What happens to your body after you don't eat for a day?

January 5, 2026 •

4 min read

Within the first 24 hours of fasting, your body undergoes a complex and predictable metabolic shift. So, what happens to your body after you don't eat for a day? This article explores the physiological changes, both positive and negative, that take place as your body adapts to a temporary absence of food.

Quick Summary

A 24-hour fast causes the body to exhaust its glucose stores and transition to burning fat for energy through ketosis, affecting energy levels and appetite. The process also triggers cellular cleanup (autophagy) but can cause side effects like headaches and fatigue, with risks varying per individual.

Key Points

Metabolic Switch: After about 18 hours, the body depletes its stored glucose and switches to burning fat for energy.
Ketosis Begins: Once fat becomes the primary fuel source, the liver produces ketones, which the brain can use as an alternative energy source.
Cellular Repair: A 24-hour fast can trigger autophagy, a process of cellular 'housekeeping' that recycles damaged components.
Insulin Sensitivity Improves: Fasting lowers insulin levels, potentially improving your cells' sensitivity to the hormone.
Side Effects are Common: Early symptoms can include fatigue, irritability, and headaches as your body adjusts to the metabolic shift.
Hydration is Key: Staying well-hydrated with water is essential during a fast to manage side effects and maintain bodily functions.
Risks for Vulnerable Groups: Fasting can be dangerous for individuals with certain health conditions, pregnant women, or those with a history of eating disorders, and should only be undertaken with medical supervision.

The Metabolic Shift: Your Body's Fuel Change

When you don't eat for a day, your body doesn't just stop; it changes its energy strategy. This metabolic switch is a fundamental adaptation for survival, moving from using readily available glucose from your last meal to tapping into stored energy reserves. This transition happens in distinct phases and is orchestrated by key hormones like insulin and glucagon.

Phase 1: The Fed and Postabsorptive State (0-18 Hours)

In the first several hours after your last meal, your body is in the fed state, actively digesting and absorbing nutrients. Your pancreas releases insulin to help cells absorb glucose for immediate energy. As time passes, your blood sugar levels begin to drop, and your body enters the postabsorptive state. To maintain a stable blood glucose level, your liver breaks down stored glycogen (the body's short-term glucose storage) and releases it into the bloodstream.

Phase 2: Glycogen Depletion and Early Ketosis (18-24+ Hours)

As the hours tick by, your liver's glycogen stores start to run low. For most people, this happens between 18 and 24 hours into a fast. At this critical point, the body initiates a process called gluconeogenesis, creating its own glucose from non-carbohydrate sources like amino acids from proteins and glycerol from fats. Simultaneously, the breakdown of fat (lipolysis) ramps up significantly. The liver begins converting these fatty acids into ketone bodies, which are an alternative fuel source for the brain and other tissues. This metabolic state, characterized by elevated ketones, is known as ketosis. During this phase, many people report a shift in mental clarity, though others experience fatigue and headaches.

Potential Benefits of a 24-Hour Fast

For healthy individuals, a supervised 24-hour fast may offer several short-term benefits, though its effects are highly individual and require further research.

Enhanced Cellular Repair (Autophagy): Fasting triggers autophagy, a cellular process where the body removes damaged or dysfunctional cells and proteins. This process is essentially the body's 'housekeeping' system, and research suggests it can promote longevity and disease prevention.
Improved Insulin Sensitivity: Periods of fasting cause insulin levels to decrease, which can improve your cells' responsiveness to insulin. This may lower the risk of type 2 diabetes.
Metabolic Flexibility: The switch from glucose-burning to fat-burning demonstrates the body's ability to use different fuel sources efficiently.
Weight Management: A 24-hour fast reduces overall calorie intake, which can lead to weight loss over time, particularly when practiced regularly in an intermittent fasting routine.
Reduced Inflammation: Some studies suggest intermittent fasting may help decrease markers of systemic inflammation.
Gut Rest: Abstaining from food gives the digestive system a break, potentially promoting gut health and balance.

Potential Risks and Considerations

While a 24-hour fast can be beneficial, it also comes with potential side effects and risks, especially if you have underlying health conditions.

Side Effects: Common side effects include fatigue, dizziness, headaches, irritability, and initial hunger pangs. These are often temporary as your body adjusts.
Nutrient Deficiencies: With longer or more frequent fasting, there is a risk of not getting enough essential vitamins and minerals.
Hormonal Disruption: Fasting can affect hormone levels, which may disrupt menstrual cycles in women.
Risk of Binge Eating: Some individuals may find themselves overeating or binge eating after a fast, undoing the potential benefits.
Contraindications: Fasting is not suitable for everyone. Individuals with diabetes, a history of eating disorders, those who are pregnant or breastfeeding, and children or adolescents should not fast without strict medical supervision.

Physiological Comparison: Fed vs. Fasted State


Feature	Fed State (after eating)	Fasted State (after ~18 hours)
Primary Energy Source	Glucose from food	Ketones from fat
Blood Glucose Levels	Elevated due to food intake	Decreased, then regulated by liver
Hormone Levels (Insulin)	High to shuttle glucose into cells	Low to signal release of stored energy
Hormone Levels (Glucagon)	Low	High to stimulate glycogen and fat breakdown
Cellular Activity	Growth and energy storage	Repair and recycling (autophagy)
Appetite	Decreased due to satiety signals	Can increase initially, but often subsides as ketosis takes over

Breaking Your Fast Safely

How you end your 24-hour fast is crucial for a smooth transition back to eating. Avoid heavy, high-sugar, or processed foods immediately after fasting, as this can overwhelm your digestive system and cause blood sugar spikes. Instead, opt for a small, balanced meal rich in protein, healthy fats, and whole foods. Examples include a broth-based soup, some yogurt, or a piece of fruit. Stay hydrated throughout the fast with plenty of water, and consider incorporating unsweetened tea or black coffee.

Conclusion: A Temporary Shift, Not a Long-Term Solution

Going without food for a day triggers a natural metabolic shift as the body transitions from burning glucose to relying on fat and ketones for fuel. While this process is generally safe for healthy adults and may offer short-term benefits like enhanced cellular repair and improved insulin sensitivity, it is not without potential side effects like fatigue and headaches. For those considering a 24-hour fast as a health strategy, it is critical to stay well-hydrated and listen to your body's signals. This practice should never be approached as a cure-all or a long-term solution for weight loss without medical guidance. If you have any underlying health conditions, always consult with a healthcare professional before attempting any form of fasting to ensure it is right for you.

Visit Healthline for more detailed information on extended fasting and how it affects the body.

Frequently Asked Questions

During a 24-hour fast, it is crucial to drink plenty of water. Other calorie-free beverages like plain, unsweetened tea and black coffee are also acceptable.

A single 24-hour fast is unlikely to cause significant muscle loss. The body primarily uses stored glycogen and fat for energy during this period. Muscle breakdown typically only occurs during more prolonged periods of fasting or starvation.

No, a 24-hour fast is not safe for everyone. Individuals with conditions like diabetes, those with a history of eating disorders, children, pregnant or nursing women, and people on certain medications should avoid fasting unless advised and supervised by a doctor.

Initial hunger pangs are common as the body is accustomed to regular feeding times. However, many people report that strong hunger often subsides as the body enters ketosis and starts burning fat for fuel.

Break your fast with a small, nutritious, and easily digestible meal. Avoid overeating or consuming heavy, sugary, or processed foods immediately. A light soup, some nuts, or a small portion of whole foods can help ease your digestive system back into action.

Ketosis is the metabolic state where your body burns fat for energy. During a 24-hour fast, ketosis typically begins after your liver's glycogen stores are depleted, usually somewhere between 18 and 24 hours.

A single 24-hour fast does not 'reset' your metabolism in a way that provides long-term effects. It primarily shifts your energy source from glucose to fat. For sustainable metabolic benefits, fasting must be part of a consistent, healthy lifestyle.