What does your body eat first when starving?

October 4, 2025 •

4 min read

While people have been known to survive for over seventy days without food under medical supervision, the body initiates a complex, multi-stage metabolic response almost immediately, which answers the question: what does your body eat first when starving. This process prioritizes energy sources to protect vital functions.

Quick Summary

During severe calorie restriction, the body systematically consumes its energy reserves in a specific order: first glycogen, then fat stores, and finally, muscle tissue. The process involves hormonal shifts, ketosis for brain fuel, and a lowered metabolic rate to prolong survival.

Key Points

Glycogen First: In the first 12-24 hours of no food, the body burns through its glycogen reserves stored in the liver and muscles for quick energy.
Fat Next: After glycogen is depleted, the body enters a state of ketosis, primarily using stored fat to create ketone bodies for fuel.
Ketones for the Brain: The brain, which usually runs on glucose, adapts to use ketone bodies as a major fuel source after a few days, preserving limited glucose.
Protein Last: When fat reserves are nearly gone, the body begins breaking down muscle protein as a last resort for survival.
Metabolic Slowdown: The body significantly lowers its metabolic rate and conserves energy to prolong survival during prolonged periods of starvation.
Endgame: Severe, long-term starvation leads to the breakdown of essential organ tissues, ultimately resulting in organ failure and death.

The First 24 Hours: Glycogen Is Your Primary Fuel

After eating, the body relies on glucose from digested food for energy. When this source runs out, the body turns to its internal storage. During the initial phase of starvation, typically lasting 6 to 24 hours after the last meal, the body primarily consumes stored glycogen. This complex carbohydrate is stored in the liver and muscles, acting as a readily accessible glucose reserve. The liver's glycogen is broken down to release glucose directly into the bloodstream, supplying energy to the brain and other essential tissues. Muscle glycogen, however, can only be used by the muscle cells themselves. Once these glycogen reserves are depleted, the body must adapt its metabolic strategy to find a new energy source.

The Days to Weeks After: The Switch to Fat

As glycogen stores dwindle, the body enters a second phase, focusing on fat for fuel. The pancreas releases less insulin and more glucagon, signaling the body to begin breaking down triglycerides stored in adipose tissue (body fat) through a process called lipolysis. These triglycerides are split into glycerol and fatty acids. While the fatty acids can't directly cross the blood-brain barrier to fuel the brain, they are used by muscles and other tissues, sparing any remaining glucose for the brain. The liver plays a crucial role during this phase, converting the fatty acids into ketone bodies via ketogenesis. After about two to three days of fasting, the brain begins to adapt, using these ketone bodies as a major fuel source. This metabolic shift, known as ketosis, is an evolutionary survival mechanism that significantly reduces the body's dependence on glucose and helps conserve muscle mass for as long as possible.

Prolonged Starvation: The Devastating Use of Protein

Once the body's fat reserves are nearly exhausted, the most severe phase of starvation begins. At this point, the body has no other option but to turn to protein for energy, primarily from muscle tissue. This process is called proteolysis. The proteins are broken down into amino acids, which are then sent to the liver to be converted into glucose through gluconeogenesis. While the body does its best to conserve vital organs, the breakdown of muscle is rapid and leads to significant muscle wasting. As this process continues, essential proteins that perform critical cellular functions are degraded, leading to organ failure. The heart, being a muscle itself, begins to deteriorate. Death from prolonged starvation is often the result of cardiac arrest or arrhythmia, brought on by severe tissue degradation and electrolyte imbalances.

Starvation Stages Comparison Table


Stage	Duration	Primary Energy Source	Brain Fuel	Notable Metabolic Process
Phase 1: Glycogen Depletion	First 12-24 hours	Stored Glycogen	Glucose	Glycogenolysis
Phase 2: Ketosis	Weeks	Stored Fat (Triglycerides)	Ketone Bodies + Glucose	Lipolysis, Ketogenesis
Phase 3: Protein Catabolism	After fat depletion	Body Protein (Muscle)	Ketone Bodies + Glucose (from protein)	Proteolysis, Gluconeogenesis

The Hormonal Response to Starvation

Understanding the hormonal changes is key to grasping the body's metabolic adaptations. Initially, as blood glucose levels drop, insulin levels decrease significantly. In contrast, the stress hormones glucagon, epinephrine, and cortisol increase. Glucagon specifically signals the liver to break down glycogen and begin gluconeogenesis. These hormonal shifts trigger the body's various energy-generating processes to keep the brain and other vital organs functioning during times of scarcity. The suppression of thyroid hormones also contributes to a lowered basal metabolic rate, which helps conserve energy.

Conserving Energy and Prolonging Survival

Beyond simply switching fuel sources, the body actively works to slow down. The basal metabolic rate (BMR) decreases significantly, sometimes by as much as 30%. This reduction in energy expenditure helps prolong survival by stretching the limited energy reserves for as long as possible. The body also reduces non-essential energy use, like shivering and physical activity. For humans, this survival response, which involves maintaining cognitive function for weeks through the use of ketones, is a powerful evolutionary adaptation that would have allowed our ancestors to continue searching for food during periods of famine. For more on the complex biochemical pathways involved, you can refer to the Wikipedia page on Starvation response.

Conclusion

When starvation sets in, the body eats its stored resources in a calculated, life-prolonging sequence. It begins with the easily accessible glycogen, then moves to the more abundant and energy-dense fat reserves. Only as a last resort, when fat stores are exhausted, does it sacrifice its own protein and muscle tissue. This multi-stage metabolic adaptation illustrates the human body's remarkable resilience and efficiency in the face of extreme deprivation, with hormonal changes and reduced metabolic rate all working together to ensure survival for as long as possible.

Frequently Asked Questions

After the initial 12 to 24 hours, when glycogen stores are depleted, the body begins transitioning to fat metabolism. This shift becomes the primary source of fuel for the body over the next days and weeks.

Neither is burned strictly first; the processes overlap. The body first uses glycogen, then a combination of fat and protein. However, the body prioritizes fat burning to preserve muscle tissue for as long as possible before eventually relying heavily on muscle protein.

Ketosis is a metabolic process that occurs during starvation when the liver converts fatty acids from fat stores into ketone bodies. The brain and other organs can then use these ketones for energy, reducing the demand for glucose and preserving muscle.

Once fat stores are exhausted, the body enters the most critical stage of starvation. It begins to rapidly break down muscle tissue and other protein structures to produce glucose, leading to severe muscle wasting, organ damage, and eventually death.

The body lowers its basal metabolic rate (BMR) as an adaptive survival mechanism. By reducing energy expenditure on non-essential functions, it can prolong life for as long as possible on its limited energy reserves.

Initially, the brain relies on glucose. During prolonged starvation, however, it becomes highly efficient at using ketone bodies, produced from fat breakdown, as its primary fuel source. This significantly reduces the brain's glucose needs and helps protect muscle mass.

Long-term starvation can lead to permanent organ damage, a weakened immune system, cardiovascular and respiratory issues, neurological problems, and severe psychological distress. Death is often caused by complications like heart failure or infection.