What are the physiological changes in starvation?

October 26, 2025 •

4 min read

Over 700 million people worldwide suffered from undernourishment in 2023, highlighting starvation as a serious global health crisis. When the body is severely deprived of calories, it activates a series of adaptive physiological changes in starvation to conserve energy and prolong survival.

Quick Summary

Starvation triggers a multi-stage metabolic shift, beginning with the depletion of glycogen stores, followed by the breakdown of fat into ketones, and eventually, the catabolism of muscle protein. This process significantly slows metabolism and affects nearly every organ system, with profound physical and psychological consequences, to prioritize the survival of the brain and heart.

Key Points

Metabolic Fuel Shift: The body progresses from burning stored glycogen to utilizing fat reserves, and finally, catabolizing its own protein for energy.
Ketone Production: During prolonged starvation, the liver produces ketone bodies from fatty acids, which become the brain's main energy source to spare muscle mass.
Metabolic Slowdown: A significant drop in the basal metabolic rate, sometimes by as much as 40%, occurs to conserve energy, leading to decreased body temperature.
Organ Deterioration: All major organ systems are affected, with critical organs like the heart and liver shrinking and immune function becoming severely compromised.
Refeeding Syndrome Risk: Reintroducing food too quickly after prolonged starvation can cause a fatal shift in electrolytes, known as refeeding syndrome, which requires careful medical management.
Neurological and Psychological Impact: Cognitive functions are impaired, and mood changes, such as irritability, depression, and anxiety, are common side effects of severe calorie restriction.

When the body is deprived of calories for a prolonged period, it initiates a complex and tightly regulated series of physiological and biochemical adaptations to conserve energy and maintain vital functions. This process, which unfolds in distinct phases, affects everything from metabolism to organ function.

The Three Stages of Starvation

Starvation progresses through predictable metabolic phases as the body exhausts its different energy reserves.

Phase 1: Glycogen Depletion (Hours to 1-2 Days)

After a typical meal, the body uses blood glucose as its primary fuel. When food intake ceases, the body must find an alternative glucose source to sustain the brain, red blood cells, and renal medulla, which are obligate glucose consumers.

Glycogenolysis: The liver breaks down its stored glycogen into glucose, releasing it into the bloodstream.
Duration: This glycogen reserve is limited and typically depleted within 24 to 48 hours.
Initial Adaptation: In the initial hours, the body may experience a temporary increase in metabolic rate due to the hormonal response of increased glucagon and catecholamines.

Phase 2: Fat and Ketone Utilization (Weeks)

Once the body's glycogen stores are exhausted, it shifts its primary energy source to fat, its largest energy reserve.

Lipolysis: Fat stored in adipose tissue is broken down into fatty acids and glycerol.
Ketogenesis: The liver converts the fatty acids into ketone bodies (acetoacetate and β-hydroxybutyrate).
Fuel for the Brain: Over several days, the brain adapts to use these ketones as a primary fuel source, significantly reducing its glucose requirement from about 80g to 30g per day.
Protein Sparing: By using fat and ketones for energy, the body minimizes the breakdown of muscle protein, entering a protein-sparing phase.

Phase 3: Protein Catabolism (After Fat Depletion)

When fat reserves are exhausted, the body has no choice but to break down its functional protein, primarily from skeletal muscle, to generate glucose.

Proteolysis: Muscle tissue is catabolized to release amino acids.
Gluconeogenesis: The liver uses these amino acids to create a small amount of glucose for critical functions.
Organ Atrophy: This widespread tissue breakdown leads to severe muscle wasting (emaciation) and shrinking of vital organs, including the heart.
Fatal Complications: As essential proteins and organ function decline, death can result from cardiac arrhythmia, infection, or multiple organ failure.

Organ-Specific Effects of Starvation

Starvation affects virtually every system in the body as it struggles to allocate limited resources.

Cardiovascular System: Heart rate and blood pressure decrease significantly. Cardiac muscle mass can shrink by as much as 25% in severe cases, increasing the risk of cardiac arrest.
Metabolism and Thermoregulation: Basal metabolic rate drops by up to 40% to conserve energy. This leads to a decreased body temperature, making the individual hypersensitive to cold.
Immune System: Immune function is severely impaired, increasing vulnerability to infections, which are often the ultimate cause of death.
Endocrine System: Hormonal levels, including thyroid hormones (T3 and TSH), insulin, and leptin, decline to slow metabolic processes and reduce appetite. In women, this can lead to the cessation of menstrual periods (amenorrhea).
Nervous System: Cognitive functions, including concentration, judgment, and emotional regulation, are impaired. Psychological symptoms such as irritability, anxiety, and depression are common. In children, this can lead to impaired brain development.
Musculoskeletal System: Beyond muscle wasting, starvation leads to poor bone health and increased risk of osteoporosis due to hormonal changes and nutrient deficiencies.

Starvation vs. The Stress Response

While starvation is an adaptive response to a lack of energy intake, it differs from the metabolic response to severe physiological stress, such as burns or sepsis.


Feature	Starvation	Stress Response
Energy Expenditure	Decreased (adaptive hypometabolism)	Increased (hypermetabolism)
Protein Catabolism	Minimized during fat utilization phase	Aggressively increased due to cytokine release
Primary Fuel Source	Glycogen -> Fat (Ketones) -> Protein	Glucose (initially), with persistent protein breakdown
Hormonal Profile	Decreased insulin and thyroid hormones; increased glucagon	Increased stress hormones (cortisol, catecholamines)
Primary Goal	Conserve energy and protein for survival	Mobilize substrates to support immune and repair processes

Refeeding and Its Risks

The reintroduction of food after prolonged starvation, especially carbohydrates, can trigger a dangerous condition known as Refeeding Syndrome. A sudden influx of nutrients shifts the body back to a carbohydrate-based metabolism, causing rapid and dangerous shifts of fluid and electrolytes, particularly potassium, phosphate, and magnesium, into cells. This can result in serious, life-threatening complications, including cardiac failure, respiratory distress, and neurological issues. Careful medical supervision and gradual refeeding are critical for recovery.

Conclusion

In conclusion, the physiological changes in starvation are a sophisticated and grim testament to the body's drive for survival in the face of profound nutritional deprivation. It is an organized, multi-stage process that systematically shifts the body's fuel sources from stored carbohydrates to fat, and eventually to its own functional protein, all while drastically slowing metabolism. This orchestrated decline impacts every major organ system, leading to severe physical and psychological consequences. The risks of refeeding syndrome underscore the complexity of recovery, emphasizing that addressing starvation requires not only food but also careful medical intervention to manage the metabolic shifts. Understanding these changes is crucial for treating malnutrition and appreciating the delicate balance of the body's energy systems.

For further reading, the World Health Organization offers extensive resources on malnutrition and its global impact.

Frequently Asked Questions

The initial metabolic change is the breakdown of stored glycogen in the liver into glucose, a process called glycogenolysis, which lasts for about 24 to 48 hours to provide energy for the brain and red blood cells.

After glycogen is depleted, the body begins breaking down fat reserves in adipose tissue for energy. The liver converts the resulting fatty acids into ketone bodies, which the brain can use for fuel, helping to conserve muscle protein.

The body begins to break down muscle protein for energy only after its fat reserves have been almost entirely exhausted. This is a last-resort survival mechanism that leads to significant muscle wasting and can be fatal.

During starvation, the heart's size and function diminish significantly. Heart rate and blood pressure drop, and in advanced stages, the loss of muscle mass in the heart can lead to cardiac arrest.

Starvation severely weakens the immune system due to the lack of nutrients required for immune cell function. This increases the individual's susceptibility to infections, which are a common cause of death in severe malnutrition.

Refeeding syndrome is a potentially fatal condition that can occur when reintroducing food after a period of prolonged starvation. The sudden shift in metabolism causes rapid changes in electrolytes like phosphate, potassium, and magnesium, leading to cardiac, respiratory, and neurological complications.

The psychological and cognitive effects of starvation, such as irritability, anxiety, and depression, are often reversible with consistent and adequate nutritional rehabilitation. However, a full recovery can take time, and medical supervision is often necessary.