What Does Your Body Start to Do When You Don't Eat? The Stages of Starvation

October 8, 2025 •

5 min read

Within just a few hours of abstaining from food, your body begins a complex and sequential metabolic process to find alternative fuel sources. Understanding what does your body start to do when you don't eat is crucial for comprehending the body's powerful and often risky survival mechanisms.

Quick Summary

The body undergoes a metabolic shift when fasting, first burning glycogen, then fat through ketosis, and finally, muscle protein to sustain energy. This process involves hormonal and physiological changes with significant health implications, including slowing metabolism and organ stress.

Key Points

Metabolic Stages: When food is absent, the body shifts from using stored glucose (glycogenolysis) to burning fat for ketones (ketosis), and finally to breaking down muscle protein for fuel in severe cases.
Ketosis as a Survival Mechanism: After depleting glycogen stores in about 24 hours, the body starts converting fat into ketones, which serve as an energy source for the brain and other tissues, helping conserve muscle mass.
Severe Starvation Effects: Prolonged food deprivation forces the body to consume muscle tissue, leading to critical protein loss, organ damage, and a weakened immune system.
Significant Risks: Beyond metabolic changes, the body's response includes a slowed heart rate, low blood pressure, hormonal imbalances, and a severe risk of refeeding syndrome upon reintroduction of food.
Psychological Impact: Not eating also affects mental health, causing mood swings, anxiety, depression, impaired cognitive function, and an unhealthy preoccupation with food.
Individual Factors Matter: The timeline and severity of the body's response vary based on individual factors like starting weight, body fat reserves, hydration, and overall health status.

When you abstain from eating, whether for a short period or extended time, your body doesn't simply shut down. Instead, it activates a series of carefully orchestrated survival mechanisms to find alternative fuel sources and conserve energy. This process unfolds in distinct metabolic stages, moving from easily accessible carbohydrates to stored fat, and eventually, to the body's own protein tissue. While short-term fasting can be a temporary, controlled event, prolonged food deprivation pushes the body into a dangerous state of starvation with severe health consequences.

The Initial Hours: Relying on Stored Glucose

In the first 6 to 24 hours after your last meal, your body primarily relies on glucose for energy. This glucose comes from two main sources: the circulating glucose in your bloodstream and the glycogen stored in your liver and muscles. As your blood sugar levels drop, your pancreas releases less insulin and more glucagon, a hormone that signals the liver to convert its stored glycogen back into glucose.

The liver holds a crucial role in maintaining stable blood glucose during this phase, ensuring that the brain—which is a heavy glucose consumer—continues to function.
Once liver glycogen is depleted, typically within about 24 hours for most individuals, the body must find a new source of energy.
During this phase, you may experience the initial symptoms of not eating, including fatigue, headaches, and irritability, due to the drop in blood sugar.

The Transition to Ketosis: Burning Stored Fat

After glycogen stores are exhausted, the body enters a state known as ketosis, usually within 48 to 72 hours. This is the body's adaptive response to the lack of carbohydrates. At this point, the liver begins to break down stored fat into fatty acids and glycerol. The fatty acids are then converted into ketone bodies, which are released into the bloodstream and used by various organs, including the brain, as an alternative fuel source.

Ketosis allows the body to conserve muscle mass, as the brain begins to rely on ketones for up to 70% of its energy needs, significantly reducing its demand for glucose.
The shift can cause side effects sometimes referred to as the 'keto flu,' with symptoms such as brain fog, fatigue, and headaches.
The rate of fat utilization depends on a person's existing fat reserves. Individuals with higher body fat can sustain this phase longer.
The body also significantly lowers its metabolic rate to conserve energy, a slowdown that can persist even after re-feeding.

The Dangers of Severe Starvation: Protein and Muscle Breakdown

If food deprivation continues after all fat reserves have been depleted, the body is left with no choice but to break down its own protein tissue for energy. This marks the beginning of the final, and most dangerous, stage of starvation. Muscle tissue, including the crucial heart muscle, is broken down to release amino acids, which are then converted into glucose by the liver.

This phase leads to rapid muscle wasting and is highly detrimental to organ function.
The body's immune system also becomes severely compromised, making the individual highly susceptible to infection.
As vital organs like the heart and kidneys lose muscle mass and protein, they begin to fail. Death from starvation often results from complications like cardiac arrhythmia or infection.

The Physiological and Psychological Toll

The effects of not eating extend far beyond the metabolic shifts within your cells. The physiological and psychological impacts are significant and wide-ranging.

Physiological Consequences

Weakened Immune System: Nutrient deficiencies cause the immune system to weaken, hindering the body's ability to fight off infections.
Hormonal Disruptions: The body's production of hormones like thyroid hormones and sex hormones decreases, leading to a slower metabolism, cold intolerance, and menstrual irregularities in women.
Cardiovascular Issues: As the body catabolizes muscle, the heart muscle weakens, leading to a dangerously slow heart rate (bradycardia) and low blood pressure (hypotension).
Digestive Problems: Gastrointestinal function slows down, resulting in issues like constipation or, in some cases, diarrhea.

Psychological and Cognitive Effects

Mood Changes: Fluctuations in blood sugar and nutrient levels can cause severe mood swings, leading to irritability, anxiety, and depression.
Impaired Cognition: The brain requires a steady supply of energy. When glucose levels are low and the body shifts to ketones, cognitive functions like concentration, problem-solving, and decision-making become impaired.
Food Preoccupation: Individuals may become obsessed with food, fixated on meal planning and food-related rituals.
Social Withdrawal: The physiological and emotional stress can cause social withdrawal, isolation, and apathy.

A Comparison of Fed vs. Fasting States


Feature	Fed State (0-4 hours)	Fasting State (24-72 hours)	Severe Starvation (Extended)
Primary Fuel Source	Dietary glucose	Stored fat (ketones)	Muscle protein
Insulin Levels	High	Low	Very Low
Glucagon Levels	Low	High	High
Metabolic Rate	Normal	Decreased	Very Low
Symptom Profile	Normal energy levels	Fatigue, headaches, mental fog	Muscle wasting, organ failure, severe weakness
Body's Priority	Storing excess energy	Conserving energy and sparing muscle	Survival at all costs, regardless of tissue damage

The Threat of Refeeding Syndrome

When a severely malnourished individual begins eating again, they face a serious risk of refeeding syndrome. This potentially fatal condition is caused by a rapid shift in fluids and electrolytes as the body tries to restart its metabolism. The sudden influx of nutrients can overwhelm the body's weakened systems, causing dangerous shifts in electrolyte levels that can lead to heart failure, neurological issues, and death. This is why medical supervision is essential for reintroducing food after a prolonged period of starvation.

Conclusion

The body's response to not eating is a remarkable testament to its survival capabilities, but it is not without significant health risks, especially during prolonged periods. From the immediate tapping of glucose reserves to the eventual and devastating breakdown of muscle tissue, the process is a cascade of metabolic adaptations designed to keep critical organs functioning for as long as possible. While short-term, controlled fasting practices may offer certain health benefits for some individuals, true starvation is a state of severe and dangerous malnutrition. The physical and psychological consequences underscore the absolute importance of proper and regular nourishment for overall health and well-being. Understanding this complex physiological journey should serve as a powerful reminder of how critically our bodies rely on consistent nutrition.

For more detailed medical information on fasting and starvation, consult the National Institutes of Health(https://www.ncbi.nlm.nih.gov/books/NBK534877/).

Frequently Asked Questions

The first thing your body uses for energy when you don't eat is the circulating glucose in your bloodstream, followed by stored glycogen from your liver and muscles.

Ketosis is a metabolic state where the body burns stored fat for energy, producing ketones as fuel. It typically starts after 48 to 72 hours of not eating, once the body's glucose and glycogen reserves are depleted.

Survival time varies significantly, but estimates suggest a person can survive for weeks to months without food, provided they have water. Without water, survival is limited to about one week.

In the initial stages of fasting, the body prioritizes conserving muscle by shifting to burning fat. However, in cases of severe or prolonged starvation, when fat stores are exhausted, the body will begin to break down muscle protein for energy, leading to muscle loss.

Lack of food can severely impact mental health, leading to irritability, anxiety, depression, impaired concentration, and a strong preoccupation with food.

Refeeding syndrome is a dangerous condition that occurs when a severely malnourished person eats too much too quickly. It causes a rapid and potentially fatal shift in fluids and electrolytes, leading to heart failure and other complications.

Yes, chronic or severe meal-skipping can be dangerous. It can disrupt metabolic hormones, cause nutrient deficiencies, lead to disordered eating patterns, and trigger anxiety and low energy due to fluctuating blood sugar levels.