The Stages Your Body Goes Through When Not Eating

October 13, 2025 •

4 min read

Records of hunger strikers suggest that the human body can survive without food for an average of one to two months, provided there is adequate hydration. This remarkable resilience is possible because of the specific stages your body goes through when not eating, systematically shifting its energy sources to preserve vital functions.

Quick Summary

The body undergoes a predictable metabolic sequence during periods without food, beginning with the use of stored glucose and transitioning to burning fat for energy through ketosis. Prolonged fasting or starvation eventually leads to the breakdown of muscle protein and a critical state of metabolic decline.

Key Points

Initial Energy Source: In the first 18 hours of not eating, your body uses stored glucose (glycogen) from the liver and muscles for energy before depleting these reserves.
Metabolic Shift to Ketosis: After approximately 18-24 hours, the body switches to burning fat for fuel, a metabolic state known as ketosis. The liver produces ketone bodies from fatty acids to power the brain and other organs.
Autophagy Benefits: Fasting for over 24 hours can trigger autophagy, a process where cells clear out and recycle damaged components, promoting cellular renewal and anti-inflammatory effects.
The Danger of Starvation Mode: Extended fasts beyond 72 hours can lead to 'starvation mode,' where muscle protein is broken down for energy after fat stores are depleted. This is highly dangerous and leads to severe muscle wasting.
Risks of Prolonged Fasting: Risks of long-term fasting include electrolyte imbalances, heart complications, and the potentially fatal refeeding syndrome when food is reintroduced too quickly.
Medical Supervision is Key: Any extended period without food should be medically supervised, particularly for those with underlying health conditions, to ensure safety and monitor metabolic changes.

The Initial Hours: Fed State and Post-Absorptive Phase

Within the first 0-18 hours after your last meal, your body is in the fed and early fasting states. This period is characterized by the consumption and storage of easily accessible energy.

Hours 0–4: The Fed State

Immediately after eating, your body digests and absorbs nutrients. Blood glucose levels rise, prompting the pancreas to release insulin, which helps move glucose from the bloodstream into your cells for immediate energy. Excess glucose is converted into glycogen and stored in the liver and muscles for later use. High insulin levels in this stage inhibit fat breakdown.

Hours 4–18: Early Fasting State

As time passes, blood glucose and insulin levels begin to fall. To maintain a steady energy supply, your body starts converting its stored glycogen back into glucose in a process called glycogenolysis. For most people, this process taps out the liver's glycogen reserves around the 18-hour mark.

The Transition to Ketosis: Fat Burning Begins

Once glucose and glycogen stores are significantly depleted, your body initiates a crucial metabolic shift to burn fat for fuel. This phase, known as ketosis, typically begins after 16 to 24 hours of fasting.

Hours 18–48: The Ketogenic Phase

As the body depletes its glycogen reserves, it increases the rate of fat breakdown (lipolysis). The liver converts fatty acids into ketones, which become the body's primary alternative energy source. The brain, which typically relies on glucose, also adapts to using ketones for a significant portion of its fuel. This metabolic flexibility is why many people who practice intermittent fasting report increased mental clarity and reduced hunger during this phase.

Benefits of Ketosis and Autophagy

Extended periods in ketosis, particularly beyond 24 hours, can trigger autophagy, a cellular self-cleaning process. Autophagy helps remove damaged cell components and proteins, effectively rejuvenating cells and contributing to anti-aging effects. This process intensifies after about 24 hours of fasting, leading to enhanced cellular efficiency and reduced inflammation.

Extended Fasting: Tapping into Protein Reserves

In the event of prolonged food deprivation, the body's fat stores become its main source of energy. However, even these reserves are finite, and once significantly depleted, the body is forced to turn to its last remaining fuel source.

After 72+ Hours: Protein Breakdown (Starvation Mode)

When fat reserves are nearly exhausted, the body enters a true state of starvation. It begins breaking down muscle tissue (protein) to convert amino acids into glucose for the brain and other vital organs. This process is highly detrimental, leading to severe muscle wasting, weakness, and a suppressed immune system. The body's metabolism slows drastically to conserve energy. Death ultimately results from organ failure and heart complications caused by tissue degradation and electrolyte imbalances.

Comparison of Fasting Stages


Stage	Timeframe	Primary Energy Source	Key Metabolic Changes	Risks
Fed State	0–4 hours	Glucose from food	Insulin rises, glucose stored as glycogen.	Minimal; normal digestion.
Early Fasting	4–18 hours	Glycogen stores	Insulin falls, glucagon rises, glycogen converted to glucose.	Mild fatigue, hunger pangs.
Ketosis	18–72 hours	Stored fat (ketones)	Glycogen depleted, liver produces ketones from fatty acids, autophagy increases.	Dehydration, "keto flu" symptoms like headache and nausea.
Prolonged Fasting	72+ hours	Protein (muscle)	Fat stores nearly exhausted, muscle tissue broken down, metabolism slows significantly.	Muscle wasting, electrolyte imbalances, heart failure, death.

Conclusion

Understanding the stages your body goes through when not eating provides crucial insight into metabolic health and the risks of prolonged starvation. The body is a highly adaptive machine, capable of efficiently switching from using glucose to burning fat for energy. Short-term fasting, which induces ketosis and autophagy, is the basis for many modern wellness practices. However, this process has a limit. Unsupervised or prolonged food deprivation, which forces the body into true starvation mode, is extremely dangerous and can lead to severe health complications and, ultimately, death. Any form of extended fasting should only be undertaken with medical supervision to ensure safety and prevent adverse health outcomes.

The Risks of Extended Fasting

For extended fasts (beyond 48 hours), risks increase substantially. Electrolyte imbalances become a serious concern and can lead to heart arrhythmias. Furthermore, refeeding syndrome, a potentially fatal complication, can occur when a malnourished person is reintroduced to food too quickly. This happens because a sudden influx of carbohydrates triggers a rapid shift in fluids and electrolytes, overwhelming the heart and other organs. Therefore, returning to normal eating after a prolonged fast requires careful medical management.

Outbound Link

For a deeper dive into the metabolic processes involved in fasting, you can explore the comprehensive research published on the National Institutes of Health's website(https://pmc.ncbi.nlm.nih.gov/articles/PMC8754590/).

How to Approach Fasting Responsibly

If you are considering any form of fasting, whether for religious purposes, weight management, or other health reasons, it is essential to consult a healthcare professional first. A doctor can help you determine if fasting is safe for you, especially if you have pre-existing conditions like diabetes, and can provide guidance on how to do so safely.

Monitoring Your Body

During any fasting period, it is crucial to stay hydrated by drinking plenty of water. Pay attention to your body's signals and stop fasting if you experience severe symptoms like dizziness, weakness, or confusion. These are signs that your body is under significant stress and may be entering a more dangerous metabolic phase.

Frequently Asked Questions

With adequate hydration, a person can typically survive for one to two months without food, though this is heavily influenced by their starting body fat percentage and overall health.

In the initial hours after a meal, your body digests food and uses glucose from that meal for energy. Once that is used, it turns to stored glucose (glycogen) in the liver and muscles.

The body typically begins to break down stored fat for energy and produce ketones between 18 and 24 hours after the last meal, once the easily accessible glycogen stores are used up.

True starvation mode is a critical state that occurs when the body's fat reserves are nearly gone. The body begins to break down muscle tissue to use its protein for energy, leading to significant muscle loss and organ damage.

No. While ketosis is a metabolic state where the body burns fat for fuel, it is not the same as starvation. Starvation is a prolonged, dangerous condition where the body starts breaking down muscle and organs after fat stores are depleted.

Yes, especially if done for prolonged periods without medical supervision. Risks include nutrient deficiencies, electrolyte imbalances, and the potentially fatal refeeding syndrome upon resuming eating.

Initial symptoms can include fatigue, irritability, difficulty concentrating, and hunger pangs as the body's blood sugar levels drop.

Autophagy is a cellular recycling process where the body cleans out damaged cells. It is stimulated during fasting and typically increases after about 24 hours of no food.

After a prolonged fast, food must be reintroduced gradually to prevent refeeding syndrome. This should be done under medical supervision, with a focus on small, nutrient-rich meals.