What happens to the body when it's starving?: The Physiological Journey

October 13, 2025 •

4 min read

During prolonged starvation, the human body can survive for weeks, but the process is a brutal journey of metabolic shifts. What happens to the body when it's starving? It systematically deconstructs itself for fuel, leading to a cascade of severe physical and psychological consequences.

Quick Summary

Starvation forces the body to exhaust its stored glucose and fat before it begins consuming its own muscle and organs for energy. This metabolic process causes severe physical and mental deterioration, organ damage, and can lead to refeeding syndrome during recovery.

Key Points

Initial Energy Source: The body first burns stored glycogen for energy, which is depleted in about 24 hours.
Fat as Fuel: After glycogen is gone, the body shifts to burning fat, producing ketones to fuel the brain and conserve glucose.
Muscle Wasting: Once fat reserves are exhausted, the body breaks down its own muscle and organ protein for energy, a critical and dangerous stage.
Organ Damage: Prolonged starvation leads to the atrophy and damage of vital organs, including the heart, which can cause fatal cardiac issues.
Refeeding Syndrome: The reintroduction of food after prolonged starvation can cause refeeding syndrome, a potentially fatal electrolyte imbalance.
Mental Deterioration: Psychological effects like irritability, anxiety, and apathy are profound, alongside impaired concentration and memory.
Immune System Collapse: Starvation weakens the immune system, increasing vulnerability to infections, which are a common cause of death.

The Body's Survival Mechanism in Crisis

When caloric energy intake drops below the level needed to maintain life, the body initiates a complex, multi-stage survival protocol. It is an orchestrated, albeit destructive, metabolic adaptation designed to preserve vital organ function for as long as possible. The effects of starvation are systemic, impacting nearly every biological process, from mental clarity to immune defense. The transition through these phases demonstrates the body's remarkable yet ultimately fragile resilience.

Phase 1: Glycogen Depletion (First 24 Hours)

In the initial hours without food, the body first turns to its most accessible energy source: glucose. After a meal, blood glucose levels are typically regulated by insulin, which prompts cells to absorb glucose for immediate use or storage as glycogen in the liver and muscles. When food intake ceases, insulin levels drop, and the pancreas releases glucagon. This hormone signals the liver to break down its glycogen stores, releasing glucose back into the bloodstream to maintain stable blood sugar levels. This energy reserve is relatively small and is typically depleted within 24 hours, depending on the individual's activity level and metabolism.

Phase 2: Fat-Burning and Ketosis (Days 2 to 7)

Once the glycogen stores are exhausted, the body transitions to its second energy source: fat. Adipose tissue, or body fat, is broken down through a process called lipolysis, which releases fatty acids and glycerol. Most body tissues can use fatty acids for energy, sparing any remaining glucose for the brain and red blood cells. The liver also converts some of the fatty acids into ketone bodies through ketogenesis. After about three days, the brain adapts to using these ketones as a primary fuel source, which significantly reduces its glucose requirements. This metabolic shift is a crucial survival tactic, as it conserves the body's limited protein stores.

Phase 3: Protein Breakdown and Organ Failure (Beyond One Week)

This is the most critical and damaging phase of starvation, occurring when the body's fat reserves are exhausted. With no fat left to convert to ketones, the body must turn to its structural and functional proteins for energy. This involves the rapid breakdown of muscle tissue, a process known as gluconeogenesis, where amino acids from muscle are converted into glucose by the liver. As muscle mass rapidly wastes away, the physical symptoms become severe, including extreme weakness and fatigue. Critically, this includes the breakdown of heart muscle, leading to a weakened cardiovascular system, arrhythmia, and eventual cardiac arrest. The immune system also collapses, leaving the body highly vulnerable to infections, which are a frequent cause of death in prolonged starvation.

Systemic Impacts of Starvation

Starvation's effects extend far beyond metabolic changes, causing profound damage to all of the body's major systems:

Cardiovascular System: The heart muscle weakens and shrinks, leading to a slow heart rate and dangerously low blood pressure. The risk of cardiac arrhythmias and heart failure increases dramatically.
Digestive System: The intestinal lining and muscles weaken, impairing nutrient absorption and causing issues like severe constipation or diarrhea. The lack of proper nutrition can also trigger pancreatitis.
Immune System: The immune system becomes severely compromised due to a lack of protein, vitamins, and minerals, making the body unable to fight off infections. Minor illnesses can become fatal.
Endocrine System: Hormone production, including thyroid hormones and reproductive hormones like estrogen and testosterone, slows down. This can result in amenorrhea in women, low libido, reduced metabolism, and a constant feeling of cold.
Neurological Function: While the brain has adapted to use ketones, cognitive abilities still decline. Symptoms include irritability, anxiety, apathy, brain fog, and difficulty concentrating. Severe cases can cause hallucinations or seizures due to electrolyte imbalances.

Comparison of Starvation Phases


Feature	Phase 1 (First 24 hrs)	Phase 2 (Days to Weeks)	Phase 3 (Prolonged)
Primary Fuel Source	Stored glycogen	Stored fat (ketones)	Muscle and organ protein
Metabolic State	Shift from glucose to fat use	Adaptive hypometabolic state	Increased catabolism; systemic breakdown
Brain Function	Maintains glucose supply	Adapts to using ketones as fuel	Impaired function due to lack of energy
Physical Symptoms	Hunger, headaches, fatigue	Weakness, dizziness, fatigue	Severe wasting, organ damage, weakness
Weight Loss	Primarily water and glycogen	Slows down as metabolism drops	Rapid, dangerous loss of muscle mass

The Hazards of Recovery: Refeeding Syndrome

Paradoxically, the process of reintroducing nutrition after a period of starvation can be life-threatening if not managed carefully. A sudden influx of carbohydrates causes a surge in insulin, which prompts cells to rapidly absorb glucose and vital electrolytes like phosphate, potassium, and magnesium. For a body that has adapted to extremely low electrolyte levels, this rapid shift can overwhelm the heart and other organ systems, leading to cardiac arrhythmias, respiratory distress, and even death. Medical supervision and a slow, controlled refeeding process are essential for recovery.

Conclusion: The Ultimate Price of Starvation

The human body, when deprived of food, enters a desperate fight for survival, systematically shutting down non-essential functions and cannibalizing its own tissues for energy. This grim journey progresses from burning accessible glycogen to converting fat into ketones, and finally, to the catastrophic breakdown of muscle and organ protein. The physical and psychological toll is immense and often leaves lasting damage, even with successful recovery. The vulnerability to infection, severe cardiovascular distress, and the risk of refeeding syndrome underscore that starvation is not merely hunger—it is a brutal and complex process that pushes the body to its absolute limits.

For more insight into the physiological mechanisms at play, consider exploring publications from institutions like the National Institutes of Health.

Frequently Asked Questions

Within the first 24 hours of not eating, your body uses up its stored glycogen, a form of carbohydrate stored primarily in the liver, to provide glucose for energy.

Ketosis is the metabolic state where the body burns fat for fuel instead of glucose. It begins after about 24 to 48 hours of starvation, once glycogen reserves are depleted, and the liver starts converting fatty acids into ketone bodies.

Muscle wasting occurs during the late stages of starvation, after the body's fat stores have been depleted. The body then breaks down its own protein, which makes up muscle tissue, to convert it into glucose for energy.

Prolonged starvation can cause permanent organ damage, including a weakened heart and compromised kidney function. It can also lead to long-term effects like poor bone health, digestive problems, and mental health issues such as depression and anxiety.

Starvation severely impacts the heart by causing the heart muscle to weaken and shrink. This can lead to a dangerously low heart rate, low blood pressure, and potentially fatal cardiac arrhythmias.

Refeeding syndrome is a potentially fatal condition that can occur when someone who has been malnourished reintroduces food. The sudden shift in metabolism causes rapid and dangerous drops in electrolyte levels, which can lead to cardiac failure and respiratory distress.

No. While people who are underweight may have less fat reserve, starvation can cause negative physiological and psychological effects in any individual whose caloric intake is severely restricted, regardless of their starting weight.