What happens in the process of starvation?

November 29, 2025 •

4 min read

According to the World Health Organization, malnutrition is the single gravest threat to the world's public health and a major contributor to child mortality. It is within this dire context that we explore what happens in the process of starvation, as the human body systematically consumes its own reserves to survive.

Quick Summary

This article details the three progressive stages of starvation, outlining how the body shifts its primary fuel source from stored carbohydrates and fat to its own muscle and organ tissue.

Key Points

Initial Response: The body first depletes its stored glucose, or glycogen, primarily from the liver, which lasts for about 24-48 hours.
Shifting to Fat: Once glycogen is gone, the body enters a state of ketosis, burning fat for fuel and producing ketones to power the brain, a phase that can last weeks.
Breaking Down Muscle: After fat stores are exhausted, the body resorts to breaking down its own muscle and organ protein for energy, leading to severe wasting.
Slowing Metabolism: To conserve energy, the body significantly lowers its basal metabolic rate, causing symptoms like fatigue and feeling cold.
Risk of Organ Failure: The breakdown of vital organ proteins, including the heart, ultimately leads to multi-system organ failure and is the eventual cause of death.
Refeeding Danger: Resuming feeding too quickly after prolonged starvation can cause potentially fatal fluid and electrolyte shifts known as refeeding syndrome.
Psychological Impact: Starvation profoundly affects mental health, causing anxiety, irritability, poor concentration, and a consuming preoccupation with food.

The Three Stages of Starvation

When the body is deprived of food for an extended period, it initiates a series of metabolic adaptations to conserve energy and prolong survival. This process is typically broken down into three distinct stages based on the body's primary fuel source.

Stage 1: Glycogen Depletion

In the first hours after the last meal, the body's primary source of energy is glucose readily available in the bloodstream. Once this is used, the body taps into its readily accessible short-term energy reserve: glycogen.

Initial Hours: Blood glucose is consumed first, and insulin levels drop as glucagon levels rise.
Glycogen Stores: The liver, which holds a store of glycogen (about 100g), breaks it down into glucose and releases it into the bloodstream to maintain blood sugar levels. Muscles also store glycogen (300-400g) but use it only for their own energy needs.
Timeline: Hepatic glycogen stores are typically exhausted within 24 to 48 hours of fasting.

Stage 2: Fat Utilization and Ketosis

After the glycogen reserves are depleted, the body makes a crucial metabolic shift. It begins to break down triglycerides from adipose (fat) tissue, a process called lipolysis, for energy.

Fatty Acid and Glycerol: Fat is broken down into fatty acids and glycerol. Most body tissues can use fatty acids for fuel. The glycerol is transported to the liver, where it is converted into a small amount of glucose through gluconeogenesis to help sustain glucose-dependent organs.
Ketone Body Production: Fatty acids cannot cross the blood-brain barrier. Therefore, the liver starts synthesizing ketone bodies from fatty acids to provide an alternative fuel source for the brain and other organs.
Duration: This stage can last for several weeks, with the duration depending heavily on the individual's initial body fat percentage.
Effects: The brain becomes more reliant on ketones, reducing its glucose needs. The basal metabolic rate (BMR) also begins to slow down significantly to conserve energy.

Stage 3: Protein Breakdown

This is the final, and most dangerous, stage of starvation. Once the body's fat reserves are exhausted, it has no choice but to break down its own functional proteins to provide energy, leading to severe and rapid muscle wasting.

Cannibalization of Tissues: The body begins a process of self-cannibalization, converting amino acids from muscle tissue into glucose to keep the brain alive.
Organ Atrophy: This widespread protein catabolism affects all organ systems, including the heart, kidneys, liver, and immune system. The heart, being a muscle, can shrink to less than half its normal size.
Systemic Failure: Organ function declines, the immune system weakens, and the body becomes susceptible to infections, which are often the ultimate cause of death. The severe breakdown of tissue and electrolyte imbalances can also lead to fatal cardiac arrhythmias.

The Body's Adaptive and Detrimental Responses

The metabolic shift during starvation is a remarkable evolutionary adaptation, but it comes with severe consequences. Here is a breakdown of the systemic effects:

Psychological and Neurological Effects: The brain, highly dependent on glucose, suffers from cognitive decline even when using ketones. Individuals often experience mood swings, irritability, difficulty concentrating, and a preoccupation with food.
Cardiovascular Effects: Blood pressure and heart rate drop to conserve energy. As the heart muscle weakens, it becomes unable to pump blood effectively, leading to heart failure or dangerous arrhythmias.
Immune System Suppression: The body's ability to fight off infections is severely compromised, making secondary infections a major threat. Wound healing is also slowed.
Endocrine Changes: Hormonal regulation is disrupted, affecting thyroid function and reproductive hormones. In women, menstrual periods may stop. This can also lead to weakened bones and osteoporosis.
Gastrointestinal Issues: The digestive system slows down, and its muscles can atrophy, leading to issues like gastroparesis (delayed stomach emptying), constipation, and bloating.
Refeeding Syndrome: A significant danger during recovery is refeeding syndrome, a potentially fatal shift in fluid and electrolytes that occurs when a severely malnourished person is fed too quickly. It can cause heart failure, respiratory distress, and seizures and requires careful medical management.

Comparison of Metabolic Stages During Starvation


Feature	Stage 1: Initial Fasting	Stage 2: Prolonged Fasting	Stage 3: Terminal Starvation
Primary Fuel Source	Glycogen (from liver & muscle)	Fat (triglycerides)	Protein (muscle & organ tissue)
Duration	Up to 48 hours	Several weeks (variable)	Until death
Brain's Fuel	Glucose	Ketones (up to 75%) & Glucose	Glucose (from protein breakdown)
Metabolic Rate	Normal to slightly elevated	Significantly decreased	Critically low
Weight Loss	Initial rapid loss (mostly water)	Slows down to conserve energy	Severe and rapid muscle wasting
Symptoms	Mild fatigue, hunger pangs	Dizziness, apathy, cognitive decline	Severe weakness, organ failure
Primary Risk	None (short-term)	Electrolyte imbalance, nutrient deficiencies	Cardiac arrest, infections, multi-organ failure

Conclusion

The process of starvation reveals the body's remarkable yet fragile capacity for survival. It represents a systematic shutdown, from the consumption of readily available glucose to the desperate breakdown of vital proteins, with each stage marking a deeper level of crisis. While metabolic adaptations like ketosis offer temporary salvation, they ultimately lead to the severe wasting of muscle mass and inevitable organ failure. The long-term physical and psychological impacts of starvation, and the risks involved in recovery, such as refeeding syndrome, underscore the profound and lasting damage caused by a lack of nutrition. Understanding this process is crucial for appreciating the critical need for proper nutrition and the dangers of extreme caloric deprivation. For more details on the specific biochemical pathways involved, consult sources like the National Institutes of Health.

Frequently Asked Questions

The body first uses glucose that is circulating in the blood. After that, it mobilizes glycogen, which is a stored form of glucose found in the liver and muscles.

Survival time varies based on an individual's body fat reserves and hydration. While the timeline is not definitive, most people can survive for weeks, potentially up to two months, with water but no food.

During the initial phase, the brain uses glucose. Once fat reserves are being metabolized, the liver produces ketone bodies, which can cross the blood-brain barrier and serve as an alternative energy source for the brain.

Prolonged starvation can cause permanent organ damage, affect brain development (especially in children), lead to osteoporosis due to low bone mineral density, and result in long-term psychological issues.

No. Fasting is the voluntary, temporary abstinence from food, typically for a shorter duration. Starvation is a prolonged, involuntary, and severe deprivation of nutrients that depletes all energy reserves and begins breaking down vital body tissues.

Refeeding syndrome is a potentially fatal condition that can occur when a severely malnourished person is fed too quickly. The sudden metabolic shift and electrolyte imbalances can overwhelm the body's systems, leading to heart failure, respiratory problems, and other complications.

Muscle wasting occurs during the final stage of starvation when the body has depleted its fat stores. It must then break down muscle protein to create glucose to fuel essential functions, leading to severe weakness and loss of tissue.