What happens the first day of fasting?

December 19, 2025 •

3 min read

Within the first 24 hours of fasting, your body fully depletes its stored glucose and transitions to burning stored fat for energy. This significant metabolic shift is the core of what happens the first day of fasting, triggering a variety of noticeable physical and mental changes.

Quick Summary

The initial 24 hours of fasting prompts a metabolic switch, with the body moving from using stored glucose to burning fat for fuel. This transition may cause temporary hunger, headaches, and fatigue as your system adjusts to the new energy source.

Key Points

Metabolic Shift: On day one, your body shifts from using glucose from food to burning its own stored glycogen, and eventually, fat for energy.
Glycogen Depletion: After 18-24 hours, your liver's glycogen stores are depleted, triggering the metabolic switch to fat-burning.
Temporary Symptoms: Expect common side effects like hunger, headaches, fatigue, and irritability, which are typically most pronounced during the first day.
Hydration is Crucial: Staying well-hydrated with water and potentially adding electrolytes is essential to combat dehydration-related symptoms like headaches and dizziness.
Adaptation is Key: The initial discomfort is often temporary, and most people report that symptoms improve as the body becomes more accustomed to fasting.
Cellular Cleanup Begins: Autophagy, the cellular repair process, begins on day one, though it intensifies in subsequent fasting periods.

The First 24 Hours: Your Body’s Timeline

The first 24 hours of fasting involve a series of metabolic adjustments as your body adapts to not receiving external food. This period can be understood in stages:

Stage 1: The Fed State (0–4 Hours)

After eating, your body is in the fed state, breaking down food and absorbing nutrients. Insulin levels rise to help cells absorb glucose for energy and store excess as glycogen.

Stage 2: The Early Fasting State (4–18 Hours)

As the energy from your last meal is used, blood sugar and insulin levels decrease. Your body starts using stored glycogen from the liver to maintain stable blood sugar. Hunger may begin as ghrelin levels increase.

Stage 3: Glycogen Depletion and Early Fat-Burning (18–24 Hours)

Liver glycogen stores become significantly depleted, prompting your body to switch to burning stored fat for fuel. The liver produces ketone bodies, signaling the start of ketosis. This metabolic shift is when many initial fasting symptoms can appear.

What to Expect: Common Symptoms and Experiences

The first day of fasting can bring various temporary physical and mental symptoms as your body adjusts.

Hunger: Expect hunger pangs and cravings as your body adjusts to the absence of food. Staying hydrated can help manage this.
Headaches: Common headaches on day one can be due to dehydration, caffeine withdrawal, or low blood sugar. Proper hydration is important.
Fatigue and Lethargy: You might feel tired as your body depletes glucose reserves, a normal part of the transition.
Irritability and Mood Swings: Initial low blood sugar can cause irritability, which for some may later improve with mental clarity as ketones are produced.
Bad Breath: As fat is burned, acetone is produced, a ketone body that can cause temporary bad breath.
Dizziness and Lightheadedness: Dehydration and blood pressure changes can cause dizziness. Drinking water and potentially adding a pinch of salt can help.

Comparison: Day 1 of Fasting vs. Later Stages


Feature	Day 1	Day 3 and Beyond
Primary Fuel Source	Stored glucose (glycogen), then transition to fat.	Primarily fat and ketone bodies.
Dominant Sensation	Strong hunger pangs, cravings, and potential irritability.	Hunger often subsides; mental clarity can increase.
Energy Levels	May feel tired or sluggish during the transition.	Often experience a more stable, focused energy.
Weight Loss	Initial weight loss is mostly water weight due to glycogen depletion.	Primarily fat loss as the body is fully in fat-burning mode.
Autophagy	Begins to ramp up gently, particularly after 16-24 hours.	Increases significantly, promoting cellular repair and recycling.

How to Prepare for and Manage the First Day

Preparing adequately can ease the challenges of your first day of fasting.

Preparing for the Fast

Eat Nutrient-Dense Meals: Consume balanced meals with complex carbs, healthy fats, and protein before fasting to promote satiety.
Hydrate Consistently: Drink plenty of water in the days prior to prevent dehydration during the fast.
Gradually Reduce Caffeine: Lower caffeine intake beforehand to minimize withdrawal headaches on day one.

During the Fast

Stay Hydrated: Drink plenty of water, herbal tea, or black coffee. Electrolytes may be helpful for longer fasts.
Listen to Your Body: If you experience severe symptoms, it is safe to end the fast gently.
Avoid Strenuous Exercise: Engage in light activities like walking during the first day.
Distract Yourself: Keep your mind occupied to manage hunger and cravings.

The Role of Autophagy on Day One

Autophagy, cellular self-cleaning, begins on the first day, typically after 16 hours, promoting repair.

Conclusion

The first day of fasting is marked by a metabolic shift from glucose to fat burning. While temporary discomforts like hunger and fatigue are common, these generally lessen as the body adjusts. Proper preparation, hydration, and listening to your body are key to navigating the initial 24 hours successfully.

For more detailed information on the physiological processes involved in fasting, you can consult the Physiology, Fasting entry on the NCBI Bookshelf.

Frequently Asked Questions

Intense hunger on the first day is a normal reaction as your body and mind are used to regular food intake. The hunger hormone ghrelin can also spike initially. This feeling often subsides as your body adapts.

Yes, fasting headaches are very common on the first day. Possible causes include dehydration, caffeine withdrawal, or a drop in blood sugar. Staying hydrated is a key way to help minimize headaches.

Many people experience fatigue or low energy during the initial transition period as the body uses up its stored glucose. This is typically temporary and often replaced by a clearer, more stable energy later in the fast.

Dizziness is often linked to dehydration or electrolyte imbalance. Ensure you are drinking plenty of water. Adding a small amount of salt to your water can help restore electrolytes and alleviate the symptoms.

No, your body is designed to burn fat for energy once glucose and glycogen stores are depleted. The breakdown of muscle tissue for energy only happens during prolonged starvation, not in the initial phases of fasting.

Yes, you can drink zero-calorie fluids like black coffee and unsweetened tea during your fast. These can also help suppress appetite.

To prepare, eat nutrient-dense, satisfying meals beforehand. Gradually reduce your caffeine intake if you are a regular consumer. And most importantly, ensure you are well-hydrated in the days leading up to your fast.