For most people, going a few hours without food is completely normal. Between meals, the body gradually shifts from using energy from food to relying on stored energy.
In a previous article in this series, we explained what happens in your body when you don’t eat for several hours and how the body begins using stored glycogen to maintain stable energy levels. If you’d like to understand that stage first, you can read “What Happens in Your Body When You Don’t Eat for a Few Hours.”
But what happens when that period extends to a full day without eating?
Around the 24-hour mark, the body begins shifting more noticeably from using recently stored energy to activating deeper metabolic processes designed to keep you functioning even when food isn’t available.
This ability to adapt has been part of human survival for thousands of years.
Glycogen Stores Begin to Run Low
During the first several hours without food, the body relies mainly on glycogen, a form of stored glucose found in the liver and muscles.
Glycogen helps maintain stable blood sugar levels and ensures the brain continues receiving energy.
However, these stores are limited. After roughly 18–24 hours without food, liver glycogen levels can become significantly depleted.
At that point, the body begins looking for other sources of energy.
The Body Starts Burning More Fat
As glycogen stores decrease, the body gradually shifts toward using fat as a primary energy source.
Fat cells release fatty acids into the bloodstream, which many tissues can use as fuel.
This metabolic shift is part of the body’s natural ability to adapt to periods without food. It allows the body to continue producing energy even when glucose from food is no longer available.
Ketones Begin to Appear
When the body starts breaking down fat more intensively, the liver produces molecules called ketones.
Ketones can serve as an alternative energy source, especially for the brain.
During the early stages of fasting, ketone production is still relatively modest, but it increases gradually as fasting continues beyond the 24-hour mark.
This process is commonly referred to as ketosis.
Hunger Hormones Rise and Fall
Hunger does not necessarily increase in a straight line.
The hormone ghrelin, which stimulates hunger, often rises around typical meal times. If food is not consumed, those hunger signals may temporarily decrease again.
This is why many people report that hunger tends to come in waves rather than continuously during fasting.
The Body Becomes More Focused on Energy Efficiency
As fasting continues, the body begins prioritizing essential functions.
Energy is directed toward processes necessary for survival, such as:
- brain function
- heart activity
- breathing
- maintaining body temperature
Less essential processes may temporarily slow down in order to conserve energy.
Blood Sugar Remains Carefully Regulated
Even after 24 hours without food, the body works hard to keep blood sugar levels within a safe range.
The liver produces glucose through a process called gluconeogenesis, using substances such as amino acids and glycerol.
This ensures that the brain and other vital organs continue receiving the energy they need.
What Happens After 48 Hours Without Food
After about two days without eating, the body becomes even more dependent on fat for energy.
Ketone levels increase further, and the brain begins using them more efficiently.
Some people may experience:
- fatigue
- mild dizziness
- difficulty concentrating
- reduced physical energy
At this stage, the body also begins conserving energy more carefully.
After Several Days Without Food
If the fasting period continues for several days, the body gradually begins using protein from its own tissues as an additional energy source.
This means muscle mass may begin to decrease.
Possible effects may include:
- weakness
- slower reactions
- difficulty performing everyday tasks
- lower body temperature
- lower blood pressure
Vital functions such as brain activity, heart function, and breathing remain the body’s top priorities.
How Long Can a Person Survive Without Food?
The exact duration varies widely from person to person.
There are documented cases in which people have survived around 30–40 days without food, provided they still had access to water.
Several factors influence how long someone can survive without eating, including:
- hydration
- body fat reserves
- overall health
- environmental conditions
- activity level
Without water, however, survival is typically limited to only a few days.
Differences by Age and Sex
Not everyone responds to fasting in the same way.
Children and Teenagers
Growing bodies generally require more frequent energy intake, which means long fasting periods are usually not recommended for children.
Adults
Healthy adults can typically tolerate short-term fasting periods because the body has several mechanisms for maintaining energy balance.
Older Adults
Older adults may have reduced muscle mass and energy reserves, which can make prolonged fasting more challenging.
Differences Between Women and Men
Hormonal systems can influence how the body responds to longer fasting periods.
Women’s reproductive hormones are closely linked to energy availability, which means severe or prolonged calorie restriction may affect hormonal balance.
Men often have greater muscle mass and higher energy expenditure, which can influence how quickly the body shifts between energy sources.
A Survival Mechanism Built Into the Body
Going without food for 24 hours may sound dramatic, but the human body is designed to handle occasional periods without eating.
For most of human history, food availability was not constant. The body developed flexible metabolic systems that allow it to shift between using food energy and stored energy.
These systems are still active today.
Balance Matters Over Time
While the body can adapt to short-term fasting, it still requires a regular supply of nutrients over the long term.
Energy balance, adequate nutrition, and regular meals help support the optimal functioning of the brain, muscles, and internal organs.
The processes that begin after about 24 hours without food are a reminder of how adaptable the human body can be — and how carefully it manages its energy resources.