What Happens if a Person Does Not Eat for 2 Days?

December 6, 2025 •

3 min read

According to scientific research, after about 12-24 hours without food, the body depletes its glucose stores and begins transitioning to a new metabolic state. This transition is the beginning of what happens if a person does not eat for 2 days, triggering a cascade of physiological changes.

Quick Summary

A 48-hour period without food triggers significant metabolic changes as the body depletes glycogen stores and shifts to burning fat for energy. This process, known as ketosis, can lead to side effects like headaches and fatigue but also offers potential benefits like improved insulin sensitivity and cellular repair. It is crucial to stay hydrated and reintroduce food slowly afterward.

Key Points

Metabolic Shift: After about 24 hours, the body depletes its glucose (glycogen) reserves and switches to burning stored fat for energy, entering a state called ketosis.
Ketosis Benefits: This fat-burning state can lead to benefits like improved insulin sensitivity, reduced inflammation, and increased cellular repair (autophagy).
Common Side Effects: During the 48-hour period, you may experience headaches, fatigue, irritability, and dizziness as your body adapts to the change in fuel source.
Stay Hydrated: Drinking plenty of water and replenishing electrolytes is crucial to avoid dehydration and minimize common side effects like headaches.
Properly Reintroduce Food: Breaking the fast should be done slowly with small, easily digestible meals like soups or broths to prevent digestive discomfort.
Consult a Doctor: Extended fasting is not suitable for everyone, especially those with pre-existing conditions like diabetes, and should be discussed with a healthcare provider beforehand.

The Body's Initial Response: The First 24 Hours

When you stop eating, your body doesn't immediately enter a state of distress. It's built with sophisticated survival mechanisms that prioritize energy needs. For the first 8 to 12 hours after your last meal, your body is still actively processing and absorbing nutrients. Following this, the body enters the post-absorptive phase where insulin decreases and glucagon increases. This triggers the liver to break down stored carbohydrates (glycogen) into glucose to maintain blood sugar. The brain, relying on glucose, is prioritized for this fuel.

The Shift to Fat-Burning: Day Two Begins

Around the 24-hour mark, liver glycogen is largely depleted. To conserve muscle, the body shifts its primary fuel source to stored fat. This involves lipolysis, breaking down fat into fatty acids and glycerol. The liver then converts these fatty acids into ketone bodies, which are released into the bloodstream. The brain can use ketones as an alternative fuel, a state called ketosis.

Potential Side Effects and Discomfort

The transition during a 48-hour fast can bring side effects, often called 'keto flu', as the body adapts to burning fat.

Common short-term side effects may include:

Headaches: Often due to low blood sugar or caffeine withdrawal.
Fatigue and Weakness: Resulting from the body using less readily available energy stores.
Irritability and Mood Changes: Low blood sugar can impact mood and cognitive function.
Nausea: Some may experience digestive discomfort.
Hunger Pangs: Cravings can be strong initially.
Dizziness: Often a sign of dehydration or low blood pressure.

Potential Health Benefits of a 48-Hour Fast

A 48-hour fast is linked to several potential health benefits, especially under medical supervision.

Benefits associated with 48-hour fasting include:

Promotes Autophagy: Around 48 hours, autophagy, a cellular cleanup process, significantly increases.
Improves Insulin Sensitivity: Fasting can enhance the body's response to insulin, aiding blood sugar regulation.
Reduces Inflammation: Short-term fasting has shown to decrease inflammatory markers.
Enhances Mental Clarity: As the brain uses ketones, some report improved focus.
Aids Weight Loss: Burning fat for energy can contribute to weight loss, though initial loss is often water.
Boosts Gut Health: Giving the digestive system rest may support gut health.

Risks and Safety Precautions

A 48-hour fast isn't suitable for everyone. Certain individuals should avoid it or do so under medical supervision.

Comparison of Normal vs. 48-Hour Fasting States


Feature	Normal Fed State	48-Hour Fasting State
Primary Fuel Source	Glucose from food	Ketones from fat stores
Metabolic Hormone Activity	High insulin, low glucagon	Low insulin, high glucagon
Energy Reserves Used	Recent meal, then liver glycogen	Stored fat (adipose tissue)
Cellular Process	Growth and repair	Autophagy (cellular cleanup)
Blood Sugar Levels	Stable (regulated by insulin)	Drops significantly
Common Sensation	Satiated, energetic	Hunger, fatigue, potential headaches

Breaking the Fast Safely

Reintroducing food after 48 hours requires a gentle approach. Overloading the digestive system can cause discomfort. Start with small, easily digestible foods like broth-based soup or steamed vegetables. Replenishing electrolytes is also important.

Conclusion

Skipping meals for two days leads to a metabolic shift, using stored fat (ketosis) instead of glucose for fuel. While temporary side effects like fatigue and headaches can occur, benefits include improved insulin sensitivity, reduced inflammation, and enhanced cellular repair (autophagy). However, it's not suitable for everyone and requires hydration and careful reintroduction of food. Individuals with pre-existing conditions, including diabetes or a history of eating disorders, should consult a healthcare professional before a prolonged fast. Understanding the body's response and reintroducing nourishment safely is crucial. For more on fasting physiology, visit Physiology, Fasting - StatPearls - NCBI Bookshelf.

Frequently Asked Questions

For most healthy adults, a 48-hour fast is not inherently dangerous but can cause temporary side effects. However, it is not recommended for individuals with certain medical conditions, pregnant or breastfeeding women, or those with a history of eating disorders.

After about 24 hours, your body's stored glucose (glycogen) is depleted, and it begins to primarily use stored fat for energy. The liver converts these fats into ketone bodies in a process called ketosis.

Common side effects include headaches, fatigue, irritability, nausea, dizziness, and hunger pangs. These often diminish as your body becomes accustomed to using fat for energy.

You should break a 48-hour fast gently to avoid overwhelming your digestive system. Start with small, simple foods like broth, steamed vegetables, or a small portion of protein. Avoid large or heavy meals initially.

While you will likely lose weight, a significant portion of this is water weight due to glycogen depletion and fluid loss. Sustainable fat loss depends on your long-term dietary habits.

Autophagy is a cellular 'self-cleaning' process where the body recycles damaged or dysfunctional cell components. Research suggests that autophagy levels are significantly increased during a fast of around 48 hours.

It is crucial to stay hydrated with calorie-free fluids during a 48-hour fast. Water is essential, and some people include black coffee, plain tea, or electrolyte supplements to prevent dehydration and mineral imbalances.