A 36-hour fast is considered an extended fast, moving beyond the typical overnight or 16-hour fasting window. This duration represents a deep and prolonged metabolic shift. By this point, the physiological processes relying on readily available energy sources are exhausted, forcing the system to pivot dramatically. The body transitions from burning external fuel to utilizing its own deep energy reserves.
Glycogen Depletion and the Fuel Transition
The body’s primary energy reserve is glycogen, which is stored glucose found mainly in the liver and muscles. During the initial hours of a fast, the liver breaks down this stored glycogen (glycogenolysis) to maintain steady blood glucose levels for the brain and other organs. For most individuals, the liver’s glycogen reserves are largely depleted after about 18 to 24 hours without caloric intake.
By 36 hours, the body is no longer relying on stored carbohydrates for fuel. The exhaustion of this quick-access glucose supply forces the system to find an alternative way to maintain blood sugar for glucose-dependent tissues. The body initiates gluconeogenesis, creating new glucose from non-carbohydrate sources, such as amino acids from protein and glycerol from fat stores. This marks a pivot toward a fat-based metabolism.
Entering Ketosis and Utilizing Fat Stores
The lack of incoming glucose and depleted glycogen prompt the liver to process fatty acids from adipose tissue. The liver converts these fatty acids into ketone bodies, specifically Beta-hydroxybutyrate (BHB) and acetoacetate, through ketogenesis. At 36 hours, the body is firmly generating and circulating these ketones, entering nutritional ketosis.
Ketones become the primary alternative fuel source, efficiently powering most of the body’s tissues, including the brain. The brain, which typically relies on glucose, can derive a significant portion of its energy needs from BHB, providing a sustained energy source. This metabolic adaptation is often associated with a reduction in perceived hunger, as ketones can suppress the hunger hormone ghrelin. The exact timing of reaching robust ketosis can vary depending on an individual’s prior diet and metabolic flexibility, with some needing closer to 48 hours for maximum ketone production.
Cellular Recycling and Repair
Beyond the fuel switch, the 36-hour mark represents a deep activation of cellular housekeeping known as autophagy. Autophagy is a biological process where cells systematically disassemble and recycle damaged or dysfunctional components, such as misfolded proteins and worn-out organelles. This “self-eating” process is essential for cellular maintenance and tissue renewal.
While initial signs of autophagy may appear earlier, the extended duration of the 36-hour fast significantly maximizes this recycling mechanism. The lack of nutrient input, specifically amino acids and glucose, signals to the cell that resources are scarce. This prompts the cell to scavenge and repurpose internal materials for survival, resulting in intensified cellular cleanup.
Hormonal and Neurochemical Responses
The lack of food intake for 36 hours results in profound shifts in the body’s chemical communication system. Insulin levels, which regulate blood sugar and nutrient storage, plummet to their lowest levels. This low insulin environment facilitates the unrestricted release of stored fat for energy and is a prerequisite for the body to switch into fat-burning mode.
Concurrently, a surge in Growth Hormone (GH) production begins, with levels potentially increasing significantly. This elevation of GH is a protective mechanism designed to preserve lean muscle mass while promoting the breakdown of stored fat. Additionally, the adrenal glands increase the output of norepinephrine, a neurochemical that acts as a stress hormone. This increase helps raise the metabolic rate, mobilize fatty acids from fat tissue, and contributes to feelings of heightened alertness and energy often reported during a prolonged fast.