What happens to the body if someone doesn't eat?

December 13, 2025 •

3 min read

Reports suggest humans can survive for weeks without food, but the process involves profound and systematic changes. The body, prioritizing survival above all else, begins a predictable progression of consuming its own resources. What happens to the body if someone doesn't eat is a story of metabolic adaptation under extreme duress.

Quick Summary

The body, when deprived of food, transitions through several metabolic phases. It first depletes its carbohydrate stores, then relies on fat reserves for energy production. When fat is exhausted, the body breaks down muscle tissue, leading to severe health complications and, eventually, systemic organ failure.

Key Points

Initial Glycogen Depletion: Within the first 24 hours, the body burns through its stored carbohydrates (glycogen) for energy, which can cause early fatigue and irritability.
Ketosis for Extended Fuel: After glycogen is gone, the body enters ketosis, a state where it converts stored fat into ketones for energy, potentially lasting weeks depending on reserves.
Muscle Breakdown as a Last Resort: When fat stores are depleted, the body begins breaking down muscle tissue and vital organs for protein, leading to severe wasting and damage.
Cognitive and Psychological Decline: Starvation significantly impacts brain function, causing mood swings, apathy, poor concentration, and obsessive food thoughts.
Systemic Organ Failure: Prolonged starvation leads to immune system collapse, electrolyte imbalances, heart damage, and multi-organ failure, with death often resulting from cardiac arrest.

The human body is an incredibly resilient machine, capable of withstanding periods of caloric deprivation far longer than many realize. However, the process is not without severe consequences. When an individual stops eating, the body enters a survival mode, systematically breaking down its own tissues to fuel vital functions. This process, known as starvation, unfolds in predictable metabolic stages, each with escalating risks.

The Initial Response: Days 1-3

Within the first 24 hours of fasting, the body's primary source of energy is its readily available glucose, which is primarily stored in the liver and muscles in the form of glycogen. This initial reserve provides a quick burst of fuel to keep the brain and other tissues functioning. Once the glycogen is depleted, a process that can occur within 18 to 24 hours, the body must find an alternative energy source.

During this early stage, physical and psychological symptoms may begin. These can include:

Fatigue and low energy
Irritability and mood changes
Headaches due to low blood sugar
Difficulty concentrating
Intense hunger pangs

Shifting to Fat: The Ketosis Phase

After exhausting its carbohydrate reserves, the body enters a state of ketosis, typically lasting for several weeks. This is a crucial metabolic shift designed to prolong survival. The liver begins converting fatty acids from stored body fat into ketone bodies, which can be used as fuel by the brain and other tissues. The efficiency of this stage depends heavily on the individual's existing fat reserves.

Psychological and Physical Changes During Ketosis

Psychological Effects: With ketone bodies fueling the brain, initial mood swings may give way to a state of emotional numbness or apathy as the body conserves energy for essential tasks. The relentless preoccupation with food, documented in historical studies of starvation, can consume a person's thoughts.
Metabolic Slowdown: The body lowers its basal metabolic rate to reduce energy expenditure, conserving its limited resources. This results in a drop in body temperature and heart rate.
Initial Rapid Weight Loss: The initial weight loss is largely water weight and electrolytes, not just fat, which can lead to complications if not properly managed.

The Final Stage: Protein and Organ Breakdown

When the body's fat stores are nearly exhausted, the system transitions to its last resort for energy: protein. This is the most dangerous stage of starvation. The body begins breaking down muscle tissue, including the heart muscle, and other vital organ tissues. This leads to severe wasting and organ dysfunction.

The Devastating Effects of Late-Stage Starvation

Severe Wasting: Visible muscle loss occurs, and the body becomes extremely weak.
Immune System Collapse: The lack of nutrients severely compromises the immune system, making the body highly vulnerable to infections.
Organ Failure: As essential protein is consumed, organs like the heart, liver, and kidneys begin to fail. Cardiac arrhythmia and heart attack are common causes of death.
Electrolyte Imbalances: Critical minerals are lost, disrupting heart and nerve function, which can be fatal.

Comparing Short-Term vs. Long-Term Starvation Effects


Feature	Short-Term Starvation (1-3 days)	Long-Term Starvation (Weeks+)
Energy Source	Primarily glycogen from liver	Fat reserves, followed by muscle tissue
Metabolic State	Shift from glucose to gluconeogenesis	Ketosis, followed by irreversible protein breakdown
Symptoms	Hunger, irritability, fatigue, headache	Apathy, cognitive decline, severe weakness, muscle wasting
Risks	Dehydration, mild electrolyte imbalance	Immune system failure, organ damage, death
Psychological Impact	Heightened anxiety and mood changes	Social withdrawal, depression, emotional numbness
Weight Loss Composition	Primarily water and electrolytes	Slowed but continuous weight loss, severe muscle loss

Conclusion

While the human body possesses remarkable survival mechanisms, the consequences of not eating for prolonged periods are catastrophic. The body's shift from consuming stored carbohydrates to fat, and ultimately to its own vital proteins, is a desperate, life-sustaining effort that ultimately leads to irreversible damage and death. Understanding this physiological response is crucial for appreciating the severity of malnutrition and the importance of regular, nutrient-rich food intake for overall health and survival.

For a deeper dive into the metabolic processes of fasting and starvation, an excellent resource is the NCBI Bookshelf's section on Physiology, Fasting.

Frequently Asked Questions

In the first 24 hours, the body primarily uses stored glucose from the liver and muscles (glycogen) for energy. As these stores are depleted, you may experience fatigue, headaches, and increased hunger.

While the exact time varies based on body fat and health, an average adult with water can survive for weeks, potentially 2 to 3 months. Without water, survival is limited to about a week.

The body will use stored fat for energy after its quick-access glycogen stores are depleted. It will only begin breaking down muscle tissue for protein when fat reserves are significantly reduced.

'Starvation mode' refers to the body's adaptive response to prolonged caloric deprivation, where it slows down metabolism to conserve energy. This is a severe state that follows the initial fasting phase.

Insufficient nutrients can lead to 'brain fog', irritability, anxiety, and depression. In severe cases, it can cause apathy, confusion, and impaired cognitive function.

Long-term risks include severe muscle wasting, weakened immune function, osteoporosis, electrolyte imbalances, organ failure, and a higher risk of heart disease.

Refeeding syndrome is a potentially fatal condition that can occur when a severely malnourished person is reintroduced to food too quickly. It involves dangerous fluid and electrolyte shifts that can cause heart and neurological problems.