depends on the amount of carbohydrates consumed. Glucose levels are higher if the food eaten was high in carbohydrates. In response to glucose levels, the pancreas secretes insulin to. Insulin is the hormone responsible for regulating blood glucose levels. Insulin stimulates and facilitates the transfer of glucose into cells and tissues for energy production and protein synthesis in muscles. Excess glucose is stored as glycogen or adipose tissue.
A low carb diet has less sugar levels and once consumed slight elevates blood glucose levels.
During the fed state, ghrelin (the hunger hormone) levels drop whereas leptin (appetite suppressor hormone) increases in response to food intake. The drop in ghrelin and rise in leptin tells the brain that you are full.
2. Early Fasting State (4-18 hours after eating)
Early fast state happens between 3-4hours after eating, lasting up to 18hours. This phase is scientifically known as the catabolic phase. Blood glucose and insulin levels drop during this stage. The body starts using its stores of glycogen. As insulin drops, glucagon (the hormone that stimulates conversion of glycogen into glucose for energy) rises. Glucose through breakdown of glycogen is still the main source of energy.
Glycogen stores start being depleted towards the end of this phase.
In response to depletion of glucose produced from breakdown of glycogen, the body must adapt and look for alternative source of energy.
The body turns to fats for energy, and lipolysis (breakdown of fats into fatty acids) starts to take place. As fat is broken down into fatty acids, PPAR-alpha (regulates fat metabolism) essential for ketogenesis is activated. Activation of PPAR-alpha promotes metabolism of fats and the use of fatty acids for energy.
Fatty acids are metabolized into ketones (mainly in the liver) and these ketone bodies are used as energy source.
3. Fasting State (18 hours-48 hours)
At this state glycogen levels have been depleted. The body now increasingly recruits and turns to proteins and fats as source of energy. Ketones become the main source of energy for the body. Ketogenesis normally takes place in the liver where fatty acids are converted into ketone bodies which are then used for energy production. There are three main types of ketone bodies: acetone, acetoacetate, and beta-hydroxybutyrate (BHB).
How quickly the body gets into ketosis is influence on the amount of carbohydrates one consumes, level of exercise, and calorie restriction. Someone who eats less carbohydrates and exercises reaches ketosis quicker compared to one who consumes a lot of carbs and less active.
The human growth hormone starts increasing during this phase.
Potentially, the body enters autophagy (a process where damaged cells and tissues are repaired). Through autophagy, mitochondria function improves immensely.
“A decrease in circulating amino acids and glucose inhibits mTOR and leads to decreased protein synthesis, and an increased mitochondrial biogenesis and autophagy—resulting in prolonged life span in experimental animals (1).”
4. Long-Term Fasting State or Prolonged Fasting State (beyond 48 hours)
Happens after 2 days of fasting. Insulin further drops and there is increased production of ketones especially beta-hydroxybutyric (BHB) levels. The body is in deep ketosis at this point.
Ketosis and autophagy have peaked at this point. Cells and tissues continue repairing themselves (maximum autophagy).
5. Fasting Beyond 72 hours
Benefits of fasting, including weight loss, metabolic health, and longevity are amplified at this stage.
Due to reduced amount of nutrients, the liver reduces production of insulin-like growth factor 1 (IGF-1). IGF-1 is a hormone that controls growth and development, but short-term reduction in its production has been linked to low oxidative state that triggers anti-cancer and anti-aging activities in the body.
Summary
The physiology of fasting is composed of five stages, but individuals move through these stages differently. There are many factors that influence the physiology of these stages, including type of food consumed, exercise levels, underlying medical conditions, and whether one is overweight/obese or not vs healthy weight. The stages are:
1. The fed state (few hours after eating, up to 4 hours).
2. Early fasting state (approximately between 4-18hours)
3. The fasting state (approximately between 18-48hours)
4. Prolonged fasting (approximately between 48-72 hours)
5. Fasting beyond 72 hours
References
1. Deleyto-Seldas, N., & Efeyan, A. (2021). The mTOR–autophagy axis and the control of metabolism. Frontiers in cell and developmental biology, 9, 655731.
2. Harshiba, H. F., & Rafi, N. M. (2024). Fasting for clearer skin: Review article investigating the impact of intermittent fasting on dermatological conditions. Cosmoderma, 4.
