A 72-hour fast involves abstaining from all caloric intake for three consecutive days, allowing only water, black coffee, or tea. This prolonged period of abstinence forces the body to undergo a metabolic transformation, shifting its primary fuel source from ingested food to internal reserves. The body’s systems, which are normally focused on processing and storing energy, switch their attention to conservation and internal maintenance. This controlled scarcity triggers a sequential breakdown of stored energy supplies, leading to distinct physiological changes over the three-day timeline.
The Initial Metabolic Transition (0-24 Hours)
The first day of the fast is characterized by the body quickly burning through its most readily available fuel: glucose. During the first 12 to 24 hours, the body relies heavily on glycogen, the stored form of glucose found primarily in the liver and muscles. The liver contains the largest reserve of accessible glycogen, which is broken down into glucose and released into the bloodstream to maintain normal blood sugar levels.
The cessation of food intake causes an immediate drop in the hormone insulin, which usually signals cells to absorb glucose. As insulin levels fall, the pancreas increases the release of glucagon, a counter-regulatory hormone that signals the liver to break down its glycogen stores. This initial phase is often accompanied by feelings of hunger, mild fatigue, and an energy dip.
Once the liver’s glycogen stores are depleted, around the 24-hour mark, the body must find a new way to generate the glucose necessary for cells that cannot use fat for fuel, such as red blood cells. This process is called gluconeogenesis, where the liver begins creating new glucose from non-carbohydrate sources like lactate, glycerol, and amino acids. This shift marks the completion of the initial transition and signals the body’s entry into a deeper, fat-burning state.
Full Shift to Ketosis (24-48 Hours)
The period between 24 and 48 hours is defined by a complete metabolic switch as the body enters nutritional ketosis. With glycogen exhausted, the liver accelerates the breakdown of stored triglycerides (body fat) into fatty acids. These fatty acids are then converted into water-soluble molecules known as ketone bodies, specifically acetoacetate and beta-hydroxybutyrate.
Ketones become the primary alternative fuel source, especially for the brain, which cannot directly use fatty acids. Ketone bodies supply much of the brain’s energy needs, replacing glucose as the preferred fuel. This metabolic state often results in a stabilization of energy levels and a reduction in hunger pangs, commonly referred to as a “second wind.”
The rise in circulating ketone bodies indicates that the body is now efficiently oxidizing fat for fuel. This utilization of fat reserves is a defining characteristic of this stage of the prolonged fast.
Cellular Recycling and Hormonal Response (48-72 Hours)
By the time the fast reaches the 48 to 72-hour window, the body is operating in a state of conservation. One of the most important processes upregulated during this phase is autophagy, a cellular self-cleaning mechanism. Autophagy involves the cell identifying, isolating, and recycling damaged or dysfunctional components, such as old proteins and worn-out organelles.
This cellular maintenance is triggered by the prolonged absence of nutrients, which effectively deactivates the nutrient-sensing pathway known as mTOR. The resulting cellular stress prompts the body to scavenge internal resources for repair and energy production. Markers of autophagy are amplified after 48 hours of fasting, leading to a more efficient and resilient cellular environment.
Concurrently, the body initiates a protective hormonal response to preserve muscle tissue while maximizing fat loss. Levels of Human Growth Hormone (HGH) show a sustained elevation during this period. This surge in HGH helps to maintain lean muscle mass by promoting the breakdown of fat for fuel and supporting protein synthesis.
Insulin levels remain suppressed, contributing to improved whole-body insulin sensitivity. The body’s overall metabolic machinery becomes efficient, utilizing fat stores to conserve essential nutrients and protein. By 72 hours, the body relies almost entirely on ketones and fat oxidation for fuel while initiating cellular repair.