What Happens If the Body Doesn't Eat? A Scientific Look at Starvation

December 7, 2025 •

4 min read

Remarkably, while humans can survive for up to 70 days without food under certain conditions, the body begins a complex and damaging process of adaptation within just hours of the last meal. These physiological changes occur as a last-ditch effort to preserve life, ultimately explaining what happens if the body doesn't eat for extended periods.

Quick Summary

The body, when deprived of food, undergoes a series of metabolic shifts, first burning glucose, then fat, and finally muscle tissue for energy. This process, known as starvation, leads to severe health complications, organ damage, and psychological distress, with survival depending on individual reserves and hydration levels.

Key Points

Initial Fasting: For the first 24 hours, the body uses stored glycogen from the liver for energy, before transitioning to other sources.
Metabolic Switch: After glycogen is depleted, the body begins burning stored fat and produces ketones for fuel, a phase that can last weeks.
Protein Wasting: In the final, most severe stage of starvation, all fat stores are exhausted, and the body consumes its own muscle tissue, including heart muscle, for survival.
Mental and Physical Effects: Lack of food causes fatigue, irritability, and cognitive impairment, while also leading to a severely weakened immune system and vital organ deterioration.
Refeeding Danger: For those who have been starved, reintroducing food must be carefully medically managed to avoid refeeding syndrome, a potentially fatal electrolyte imbalance.

The Initial Hours: Burning Stored Glucose

Within the first few hours after eating, your body operates on the glucose derived from your last meal. This readily available energy source is what fuels all immediate bodily functions, from thinking to moving. As the time since your last meal increases, and no new glucose is ingested, the body turns to its internal storage. The liver stores a small amount of glucose in the form of glycogen, which is used to keep blood sugar levels stable. This process, known as glycogenolysis, can only last for about 24 hours before these stores are depleted. This initial stage may result in minor symptoms like hunger pangs, irritability, and slight fatigue as blood sugar levels begin to drop.

The Metabolic Shift: Turning to Fat for Fuel

Once the body exhausts its glycogen reserves, it must find an alternative energy source to sustain its functions. At this point, a critical metabolic shift occurs, moving from carbohydrate-based energy to fat-based energy.

Lipolysis and Ketogenesis: The body begins breaking down triglycerides from fat cells into fatty acids and glycerol. The liver converts these fatty acids into ketone bodies, which are then released into the bloodstream. The brain and other organs can use these ketones as a primary fuel source, significantly reducing the body's dependence on glucose. This phase can last for several weeks, with the duration depending heavily on the individual's body fat percentage.
Slowing Metabolism: To conserve energy, the body's metabolic rate slows down. This adaptive response helps prolong survival but can also lead to a persistent feeling of cold, lethargy, and a reduction in non-essential bodily functions.

The Third Stage: The Dangers of Protein Wasting

After fat reserves are depleted, the body enters the most dangerous and final phase of starvation, known as protein wasting. With no other energy sources left, the body begins to break down muscle tissue, including the heart muscle, to convert its protein (amino acids) into glucose.

Rapid Muscle Loss: Muscles, the largest source of protein, are depleted first, leading to severe weakness, extreme weight loss, and muscle atrophy.
Systemic Deterioration: Essential organ functions begin to fail as the body consumes its own critical tissues. The heart shrinks, affecting blood pressure and heart rate, and the immune system becomes severely compromised, making the individual highly susceptible to infection.

Comparison of the Body's Fuel Sources During Starvation


Feature	Glycogen (Initial Phase)	Fat (Intermediate Phase)	Protein (Final Phase)
Availability	Very short-term (approx. 24 hours)	Long-term, depending on reserves	Emergency, last resort
Energy Source	Glucose	Ketone bodies from fatty acids	Glucose from amino acids
Physical Effect	Minor fatigue, hunger, irritability	Weight loss, lethargy, feeling cold	Severe weakness, muscle wasting, organ failure
Primary Organs Affected	N/A (normal function)	Metabolic rate slows to conserve energy	Heart, lungs, immune system, brain

Psychological and Cognitive Effects

Starvation doesn't just affect the body; it profoundly impacts mental and psychological health. Initial symptoms often include irritability, anxiety, and a constant preoccupation with food. As the condition worsens, cognitive functions decline, leading to brain fog, difficulty concentrating, and impaired memory. In later stages, apathy and social withdrawal are common, and in extreme cases, individuals may experience delusions, confusion, and other psychiatric symptoms. This is largely due to the brain being deprived of its optimal energy source, glucose, and adapting to less efficient fuel.

The Risk of Refeeding Syndrome

For individuals experiencing prolonged starvation, the process of reintroducing food must be managed with extreme care to prevent a potentially fatal condition called refeeding syndrome. When a malnourished person is given food too quickly, a sudden shift in fluids and electrolytes can overload the cardiovascular system, leading to cardiac arrhythmias, respiratory distress, and other complications. Medical supervision is crucial to reintroduce nutrients slowly and safely, often requiring intravenous fluids and supplements.

Conclusion

Understanding what happens if the body doesn't eat provides a stark picture of the body's incredible yet ultimately finite survival mechanisms. From the initial depletion of glycogen to the dangerous, irreversible consumption of muscle tissue, starvation represents a systematic failure of the body. The health consequences extend far beyond physical wasting, impacting cognitive function, mood, and organ health. This highlights the fundamental necessity of consistent nutrition for both immediate well-being and long-term survival, and why extreme fasting or food deprivation should never be undertaken without professional medical guidance. For more information on the health effects of prolonged starvation, you can consult sources like the Cleveland Clinic.

List of Consequences from Lack of Food

Metabolic slowdown: The body lowers its basal metabolic rate to conserve energy.
Weight loss: Initial loss of water and glycogen, followed by significant fat and muscle mass reduction.
Fatigue and weakness: Low energy stores lead to persistent tiredness and reduced physical strength.
Nutrient deficiencies: Lack of food intake results in critical shortages of vitamins and minerals.
Hair and skin issues: Deficiencies can cause hair loss, brittle nails, and dry skin.
Weakened immune system: The body's ability to fight infection is severely compromised.
Cognitive decline: Reduced glucose to the brain impairs concentration, memory, and can cause mood swings.
Cardiovascular complications: Prolonged starvation can lead to a slow heart rate, low blood pressure, and heart failure.
Organ damage: In the final stages, vital organs like the heart and kidneys begin to fail.
Refeeding syndrome: The dangerous electrolyte shift that can occur when food is reintroduced too quickly after a long period of starvation.

Frequently Asked Questions

Experts estimate that humans can survive for up to two months without food, provided they have adequate water. However, the exact duration varies greatly depending on an individual's starting body fat, health, and hydration level.

Initially, the body uses glucose from the last meal. Once that's depleted, it turns to the glycogen stored in the liver for the first 24 hours.

Ketosis is the process where the liver breaks down fatty acids into ketone bodies for energy. This typically begins after about 24-48 hours of not eating, once glycogen stores are exhausted.

The brain primarily runs on glucose. When blood sugar levels drop from lack of food, brain function is impaired, leading to symptoms like irritability, confusion, and difficulty concentrating.

Yes, prolonged starvation severely affects the cardiovascular system. It can cause a reduced heart size, slow heart rate, low blood pressure, and eventually, heart failure as the body begins to consume its own heart muscle for energy.

Refeeding syndrome is a potentially fatal condition that can occur when severely malnourished individuals are fed too quickly. It causes a sudden and dangerous shift in fluids and electrolytes, which can lead to cardiac and respiratory failure.

Yes, starvation has profound psychological effects, including anxiety, depression, apathy, and social withdrawal. The constant stress on the body and brain can lead to severe mood and behavioral changes.