What Happens to the Body When Deprived of Carbohydrates, Lipids, and Proteins?

January 8, 2026 •

3 min read

According to the World Health Organization, malnutrition includes deficiencies in energy and nutrients, and deprivation of all macronutrients is a severe form that initiates a dramatic survival response. This article explores what happens to the body when deprived of carbohydrates, lipids, and proteins, outlining the physiological consequences that ensue.

Quick Summary

When the body is deprived of all macronutrients, it initiates a series of metabolic adaptations to survive. It burns stored energy reserves, eventually breaking down muscle and vital tissues. This process leads to severe malnutrition, hormonal imbalances, organ damage, and can ultimately be fatal if not corrected.

Key Points

Initial Energy Source: The body first burns its glycogen (stored carbohydrate) reserves within about 24 hours of deprivation.
Fuel Switch to Fat: After glycogen is depleted, the body breaks down fat into ketones for energy through a process called ketogenesis.
Muscle Wasting for Glucose: Once fat stores are low, muscle and other vital proteins are broken down for glucose via gluconeogenesis, leading to significant muscle loss.
Organ Damage: Prolonged protein breakdown results in damage to organs like the heart, liver, and kidneys, threatening their function.
Metabolic Slowdown: To conserve energy, the body’s metabolic rate decreases significantly, and hormonal changes increase hunger and stress.
Immune System Failure: The immune system becomes severely compromised, making the body highly vulnerable to infections.
Mental and Emotional Decline: Starvation causes psychological and cognitive issues, including depression, anxiety, apathy, and impaired concentration.

The Body's Survival Mechanism: A Phased Breakdown

When faced with complete deprivation of carbohydrates, lipids, and proteins, the body shifts into a survival state, systematically breaking down its own tissues for fuel. This multi-phase process is a testament to the body's adaptive capabilities but carries severe health risks.

Phase 1: Glycogen Depletion (First 24 hours)

In the initial stages, the body first turns to its most readily available energy source: stored carbohydrates in the form of glycogen. Glycogen is stored primarily in the liver and muscles. The liver's glycogen is broken down into glucose, which is released into the bloodstream to maintain blood sugar levels and fuel the brain.

Phase 2: Lipolysis and Ketosis (After 24 hours)

Once glycogen reserves are exhausted, the body enters a phase of fat mobilization, known as lipolysis. Stored triglycerides in adipose tissue are broken down into glycerol and fatty acids. While most tissues can use fatty acids for energy, the brain cannot. Therefore, the liver converts fatty acids into ketone bodies through a process called ketogenesis. The brain and other tissues then adapt to use these ketones as an alternative fuel source, reducing the dependence on glucose and helping conserve protein.

Phase 3: Protein Catabolism (After several days)

As fat stores become depleted, the body escalates its breakdown of muscle tissue and other proteins to produce glucose, a process called gluconeogenesis. Glucogenic amino acids from the breakdown of muscle protein are converted into glucose in the liver. While this process is vital for providing fuel to glucose-dependent tissues like red blood cells and parts of the brain, it comes at a tremendous cost. Prolonged protein catabolism leads to severe muscle wasting, a hallmark of advanced starvation, and damages vital organs.

The Health Consequences of Macronutrient Deprivation

Without a constant supply of macronutrients, every system in the body is negatively impacted. The most visible manifestation of this is marasmus, a severe form of protein-energy malnutrition characterized by extreme emaciation, muscle wasting, and stunted growth in children. Other severe consequences include:

Cardiovascular System: Reduced heart size, slowed heart rate, and lower blood pressure increase the risk of heart failure and cardiac arrhythmia.
Immune System: Immune function is compromised, leaving the body highly susceptible to infections.
Digestive System: The digestive tract can atrophy, leading to malabsorption problems even when food becomes available again.
Hormonal Changes: Drastic changes in hormone levels occur, including decreased leptin, increased ghrelin, and altered thyroid hormone production, which affects metabolism, hunger, and body temperature.
Psychological Effects: Starvation has profound psychological impacts, causing irritability, anxiety, depression, apathy, and cognitive impairment.

A Comparative Look at Macronutrient Deficiency


Feature	Complete Macronutrient Deprivation	Selective Carbohydrate Deprivation	Selective Protein Deprivation (with enough carbs)
Initial Energy Source	Glycogen stores first, followed by fat and protein.	Glycogen stores first, then fat and ketones.	Remaining carbohydrates and fat.
Long-Term Fuel	Catabolism of muscle and organ tissue.	Ketones from fat breakdown, and some glucose from gluconeogenesis.	Muscle wasting (kwashiorkor) as protein is used for critical functions.
Key Outcome	Marasmus; severe emaciation and multi-organ failure.	Ketosis, 'keto flu' symptoms, and eventual adaptation.	Edema (swelling) and fatty liver.
Primary Risk	Death from organ damage and immune failure.	Long-term risk of potential kidney or heart issues.	Impaired growth and high susceptibility to infection.
Body Appearance	Emaciated, visible wasting of muscle and fat.	Weight loss (initially water), potential fatigue.	Swollen belly and face due to fluid retention.

Conclusion: A Perilous Path

Total deprivation of macronutrients—carbohydrates, lipids, and proteins—is a perilous path for the human body, triggering a cascade of metabolic adaptations that prioritize survival at a devastating cost. The systematic breakdown of fat and, crucially, muscle tissue to generate energy is a short-term survival strategy that inevitably leads to severe, and often irreversible, organ damage, immune suppression, and profound mental health issues. The body's incredible resilience in the face of starvation is overshadowed by the ultimate and inevitable failure of its most vital systems if proper nutrition is not restored. The extreme and prolonged process highlights the fundamental importance of a balanced intake of all three macronutrients for sustaining life and health. Medical supervision is essential for refeeding, as refeeding syndrome poses an immediate danger to those recovering from severe malnutrition.

Frequently Asked Questions

The body first uses its stored carbohydrate reserves, known as glycogen, which are located in the liver and muscles. These reserves are typically exhausted within the first 24 hours of deprivation.

When carbohydrates are unavailable, the liver starts converting fat into ketone bodies. The brain can adapt to use these ketones as a primary fuel source, though it still requires a small amount of glucose.

During prolonged starvation, the body must break down muscle and other proteins into amino acids. These amino acids are then converted into glucose by the liver through a process called gluconeogenesis.

The term 'starvation mode' is often misunderstood. The body does enter a state of 'adaptive thermogenesis,' slowing metabolism to conserve energy, but it does not stop burning fat entirely. It just becomes more efficient, making weight loss harder, not impossible.

Long-term consequences include severe muscle wasting (marasmus), compromised immune function, atrophy of the digestive system, permanent organ damage, and significant psychological distress.

Hormone levels shift dramatically: leptin (satiety hormone) decreases, ghrelin (hunger hormone) increases, and thyroid hormone levels may drop. These changes increase hunger, slow metabolism, and affect mood.

Refeeding syndrome is a life-threatening condition that can occur when an undernourished person is reintroduced to food too quickly. It involves dangerous fluid and electrolyte shifts that can lead to heart failure and other complications.