What will happen to your body if you fast for 18 hours?: A Comprehensive Guide

October 13, 2025 •

5 min read

Recent research suggests that a time-restricted eating pattern, like the 18:6 method, can trigger significant metabolic shifts. This guide explains what will happen to your body if you fast for 18 hours, transitioning from using glucose for fuel to burning stored fat and initiating cellular cleanup.

Quick Summary

An 18-hour fast prompts the body to transition its primary fuel source from glucose to fat, initiating a process called the metabolic switch. The fast also triggers the beginning of cellular repair processes and can improve insulin sensitivity and support weight management.

Key Points

Metabolic Switch: An 18-hour fast pushes your body beyond its glucose reserves to burn stored fat for energy, a process called the metabolic switch.
Ketone Production: As fat is burned, the liver produces ketone bodies, which serve as an efficient fuel source for both the brain and muscles.
Cellular Renewal (Autophagy): This fasting duration is long enough to activate autophagy, where cells clean out and recycle damaged components, promoting cellular health and longevity.
Insulin Sensitivity: By resting your pancreas and keeping insulin levels low, an 18-hour fast can improve your body's sensitivity to insulin, aiding in blood sugar control.
Increased Growth Hormone: Fasting can boost human growth hormone levels, which is beneficial for muscle maintenance and recovery.
Potential Side Effects: Initial side effects may include hunger, headaches, or fatigue, which typically resolve as the body adapts to fat-burning.
Safe Practice: Safe 18-hour fasting involves starting gradually, staying hydrated, and eating nutrient-dense foods during your eating window.

The Body's Transition: From Fed to Fasting

For the first several hours after a meal (the fed state), your body processes and absorbs nutrients, primarily using glucose from carbohydrates for energy. Insulin levels are high to help transport this glucose to cells. During an 18-hour fast, your body works through a predictable set of metabolic phases, each marked by changes in hormones and fuel sources. The 18-hour mark represents a crucial point, moving you from the postabsorptive state into a more advanced fasting state.

The Early Fasting State (4–16 Hours)

After about 4 hours, blood sugar and insulin levels begin to decline, signaling the start of the early fasting state. Your body turns to its stored form of glucose, known as glycogen, primarily located in the liver and muscles, for fuel. Your pancreas secretes more glucagon, a hormone that signals the liver to release this stored glycogen to maintain steady blood sugar levels. During this phase, you might start to feel the initial pangs of hunger, but these often subside as the body adapts.

The Metabolic Switch (16–18 Hours)

As you approach the 18-hour mark, your liver's glycogen stores become significantly depleted. This is the key moment often referred to as the “metabolic switch.” Your body no longer has a readily available supply of glucose and must shift to an alternative energy source: stored body fat. This process, known as lipolysis, intensifies, and the liver begins converting fatty acids into ketone bodies. Ketones are a highly efficient fuel source for both the brain and muscles. This transition is a hallmark of time-restricted eating methods like the 18:6 protocol.

Autophagy is Activated (Around 18 Hours)

Around the 16 to 18-hour window, the body also initiates a crucial cellular process called autophagy. This term, which means "self-eating," describes how cells break down and recycle their damaged, old, or dysfunctional components to generate energy and build new parts. Activating autophagy supports cellular health, reduces inflammation, and may even have anti-aging effects. This cellular cleansing mechanism is a key reason many people practice intermittent fasting for health benefits beyond weight loss.

Potential Benefits of an 18-Hour Fast

Increased Fat Burning: The metabolic switch directly promotes using stored body fat for fuel, a primary driver for weight loss and improved body composition.
Improved Insulin Sensitivity: By keeping insulin levels low for extended periods, the body becomes more responsive to the hormone when you do eat, which helps with blood sugar management and may lower the risk of type 2 diabetes.
Reduced Inflammation: Fasting can lead to a reduction in systemic inflammation, a key risk factor for many chronic diseases.
Enhanced Brain Function: The production of ketones can be beneficial for brain health, potentially leading to increased mental clarity, focus, and improved memory recall.
Cellular Repair and Longevity: The activation of autophagy helps clear out cellular debris, protecting against neurodegenerative diseases and supporting overall cellular health.
Boost in Growth Hormone: Fasting can increase human growth hormone (HGH) levels, which promotes muscle repair and preservation.

Navigating Potential Side Effects

While generally safe for most healthy adults, 18-hour fasting can cause temporary side effects, especially in the initial days as your body adapts. These may include:

Hunger and Cravings: These are most prominent in the beginning but typically decrease as your body adjusts.
Headaches: Often related to caffeine withdrawal or low blood sugar. Staying hydrated is key.
Fatigue and Low Energy: Your body needs time to become efficient at burning fat for fuel. Energy levels often stabilize or increase as you become more fat-adapted.
Irritability and Mood Changes: Fluctuations in blood sugar can affect mood, but a sense of accomplishment and improved self-control often follow the initial adjustment period.
Bad Breath: A side effect of ketone production, sometimes called “keto breath”.
Digestive Issues: Some people may experience constipation or bloating initially as their digestive routine changes.

Comparison of Fuel Sources: Fed State vs. 18-Hour Fasted State


Feature	Fed State (0-4 hours)	18-Hour Fasted State (Post-Glycogen Depletion)
Primary Fuel Source	Glucose from food and liver glycogen	Stored body fat (ketones)
Key Hormones	High insulin, low glucagon	Low insulin, high glucagon
Metabolic State	Glucose-burning, energy storage	Fat-burning, cellular repair
Appetite Sensation	Satiated, ghrelin (hunger hormone) is low	Hunger pangs may subside; ketones suppress appetite
Cellular Activity	Focus on growth and synthesis	Autophagy is initiated for cellular cleanup

How to Approach an 18-Hour Fast Safely

If you are considering an 18-hour fast, it is best to approach it gradually and with a focus on nutrient density during your eating window.

Start with Shorter Fasts: Ease your body into it with a 12:12 or 16:8 schedule first. This allows your metabolism to adjust slowly.
Stay Hydrated: Drink plenty of water throughout the fasting window to prevent headaches and dehydration. Calorie-free beverages like plain tea or black coffee are typically acceptable.
Eat Nutrient-Dense Meals: During your 6-hour eating window, focus on high-quality, whole foods rich in protein, healthy fats, and fiber. This provides sustained energy and minimizes cravings.
Listen to Your Body: Pay close attention to how you feel. If you experience persistent dizziness, extreme fatigue, or other concerning symptoms, it may not be the right method for you.

Who Should Avoid Fasting?

While beneficial for many, intermittent fasting is not suitable for everyone. Certain individuals should avoid fasting or consult a healthcare professional first:

Individuals with a history of disordered eating
Pregnant or breastfeeding women
Individuals with blood sugar issues, including diabetes
Children and teenagers
Those taking certain medications for blood pressure or heart disease

Conclusion

An 18-hour fast initiates a powerful metabolic transformation in the body, shifting it from relying on glucose to burning stored fat. This process, which begins after glycogen stores are depleted, promotes significant health benefits, including enhanced fat burning, improved insulin sensitivity, and the activation of cellular repair mechanisms like autophagy. While potential side effects such as hunger and irritability can occur initially, they often fade as the body adapts. Approaching fasting thoughtfully by starting gradually, staying hydrated, and prioritizing nutrient-dense foods during your eating window can help maximize benefits and minimize risks. Always consult a healthcare provider before starting any new fasting regimen, especially if you have pre-existing health conditions, to ensure it is the right and safe choice for you. For more information, you can explore research on the metabolic effects of intermittent fasting via trusted sources like the National Institutes of Health.

Frequently Asked Questions

The primary change during an 18-hour fast is the metabolic switch, where your body shifts its main energy source from glucose (stored in glycogen) to fat, producing ketones.

Yes, an 18-hour fast can induce a mild state of ketosis, though the speed depends on individual metabolism and diet. It pushes the body into producing and using ketones for fuel.

Autophagy is the body's process of cellular cleanup and repair. Research suggests it becomes significantly activated around the 16-18 hour mark of fasting, recycling old and damaged cell parts.

Yes, it is normal to experience hunger pangs, headaches, or irritability in the initial days of fasting. These symptoms are part of the adjustment period as your body becomes accustomed to burning fat for fuel.

You can drink calorie-free beverages, including water, black coffee, and unsweetened tea, to stay hydrated. Plain water is the most recommended choice.

During your eating window, focus on nutrient-dense foods. A balanced diet with lean proteins, healthy fats, fiber-rich vegetables, and whole grains is recommended for optimal results.

People with a history of eating disorders, those who are pregnant or breastfeeding, children and teens, and individuals with blood sugar conditions like diabetes should avoid fasting unless supervised by a healthcare professional.