What happens to your insides when you don't eat?: The body's emergency fuel plan

October 10, 2025 •

3 min read

Within just 8 to 24 hours of not eating, your body shifts from using glucose to breaking down stored glycogen, triggering a cascade of metabolic events. This metabolic transition is just the beginning of what happens to your insides when you don't eat, setting off a complex series of physiological and psychological changes.

Quick Summary

The body initiates a sophisticated survival mechanism when food is absent, depleting stored glucose before transitioning to burning fat and eventually breaking down muscle for energy. This has significant effects on metabolism, organ function, and mental state.

Key Points

Metabolic Shift: Your body first depletes its stored glycogen (glucose) in the liver and muscles for energy, a process that can last up to 24 hours.
Ketosis Activation: After glycogen is used, the body begins breaking down fat stores and converting them into ketone bodies for fuel, entering a state called ketosis.
Muscle Breakdown: In prolonged starvation, when fat reserves are exhausted, the body resorts to breaking down muscle tissue for protein, leading to muscle wasting.
Organ Damage: Severe, long-term malnutrition can lead to serious damage and muscle atrophy in vital organs, including the heart, liver, and brain.
Cognitive Impairment: A lack of proper nutrition impairs brain function, leading to 'brain fog', memory issues, difficulty concentrating, anxiety, and depression.
Autophagy Benefits: Short-term fasting can trigger autophagy, a cellular "self-cleaning" process that recycles damaged cell parts, but this is overwhelmed by the long-term effects of starvation.
Refeeding Syndrome Risk: Reintroducing food too quickly after prolonged starvation can cause refeeding syndrome, a dangerous electrolyte and fluid shift that can lead to heart failure and other complications.

The human body is an incredibly adaptable machine, built with complex survival mechanisms to cope with periods of food scarcity. When you stop eating, your body doesn't simply shut down; it meticulously and systematically shifts its energy sources to keep vital functions running. This process is dictated by a decline in glucose and insulin levels and a rise in hormones like glucagon.

The Short-Term Fast: Using Stored Energy

During the initial hours without food, the body first utilizes glucose from the bloodstream. As these levels drop, the body turns to stored glycogen in the liver and muscles. This glycogen is converted back into glucose to maintain blood sugar. Once glycogen stores are depleted (typically within 12-24 hours), the body begins to break down fat into fatty acids. The liver then converts these fatty acids into ketone bodies, which can be used by the brain and other tissues for energy, entering a state called ketosis.

The Long-Term Effects: The Starvation State

If food deprivation continues for an extended period, the body enters a starvation state with severe consequences. After fat stores are significantly reduced, the body starts breaking down muscle tissue for protein, serving as a last-resort energy source. This muscle wasting affects not only skeletal muscles but also vital organs like the heart, potentially leading to weakened function, slower heart rate, and lower blood pressure.

Prolonged starvation impacts nearly all bodily systems:

Brain: Lack of adequate fuel leads to cognitive issues such as difficulty concentrating, impaired memory, anxiety, and depression. Chronic malnutrition can cause structural changes in the brain.
Digestive System: Without food, digestive processes slow down, potentially causing issues like bloating or constipation. The gut microbiome can also be negatively affected.
Endocrine System: Hormone production decreases, impacting fertility, mood, and metabolism. Women may experience missed periods, and thyroid function can decline.
Immune System: The body becomes more vulnerable to infections due to a weakened immune response.
Bones: Reduced nutrient intake and hormonal changes can decrease bone density, raising the risk of osteoporosis.

The Cellular Cleaning Process (Autophagy)

Fasting can induce autophagy, a process where the body recycles damaged cell parts to produce new cells. While beneficial in controlled fasting, the detrimental effects of long-term starvation outweigh these benefits.

Comparison of Short-Term Fasting vs. Prolonged Starvation


Feature	Short-Term Fasting (e.g., intermittent fasting)	Prolonged Starvation (severe calorie restriction)
Energy Source	Shifts from glucose (glycogen) to stored fat (ketosis).	Depletes fat stores and begins breaking down muscle and organ protein.
Metabolism	Shifts to fat-burning mode, potentially improving metabolic flexibility.	Significantly slows down to conserve energy, hindering weight management long-term.
Organ Function	Provides a period of rest for the digestive system, potentially reducing inflammation.	Leads to muscle loss in vital organs like the heart, causing functional decline.
Cognitive Effects	Can lead to increased focus and mental clarity for some, though initial brain fog is common.	Impairs concentration, memory, and mood, increasing anxiety and depression.
Cellular Impact	Can trigger the beneficial process of autophagy, recycling damaged cells.	Leads to cell and tissue degradation without the ability to properly regenerate.
Associated Health Risks	Generally low for healthy individuals when done safely.	High risk of malnutrition, nutrient deficiencies, organ damage, and other severe health complications.

Long-Term Recovery and the Dangers of Refeeding

For individuals recovering from prolonged starvation, reintroducing food requires careful medical supervision to prevent refeeding syndrome. This dangerous condition involves rapid shifts in electrolytes and fluids that can lead to severe complications affecting the heart, brain, and other organs.

Conclusion

What happens to your insides when you don't eat showcases the body's remarkable survival mechanisms. However, this process comes at a significant cost during prolonged periods without food. While short-term, controlled fasting might offer certain health benefits like cellular repair, extended starvation leads to a severe decline in health, impacting all major organ systems. Maintaining a balanced diet is crucial for overall health and preventing the body from entering this emergency state. For more information, see {Link: Cleveland Clinic https://my.clevelandclinic.org/health/diseases/22987-malnutrition}.

Frequently Asked Questions

The first fuel source your body uses when you don't eat is glucose from your bloodstream. Once that is low, it taps into your stored glycogen from your liver and muscles for energy, which typically lasts up to 24 hours.

After about 12 to 24 hours without food, your body begins to transition into burning fat for fuel, a process known as ketosis. The duration can vary based on your metabolism and how much you ate last.

Yes, not eating significantly affects your brain. Initially, you might experience "brain fog" or difficulty concentrating due to low blood sugar. In the long term, severe malnutrition can alter the brain's structure and lead to cognitive impairment, anxiety, and depression.

No, your stomach does not physically shrink from not eating. However, when you go without food for a while, you may feel full with less food when you resume eating, which can create the sensation that your stomach has shrunk.

With prolonged starvation, your heart, like other muscles, will lose tissue mass. This can result in a dangerously slow heart rate (bradycardia), low blood pressure (hypotension), and other severe cardiac issues due to electrolyte imbalances and decreased cardiac muscle.

Autophagy is a natural cellular process where the body 'eats' and recycles damaged cell components to renew them. Fasting, especially for periods over 16-18 hours, can trigger and enhance autophagy, potentially offering cellular health benefits.

Reintroducing food too quickly after a prolonged period of starvation can cause refeeding syndrome. This is a potentially fatal condition where rapid metabolic shifts cause dangerous electrolyte and fluid imbalances that can severely stress the heart and other organs.

In some cases, controlled and safe fasting periods, such as intermittent fasting, can benefit the digestive system. It can provide a rest period for your gut, reduce inflammation, and help restore a healthy balance of gut bacteria.