What will happen to your body if you didn't eat? The science of starvation

November 15, 2025 •

3 min read

Remarkably, some individuals have survived for over 70 days with water but no food, though most survival estimates are far shorter. When you don't eat, your body initiates a systematic and predictable process of consuming its own reserves to survive, a state known as starvation.

Quick Summary

The human body systematically breaks down its own energy reserves when food is unavailable, moving through phases from consuming glycogen and fat to eventually breaking down muscle and vital organ tissue. This process leads to severe physical and mental decline and, if prolonged, can be fatal.

Key Points

Initial Energy Burn: Within 24 hours of not eating, your body depletes its readily available glycogen stores from the liver and muscles, causing a drop in blood sugar and initial fatigue.
Ketosis Phase: After glycogen is gone, the body shifts to burning fat for energy, producing ketones to fuel the brain and other tissues; the duration depends on existing fat reserves.
Muscle Wasting: Once fat reserves are exhausted, the body begins breaking down muscle and organ tissue for protein, leading to severe weakness and organ damage.
Immune System Failure: Starvation severely weakens the immune system, making the body extremely vulnerable to infections, which are a major cause of death.
Psychological Effects: Mental and emotional distress, including irritability, apathy, and cognitive decline, are common as the brain is deprived of proper fuel.
Refeeding Syndrome Risk: Reintroducing food too quickly after prolonged starvation can cause a fatal metabolic shift known as refeeding syndrome.
Long-term Consequences: Survivors of prolonged malnutrition can face lifelong health issues, including permanent organ and brain damage.

The Body's Survival Mechanism: A Three-Phase Process

When faced with a lack of food, the human body is designed with a powerful, albeit ultimately self-destructive, survival mechanism. It enters a state of starvation, initiating a multi-stage process to sustain life for as long as possible by consuming its own tissues. This physiological progression is a remarkable testament to the body's resilience but also highlights the severe and irreversible damage that prolonged starvation can cause. Understanding these phases is crucial to grasp the full extent of what will happen to your body if you didn't eat.

Phase One: Glycogen and Glucose Depletion (First 24 hours)

In the initial hours after your last meal, your body relies on its most readily available energy source: glucose. This glucose is obtained from the breakdown of carbohydrates and is stored in the liver and muscles as glycogen. This reserve is quickly consumed, typically lasting less than 24 hours. Once the liver's glycogen stores are depleted, your blood sugar levels will drop, which can lead to initial symptoms such as headaches, weakness, dizziness, and irritability as your brain's primary fuel source diminishes. Your body will also begin to release stored glucose by breaking down fats and proteins to maintain blood sugar, but these sources are not yet the primary fuel.

Phase Two: Ketosis and Fat Burning (Days to Weeks)

After the first day, the body's metabolism makes a significant shift to survive. It enters a state of ketosis, where it begins breaking down stored fat reserves to produce ketone bodies. These ketones can be used as an alternative fuel source by the brain and muscles, significantly reducing the brain's dependence on glucose and helping to conserve the body's remaining protein. The duration of this phase depends heavily on the individual's body fat percentage; the more fat reserves, the longer this phase lasts. During this time, weight loss is rapid, particularly in the first week due to water and carbohydrate loss, and later slows down. Symptoms may include persistent fatigue, low blood pressure, and a lowered heart rate as the body conserves energy.

Phase Three: Protein Breakdown and Organ Failure (Weeks to months)

This is the final, and most dangerous, stage of starvation. Once the body's fat stores are almost completely exhausted, it turns to its last remaining resource: protein. This means the body begins to break down muscle tissue, including that of vital organs, for energy. This causes extreme muscle wasting and can lead to severe organ damage, including the heart. The immune system becomes severely compromised, making the individual highly susceptible to infections, which are a common cause of death during this stage. Other symptoms include severe edema (swelling), neurological damage, hair loss, and a complete breakdown of cellular function. Death often results from cardiac arrhythmia, organ failure, or opportunistic infection.

Comparison of Energy Source Utilization During Starvation


Feature	Phase One (First ~24 hrs)	Phase Two (Days to Weeks)	Phase Three (Final Stage)
Primary Energy Source	Glycogen and some fat/protein	Stored Fat (Ketone bodies)	Protein (muscle and organs)
Metabolic State	Normal, shifting to ketosis	Ketosis	Severe protein wasting
Mental State	Irritability, confusion, fatigue	Apathy, cognitive decline	Extreme psychological distress
Immune System	Initially unaffected	Weakened due to nutrient deficiency	Severely compromised; high infection risk
Physical Effects	Weakness, dizziness, headaches	Rapid weight loss, low blood pressure	Muscle wasting, organ failure, edema

Refeeding Syndrome: A Hidden Danger

For those who survive prolonged starvation, the reintroduction of food can be life-threatening. A sudden increase in carbohydrates and nutrients can cause a drastic shift in electrolytes, leading to refeeding syndrome. This can result in heart failure, neurological problems, respiratory issues, and other severe complications. Medical intervention is crucial to reintroduce nutrients slowly and safely.

Conclusion

Understanding what will happen to your body if you didn't eat reveals a complex and dangerous physiological journey. While the body has remarkable adaptive mechanisms to survive, prolonged starvation will inevitably lead to the consumption of vital tissues and, ultimately, death. The process is characterized by distinct phases of metabolic shifts, from burning glucose and fat to breaking down protein, each with its own set of severe physical and mental consequences. It is a brutal and final testament to the body's dependence on regular and adequate nutrition for long-term health and survival.

Visit the National Centre for Eating Disorders for support and information on the effects of under-eating.

Frequently Asked Questions

While individual factors vary, estimates suggest a person with adequate hydration could survive without food for anywhere from 8 to 21 days, and potentially longer depending on their initial body fat stores.

Ketosis is a metabolic state where the body breaks down stored fat into molecules called ketone bodies for energy. It is a survival mechanism that begins after a day or so of not eating, providing an alternative fuel source when glucose is scarce.

Yes, starvation has a profound effect on the brain. The brain is deprived of its primary fuel (glucose), leading to cognitive decline, irritability, apathy, and difficulty concentrating.

Refeeding syndrome is a potentially fatal shift in electrolytes that can occur when a severely malnourished person is reintroduced to food too quickly. The rapid metabolic changes can overwhelm the body, leading to heart failure and other complications.

Severe protein deficiency can lead to a condition called kwashiorkor, which causes fluid retention and a swollen, bloated abdomen. This occurs in the later, more fatal stages of starvation.

Yes, once the body's fat reserves are depleted, it begins to break down muscle tissue for energy in the final stage of starvation. This leads to significant and rapid muscle loss.

Early symptoms often include headaches, fatigue, weakness, dizziness, and irritability. Your body will also begin to slow its metabolism to conserve energy.