A 36-hour fast, often called a “fasting day and a half,” moves the body past the initial stages of food deprivation. This duration triggers a profound metabolic shift, as the body fully transitions from relying on easily accessible sugar reserves to a deeper, adaptive state. This extended period without caloric intake sets the stage for significant changes in cellular function and hormonal signaling, reflecting the body’s ability to maintain energy during resource scarcity.
Fueling the Body: Full Transition to Ketosis
By 36 hours, the body’s primary fuel source has fundamentally changed, marking a complete departure from a glucose-dependent state. The liver’s glycogen stores, the body’s short-term sugar reserve, are typically exhausted around the 24-hour mark. This depletion signals the body to begin relying heavily on stored fat for energy production.
This shift involves lipolysis, where stored triglycerides are broken down into free fatty acids (FFAs). These FFAs travel to the liver and are converted into ketone bodies, primarily beta-hydroxybutyrate (BHB), through hepatic ketogenesis. BHB now becomes the brain’s main alternative fuel source.
This metabolic state, known as ketosis, is distinct from the mixed fuel state dominating the initial 12 to 24 hours of fasting. The reliance on ketones is an efficient energy strategy that helps preserve muscle protein. At 36 hours, ketone levels are measurably elevated and serve as the dominant circulating fuel, supporting sustained fat oxidation and metabolic function.
Activating Cellular Renewal: Autophagy
The 36-hour mark is often cited as a threshold where autophagy, the cellular cleanup process, becomes substantially activated. Autophagy, which means “self-eating,” is the body’s mechanism for clearing out damaged or dysfunctional cellular components. It involves recycling old proteins, organelles, and waste products to generate new building blocks and promote cellular efficiency.
The dramatic drop in insulin and lower levels of the growth factor IGF-1 act as powerful signals to initiate this deep cellular maintenance. When nutrients are scarce, cells enter a survival and recycling mode, which is the core function of autophagy. This process is thought to contribute to cellular health and may play a role in improved longevity.
The sustained nutrient deprivation at 36 hours drives a robust and widespread activation of autophagy across various tissues. This process optimizes cellular function by removing inefficient components, allowing the body to conserve energy and resources.
Endocrine Response and Growth Hormone
The extended fasting period generates a hormonal response that governs the body’s metabolic adaptations. Sustained low levels of insulin and glucose remove inhibitory signals on the pituitary gland, leading to a marked increase in Human Growth Hormone (HGH) secretion. Fasting can elevate HGH levels multifold, with the peak effect often occurring within the 36-to-48-hour window.
This surge in HGH is a protective measure designed to preserve muscle mass by encouraging the use of fat for fuel. HGH works synergistically with other counter-regulatory hormones to mobilize fatty acids from fat stores. Additionally, the sympathetic nervous system becomes more active, leading to increased levels of noradrenaline (norepinephrine).
Noradrenaline stimulates the breakdown of triglycerides in adipose tissue, directly supporting the lipolysis needed to produce ketones. The combined effect of low insulin, high HGH, and elevated noradrenaline ensures the body’s energy needs are met primarily through fat oxidation. This hormonal environment also enhances insulin sensitivity, an adaptation that persists after the fast is broken.
Subjective Experience and Safe Refeeding
The subjective experience of a 36-hour fast differs from shorter fasts. Initial hunger pangs, typically driven by ghrelin, often subside as the body enters ketosis and ketone bodies circulate. Many individuals report a sense of mental clarity or focused energy at this stage, likely due to the brain utilizing ketones as fuel.
Conversely, some people may experience mild fatigue, headache, or dizziness, often traced to dehydration or electrolyte imbalance. It is important to maintain adequate intake of water and mineral salts, such as sodium, potassium, and magnesium, throughout the fast. These symptoms are signs of metabolic adjustment, but they warrant attention to hydration status.
When concluding the fast, a careful approach to reintroducing food is advisable to prevent digestive discomfort. Start with small portions of easily digestible foods, such as bone broth, fermented foods, or steamed non-starchy vegetables, to gently reactivate the digestive system. Breaking the fast too quickly with a large, heavy meal, especially one high in refined carbohydrates, can shock the system and lead to bloating or digestive distress.