What happens to your body during a 4 day fast?: A detailed look at the metabolic journey

October 12, 2025 •

4 min read

Within the first 24 hours of fasting, your body fully depletes its stored glucose and begins a metabolic shift to use fat for fuel. This metabolic flexibility is a key aspect of what happens to your body during a 4 day fast, triggering significant physiological and cellular changes over the course of 96 hours.

Quick Summary

During a 4-day fast, the body transitions from glucose dependence to fat-derived ketones, initiating ketosis and cellular cleanup known as autophagy. Hormonal levels, including insulin and human growth hormone, are significantly altered, impacting weight, energy, and overall cellular function. Potential side effects like fatigue and headaches often subside, but proper hydration and post-fast nutrition are crucial.

Key Points

Metabolic Switch: The body moves from burning glucose to burning stored fat for energy within 12-48 hours, entering a state of ketosis.
Autophagy Activation: By day three, cellular cleanup and repair, known as autophagy, is significantly ramped up, helping to clear out damaged cellular components.
Hormonal Shifts: Fasting causes insulin levels to plummet while human growth hormone (HGH) levels increase, which helps preserve lean muscle mass.
Significant Water and Weight Loss: Initial weight loss is largely due to shedding water and depleted glycogen, with significant fat burning occurring during later stages.
Managing Side Effects: Common initial side effects like headaches, fatigue, and hunger often subside after the first couple of days, but staying hydrated is crucial.
Careful Reintroduction of Food: Breaking a 4-day fast requires a gradual and careful approach with easily digestible foods to prevent gastrointestinal distress.
Safety First: A 4-day fast is not suitable for everyone and requires medical consultation, especially for those with pre-existing conditions.

The Initial Stages: From Fed State to Ketosis

Days 1–2: Depletion and Transition

The initial phase of a 4-day fast focuses on using the body's primary energy source: glucose. After your last meal, the body enters the 'fed state,' where insulin levels rise to facilitate glucose absorption. As time passes without food, insulin levels drop, and the body begins to rely on glycogen, the stored form of glucose in the liver. For most people, these glycogen stores are fully depleted within 12–24 hours, depending on their diet and activity level.

Once glycogen is used up, the body must find an alternative fuel source. This is the critical transition point into a state of ketosis, which typically begins within the first 48 hours. The liver starts breaking down fat into ketone bodies, which are then used for energy by the brain and other tissues.

The Shift to Ketosis

Ketosis is the hallmark metabolic state of prolonged fasting. The increase in ketone bodies, particularly beta-hydroxybutyrate (BHB), has several effects:

Appetite Suppression: Ketones act as an appetite suppressant, which is why many fasters report a reduction in hunger after the initial 48 hours.
Mental Clarity: Some studies and anecdotal reports suggest that the brain can use ketones more efficiently than glucose, leading to improved cognitive function and mental clarity during later stages of the fast.
Fat Burning: The body prioritizes the use of stored fat for fuel, which is a major driver of weight loss during a prolonged fast.

Day 3: Autophagy and Hormonal Changes

By day three (around 48–72 hours), the body's processes become more pronounced. In addition to deep ketosis, one of the most significant physiological effects is the activation of autophagy.

Autophagy: Cellular Renewal

Autophagy is a Greek word meaning "self-eating," and it is the process by which the body cleans out and recycles old, damaged, and unnecessary cellular components. Think of it as a natural cellular housecleaning. The stress of nutrient deprivation forces cells to become more efficient by breaking down and reusing faulty proteins and organelles. Benefits associated with autophagy include potential anti-aging effects, reduced inflammation, and protection against neurodegenerative diseases.

Human Growth Hormone (HGH) Increase

Studies show that human growth hormone (HGH) levels can increase significantly during prolonged fasting. HGH plays a vital role in:

Metabolism and weight loss
Muscle growth and repair
Promoting cellular regeneration

This rise in HGH is part of the body's adaptive response to conserve muscle mass during extended periods without food, a common misconception about long-term fasting.

Day 4: Deep Adaptation and Sustained Effects

By the end of the 4-day period, the body is fully adapted to using ketones for fuel. The initial hunger pangs and fatigue experienced in the first days have often subsided. However, other side effects, such as headaches, fatigue, and mood changes, can persist.

At this stage, the body continues to prioritize essential functions, slowing down metabolism to conserve energy. Hormone levels, including insulin and insulin-like growth factor (IGF-1), remain low, further contributing to cellular repair and regeneration.

Potential Benefits and Risks of a 4-Day Fast


Aspect	Potential Benefits	Potential Risks
Weight Management	Significant short-term weight loss, primarily from water, glycogen, and fat. Increased fat burning due to ketosis.	Potential for muscle loss if improperly managed. Risk of overeating post-fast, which can reverse weight loss.
Metabolic Health	Improved insulin sensitivity. Reduced blood sugar and insulin levels. May lower blood pressure and triglycerides.	Dehydration due to increased water and electrolyte excretion. Can be dangerous for individuals with diabetes.
Cellular Health	Triggers autophagy, the body's cellular recycling process. May reduce inflammation. Increases human growth hormone.	Can trigger inflammatory responses in some individuals. Prolonged fasts carry a risk of malnutrition.
Mental and Mood	Some report increased mental clarity and focus after the initial days. Reports of increased vigor and reduced tension in some studies.	Common side effects include fatigue, irritability, headaches, and dizziness. Sleep disturbances are also possible.

Conclusion: Navigating a 4-Day Fast Safely

A 4-day fast can lead to a profound metabolic shift, transitioning the body into ketosis and activating powerful cellular processes like autophagy. While potential benefits like weight loss, improved insulin sensitivity, and cellular repair are significant, the process is not without risks. Proper preparation and a cautious reintroduction of food are essential to maximize benefits and minimize side effects.

Individuals should prioritize hydration with water and electrolytes throughout the fast. Breaking the fast should be done slowly with easily digestible foods to avoid digestive upset. For anyone with pre-existing health conditions like diabetes, or if you are pregnant, breastfeeding, or underweight, a prolonged fast should only be considered under the guidance of a healthcare professional. Research shows that even short-term fasting, like the 48-hour period, can have significant effects, but longer durations require careful consideration and monitoring. For further reading on the effects of prolonged fasting, see the study on a five-day water fast published in PLOS ONE.

Frequently Asked Questions

After approximately 48 hours of fasting, your body fully transitions from using glucose to burning fat for energy. This metabolic state, known as ketosis, relies on ketone bodies produced by the liver from fat stores.

Yes, a 4-day fast does induce autophagy. Research suggests that this cellular renewal process begins around 24–48 hours and becomes more amplified during extended fasting periods, helping the body to clean out damaged cells.

Yes, it is very common to experience fatigue and low energy, especially during the first couple of days of a fast, as your body is adjusting to the metabolic shift. This feeling often decreases as the body adapts to using ketones for fuel.

While a true water fast restricts intake to only water, some people opt for 'modified' fasts that include calorie-free beverages. Plain black coffee or green tea without sugar can help curb hunger, but anything with calories will technically break the fast.

Common side effects include headaches, dizziness, fatigue, irritability, and potential sleep disturbances. Staying hydrated and ensuring adequate electrolytes is important to mitigate these symptoms.

You should break a 4-day fast slowly to prevent refeeding syndrome and digestive shock. Start with small, easily digestible foods like bone broth, vegetable soup, or a simple smoothie. Gradually reintroduce other food groups over the next day or two.

No, a 4-day fast is not safe for everyone. People who are pregnant, breastfeeding, underweight, or have certain health conditions like diabetes, heart disease, or a history of eating disorders should avoid prolonged fasting unless under strict medical supervision.