What Happens to Your Organs When You're Starving?

January 11, 2026 •

4 min read

According to the World Health Organization, hunger is the single gravest threat to global public health, highlighting the severe consequences of extreme malnutrition. When you're starving, your body initiates a series of metabolic changes, shifting from using external fuel to consuming its own internal energy stores to survive.

Quick Summary

The human body undergoes distinct metabolic phases in response to starvation, initially depleting glycogen before transitioning to fat and protein for energy. This physiological adaptation affects vital organ function, leading to significant health deterioration and, in severe cases, life-threatening complications, as the body struggles to maintain energy homeostasis. Critical organs like the heart, liver, and kidneys are particularly vulnerable.

Key Points

Stages of Starvation: The body burns fuel in a specific order: first glycogen, then fat, and finally muscle protein to sustain critical functions.
Brain Adaptation: The brain adapts to use ketone bodies derived from fat as an alternative energy source, reducing its dependence on glucose.
Muscle Wasting: In prolonged starvation, the body catabolizes its own muscle tissue for energy, leading to extreme weakness and deterioration.
Heart Damage: The heart, being a muscle, shrinks and weakens during starvation, increasing the risk of arrhythmias and heart failure.
Immune System Collapse: Malnutrition severely compromises the immune system, making a starving individual highly vulnerable to fatal infections.
Metabolic Slowdown: The body's basal metabolic rate significantly decreases to conserve energy and prolong survival.

The Body's Metabolic Adaptation to Starvation

When deprived of food, the human body enters a survival mode, triggering a cascade of metabolic and physiological adaptations. This process is not a sudden shutdown but a staged response designed to prolong life by conserving energy. The body's energy sourcing shifts dramatically, prioritizing fuel for the most critical organs, such as the brain, while catabolizing less-essential tissues.

Stage 1: Glycogen Depletion (First 24-48 Hours)

In the initial hours of starvation, your body's primary energy source is glucose, derived from the digestion of carbohydrates. Once this dietary glucose is used up, the body turns to its glycogen reserves, stored predominantly in the liver and muscles. The liver's glycogen is broken down into glucose and released into the bloodstream to maintain stable blood sugar levels, especially for the brain. This reserve is relatively small and is typically depleted within 24 to 48 hours.

Stage 2: The Shift to Fat Metabolism (After 2-3 Days)

Once glycogen is exhausted, the body enters a state of ketosis, a metabolic phase where it begins breaking down fat reserves (adipose tissue) for energy. The liver converts fatty acids from fat cells into ketone bodies, which can be used as an alternative fuel source by many tissues, including the brain. This shift is a crucial survival mechanism that spares protein and conserves muscle mass in the short term. However, this process can lead to ketosis side effects, such as 'keto breath' from acetone excretion.

Stage 3: Protein Catabolism (Extended Starvation)

After weeks of sustained starvation, fat reserves are severely depleted. At this critical juncture, the body has no choice but to break down proteins from non-essential tissues to produce glucose through a process called gluconeogenesis. This means that muscle tissue, which is the body's largest protein store, is cannibalized. This rapid muscle wasting results in severe weakness and deterioration of body function.

Specific Organ Responses During Starvation

Different organs are impacted at varying stages of starvation, with some being protected longer than others.

Heart: As the body degrades muscle tissue, the heart, which is a muscular organ, begins to shrink and weaken. This results in a decreased heart rate, low blood pressure, and an increased risk of arrhythmia. In the final stages, severe tissue degradation and electrolyte imbalances can lead to heart failure, a common cause of death.
Brain: The brain is the body's most critical organ and is prioritized for fuel. Initially, it uses glucose from glycogen and later adapts to use ketone bodies for up to 75% of its energy needs. However, cognitive function is still impaired, leading to irritability, difficulty concentrating, and apathy.
Liver: The liver plays a central role in all stages of starvation, managing the conversion of glycogen to glucose and fatty acids to ketones. Prolonged nutrient deprivation can lead to acute liver injury, impairing its ability to detoxify the body and regulate nutrients.
Kidneys: Kidney function becomes impaired as starvation progresses. The kidneys play a role in maintaining fluid and electrolyte balance, which can become severely disrupted.
Immune System: Starvation severely compromises the immune system due to a lack of vital nutrients like vitamins and minerals. This leaves the body highly susceptible to opportunistic infections, such as pneumonia, which can be a primary cause of death.
Gastrointestinal Tract: The digestive system can be damaged by chronic nutrient deprivation. The muscular walls of the intestines can atrophy, hindering digestion and absorption. This can lead to severe diarrhea or constipation, further exacerbating malnutrition.

Comparison of Organ Function: Fed vs. Starving


Organ/System	Normal (Fed State)	Starvation (Advanced Stage)
Heart	Strong, consistent pumping; normal heart rate and blood pressure.	Reduced size and strength; slow heart rate; low blood pressure; high risk of arrhythmias.
Liver	Stores glycogen; produces glucose; metabolizes fats; detoxifies.	Depletes glycogen; produces ketones from fat; eventually impaired detoxification.
Brain	Uses glucose as primary fuel source; high cognitive function.	Adapts to use ketones; impaired cognitive function; apathy; irritability.
Skeletal Muscle	Uses glucose and fatty acids for energy; maintains mass.	Protein is catabolized for energy (gluconeogenesis); severe muscle wasting and weakness.
Immune System	Robust function; fights off infection effectively.	Severely compromised; highly susceptible to infection; poor wound healing.

Conclusion: The Final Stages of Organ Failure

The process of starvation pushes the body to its absolute limits, eventually leading to systemic organ failure. After fat reserves are exhausted, the body's consumption of its own vital proteins, including the heart muscle, leads to catastrophic system collapse. Death in starvation is often not from the lack of food itself, but from secondary infections that overwhelm the body's collapsed immune system or from cardiac arrest caused by severe tissue degradation and electrolyte imbalances. The human body's resilience is remarkable, but the long-term effects of malnutrition are devastating and can cause irreversible damage even with subsequent re-feeding. Organizations like Action Against Hunger provide crucial support in combating the deadly effects of malnutrition.

The Risks of Re-feeding

It's important to note the severe danger of reintroducing food too quickly after a prolonged period of starvation. This can lead to refeeding syndrome, a potentially fatal shift in fluids and electrolytes that can cause cardiac arrhythmia and organ dysfunction. Medical supervision is essential to manage a slow, controlled reintroduction of nutrients to prevent this complication.

Frequently Asked Questions

The body first uses glucose from the last meal. Once that is gone, it depletes its small stores of glycogen from the liver and muscles, which typically lasts for about 24 to 48 hours.

The body progresses through distinct metabolic phases: initially using glycogen, then switching to stored fat for ketones, and finally breaking down protein from muscle and other tissues for energy.

When the body's fat reserves are exhausted, it resorts to breaking down muscle tissue, which is its largest protein store, to convert the amino acids into glucose for the brain and other essential functions.

Yes, as starvation progresses and the body begins to break down protein, the heart muscle can shrink and weaken. This can lead to a dangerously slow heart rate, low blood pressure, and potentially fatal cardiac arrhythmia.

Refeeding syndrome is a potentially fatal condition that occurs when a severely malnourished person is fed too quickly. It causes dangerous shifts in fluids and electrolytes that can lead to heart failure or organ dysfunction.

Yes. While the brain can adapt to use ketones, cognitive functions are still impaired. Psychological effects like apathy, irritability, and depression are common, along with difficulty concentrating.

Often, death is caused by secondary infections or cardiac arrest rather than the lack of food directly. The weakened immune system and severe tissue degradation, particularly of the heart, are major contributors to fatality.