What happens to your body when you have not eaten?

December 5, 2025 •

4 min read

Within hours of your last meal, your body initiates a sophisticated process to find alternative energy sources. This adaptive response explains what happens to your body when you have not eaten and how it prioritizes which fuel to burn to sustain vital functions.

Quick Summary

The body, in the absence of food, undergoes distinct metabolic changes, progressing from glucose consumption to utilizing stored fat for energy through ketosis, and eventually breaking down muscle tissue in severe cases. This can lead to a host of physical and mental effects, including fatigue, cognitive changes, and malnutrition.

Key Points

Initial Fasting: For the first 24 hours, the body uses blood glucose and stored glycogen for energy, which is primarily to fuel the brain.
Ketosis Transition: After glycogen is depleted, the body enters ketosis, converting stored fat into ketone bodies for fuel.
Muscle Breakdown: Prolonged fasting (beyond 72 hours) forces the body to break down muscle protein for glucose, leading to muscle wasting and organ deterioration.
Metabolic Slowdown: The body conserves energy by lowering its metabolic rate, which can lead to feeling cold.
Mental Effects: Lack of glucose and nutritional deficiencies cause cognitive issues like brain fog, irritability, and difficulty concentrating.
Refeeding Risk: A sudden reintroduction of food after prolonged starvation can cause a potentially fatal electrolyte imbalance known as refeeding syndrome.
Long-term Consequences: Persistent malnutrition can result in permanent damage, including stunted growth, bone loss, and severe emotional distress.

The Body's Emergency Fuel Plan

When you stop eating, your body doesn't just shut down; it enters a well-orchestrated sequence of metabolic shifts to keep your organs functioning. This process is highly dependent on your body's stored reserves, which are tapped in a specific order to maintain energy levels for as long as possible.

Stage 1: The Initial Fast (0–24 hours)

In the first several hours after your last meal, your body relies on the easiest and most readily available source of energy: glucose from your bloodstream. As blood glucose levels begin to drop, the pancreas releases less insulin, and the body's 'fight or flight' hormone, glucagon, increases. This signals the liver to convert its stored glycogen back into glucose and release it into the bloodstream, a process called glycogenolysis. This ensures that your brain, which is a significant consumer of glucose, has a steady supply of fuel. During this initial period, you might feel some hunger pangs and notice some mild fatigue.

Stage 2: The Shift to Fat (12–72 hours)

After approximately 12 to 24 hours, the liver's glycogen stores are largely depleted. At this point, the body begins a metabolic shift toward using stored fat for energy. This process, called ketosis, involves the liver converting fatty acids into ketone bodies. While some brain cells can use ketones for fuel, the brain still requires a small amount of glucose, which the liver continues to produce through gluconeogenesis by breaking down muscle protein. Symptoms during this stage can include headache, fatigue, and irritability, often referred to as the 'keto flu'.

Stage 3: Prolonged Fasting and Starvation (Beyond 72 hours)

For periods extending beyond three days, the body has fully transitioned into using fat and, crucially, begins to accelerate muscle and protein breakdown. While ketosis provides a more stable energy source, the body will now cannibalize its own muscle tissue, releasing amino acids that can be converted into glucose for the brain. This is a dangerous phase where significant muscle wasting occurs, including the breakdown of cardiac muscle. A lowered metabolic rate also sets in, as the body conserves energy for survival. Physical deterioration becomes more evident, leading to a host of severe health complications.

A Comparison of Fuel Sources During Fasting


Fuel Source	Initial Fast (0-24 hrs)	Prolonged Fast (24+ hrs)
Carbohydrates (Glycogen)	Primary fuel source. Quickly depleted.	Reserves are exhausted.
Fat (Ketosis)	Minimal use.	Primary fuel source. Becomes central to energy production.
Protein (Muscle)	Minimal breakdown for minor glucose needs.	Accelerated breakdown for glucose, leading to muscle wasting.

Physical and Mental Ramifications

Prolonged lack of food severely impacts both your physical and mental health. Physically, the body weakens considerably, leading to symptoms like:

Fatigue and Weakness: Non-stop tiredness as the body's energy reserves are depleted.
Cold Intolerance: A slowed metabolism means less body heat is generated, causing you to feel cold.
Hair and Skin Changes: Brittle hair, hair loss, dry skin, and a pale complexion are common signs of malnutrition.
Digestive Issues: The digestive system can become sluggish, resulting in constipation.

Mentally, the brain's reliance on glucose makes it particularly vulnerable:

Cognitive Decline: You may experience brain fog, poor concentration, and impaired judgment.
Mood Changes: Cortisol levels increase during fasting, which can lead to anxiety, irritability, and stress.
Obsessive Thoughts: A constant preoccupation with food and cravings can take over, disrupting mental clarity.

A Serious Medical Condition: Refeeding Syndrome

For individuals who have undergone a period of prolonged malnutrition, a careful reintroduction of food is critical to prevent a dangerous condition called refeeding syndrome. This can occur when the sudden intake of carbohydrates causes a rapid shift of fluids and electrolytes, such as phosphate, potassium, and magnesium, into the cells. This can overwhelm the body, leading to serious complications, including:

Heart failure
Respiratory distress
Seizures
Coma

Conclusion: Your Body's Adaptive but Vulnerable State

In summary, what happens to your body when you have not eaten is a complex physiological progression designed for short-term survival. The body cleverly shifts its energy source from quick-burning glucose to more sustainable fat stores through ketosis. However, this is not a permanent solution. Prolonged deprivation leads to the breakdown of vital muscle tissue, significant health complications, and the potential for a dangerous refeeding syndrome upon resuming eating. It is crucial to recognize that while the body is remarkably resilient, it can only sustain itself for a limited time without proper nutrition. Consulting a healthcare provider is essential for anyone considering a restrictive diet or experiencing symptoms of prolonged fasting, especially those with pre-existing conditions like diabetes.

This article is for informational purposes only and does not constitute medical advice. Please consult with a healthcare professional before making any changes to your diet.

Frequently Asked Questions

While individual factors vary, people can generally survive for up to two to three months without food, provided they have adequate water intake. The body has sufficient fat stores to provide energy for a significant period.

In the initial 24 hours, your body first uses the glucose from your last meal. As blood sugar drops, your liver releases stored glycogen to maintain glucose levels, particularly for the brain.

Ketosis is a metabolic state where the body burns stored fat for energy. It typically begins after the body's glycogen stores are largely depleted, which is usually within 12 to 24 hours of not eating.

Yes, after extended periods without food, once fat reserves are significantly reduced, the body will begin to break down muscle tissue to convert its protein into glucose for energy.

Mental effects include increased irritability, anxiety, brain fog, and difficulty concentrating, largely due to the brain's reduced access to its preferred fuel, glucose.

Refeeding syndrome is a dangerous condition that can occur when severely malnourished individuals begin eating again. It involves a rapid and life-threatening shift in electrolytes that can lead to heart failure and other complications if not medically managed.

Yes, if food intake is severely restricted over a longer period, your body may enter a conservation state, slowing down your metabolism to preserve energy.