What Happens If You Skip One Day of Eating?

October 11, 2025 •

5 min read

After around 18 to 24 hours of fasting, your body fully depletes its stored glucose reserves, triggering a metabolic shift toward burning fat for energy. Understanding what happens if you skip one day of eating is crucial for anyone considering this form of intermittent fasting.

Quick Summary

A 24-hour fast involves the body transitioning from using glucose to burning stored fat for fuel after depleting glycogen reserves. This can lead to weight loss, cellular repair, and temporary side effects like hunger and fatigue. Certain individuals, such as those with diabetes or a history of eating disorders, should avoid fasting without medical supervision.

Key Points

Metabolic Shift: After 18-24 hours without food, your body depletes its glycogen and switches to burning fat for energy, a process known as ketosis.
Fatigue and Hunger: Common side effects include fatigue, headaches, and increased hunger caused by hormonal shifts, which often subside as the body adapts.
Cellular Repair: Fasting for a day can induce autophagy, a cellular process that recycles damaged components and promotes overall cellular health.
Insulin Sensitivity: Short-term fasting may improve insulin sensitivity and help regulate blood sugar levels, benefiting metabolic health.
High-Risk Groups: Individuals with diabetes, pregnant women, and those with a history of eating disorders should avoid fasting without medical supervision due to significant health risks.
Not a Long-Term Solution: While effective for short-term calorie reduction, fasting is not necessarily superior for long-term weight loss and can be harder to maintain than consistent calorie restriction.

The Body's Transition: From Glucose to Fat

When you stop eating for a full day, your body goes through several metabolic stages. The initial hours are spent using the glucose from your last meal. After this, your body begins to tap into its stored glucose, called glycogen, primarily located in the liver and muscles.

The Stages of a 24-Hour Fast

The Fed State (0-4 hours): Immediately after your last meal, your body is in the fed state, digesting food and absorbing nutrients. Blood glucose and insulin levels are elevated, and the body uses this glucose for energy.
The Early Fasting State (4-12 hours): As time passes, blood glucose and insulin levels begin to drop. Your body accesses its glycogen stores for energy to maintain stable blood sugar levels.
Gluconeogenesis and Ketosis (12-24 hours): By the 12 to 24-hour mark, your liver's glycogen reserves are typically depleted. The body enters gluconeogenesis, producing new glucose from non-carbohydrate sources like protein, and shifts into a mild state of ketosis, breaking down fat for fuel. This metabolic switch to burning fat can offer benefits such as weight loss and improved insulin sensitivity.

Potential Benefits of a Single-Day Fast

Many people practice a 24-hour fast as a form of intermittent fasting (IF) and report various benefits.

Weight Loss: By restricting calorie intake for a day, you can reduce your overall weekly calorie count, leading to weight loss. The metabolic shift to burning fat also contributes to this. However, it is crucial not to overeat on non-fasting days to negate this effect.
Improved Metabolic Health: Research suggests that intermittent fasting can improve insulin sensitivity and help regulate blood sugar levels, potentially reducing the risk of type 2 diabetes.
Enhanced Cellular Repair (Autophagy): Fasting triggers autophagy, a cellular cleaning process where the body removes and recycles damaged cells and proteins. This can contribute to overall health and longevity.
Cardiovascular Health: Some studies indicate that intermittent fasting may help improve heart health by reducing risk factors like blood pressure and cholesterol levels.

Side Effects and Risks to Consider

While generally safe for healthy adults, skipping one day of eating is not without potential side effects.

Hunger and Irritability: The most common side effect is intense hunger, which can lead to irritability and mood swings, often referred to as being "hangry." These feelings are caused by spikes in the hunger hormone, ghrelin, but typically subside as the fast progresses.
Fatigue and Headaches: You may experience fatigue, low energy, and headaches, especially during your first fasts. These symptoms are often a result of your body adjusting to the shift in its primary fuel source from glucose to fat.
Dehydration: Without consuming food, you miss out on a significant source of water. This makes it essential to drink plenty of fluids, including water and herbal teas, during your fasting period to prevent dehydration.
Disordered Eating Patterns: For some individuals, especially those with a history of eating disorders, fasting can trigger unhealthy binge-restrict cycles and psychological distress.

Comparison Table: 24-Hour Fast vs. Standard Calorie Restriction


Feature	24-Hour Fast	Daily Calorie Restriction (Traditional Diet)
Mechanism	Calorie intake is restricted on fasting days, prompting a metabolic switch to fat burning (ketosis).	A small reduction in daily caloric intake is maintained over time.
Weight Loss Effectiveness	Can lead to significant short-term weight reduction, largely from water weight initially. Long-term effectiveness is comparable to daily calorie restriction.	A consistent and slow-paced method for weight loss over a longer period.
Metabolic Shift	Pushes the body into a state of ketosis, improving metabolic flexibility and insulin sensitivity.	Relies on a slight, consistent caloric deficit, without a major shift in fuel source.
Hormonal Response	Creates fluctuations in hunger hormones (ghrelin) and stress hormones (cortisol). Can also boost growth hormone temporarily.	More stable hormone levels, though continuous dieting can also affect them.
Sustainability	Can be difficult to maintain for some due to intense hunger and side effects. Binge eating risk is a factor.	Often considered easier to sustain long-term for many people, depending on the severity of the deficit.
Digestive Impact	Allows the digestive system to rest for a full 24 hours, potentially reducing bloating for some.	Continuous processing of food, without extended rest periods.

Who Should Not Fast for a Day?

While fasting is relatively safe for many, certain populations should avoid or exercise extreme caution when considering a 24-hour fast, always consulting a healthcare provider first.

Individuals with Diabetes: Fasting can cause dangerously low blood sugar levels (hypoglycemia), especially for those on insulin or other diabetes medications.
People with a History of Eating Disorders: Fasting can be a trigger for relapse or worsening symptoms for individuals with a history of anorexia, bulimia, or binge eating disorder.
Pregnant or Breastfeeding Women: Fasting can deprive both the mother and child of essential nutrients and calories needed for proper development and milk production.
Children and Adolescents: Fasting is not recommended for children and teens, as they are in a critical period of growth and development that requires consistent nourishment.
Individuals on Certain Medications: Some medications, especially those that need to be taken with food, can have altered effects or cause side effects when taken on an empty stomach.
Those Undergoing Cancer Treatment: For some situations and cancer types, fasting could have negative effects, and should only be done under strict medical supervision within a clinical trial setting.

Conclusion: A Matter of Caution and Context

Skipping one day of eating sets off a series of predictable physiological changes in the body, primarily shifting its energy source from glucose to stored fat. For healthy adults, a 24-hour fast may offer metabolic benefits like weight loss and cellular repair, but it is not a universally appropriate or sustainable strategy. Side effects such as hunger, fatigue, and irritability are common, and for certain populations, including those with pre-existing medical conditions or a history of eating disorders, the risks can be significant. The decision to try a 24-hour fast should always be made with careful consideration of one's own health, lifestyle, and with prior consultation with a healthcare professional to ensure it is the right and safest approach.

For more detailed information on metabolic health during fasting, consult sources like the National Institutes of Health.

Frequently Asked Questions

For a single 24-hour period, significant muscle loss is unlikely. The body first uses stored glucose (glycogen) and then turns to fat for energy. Substantial muscle catabolism typically only begins after several consecutive days of fasting.

For most healthy adults, a 24-hour fast once or twice a week is generally safe, provided they stay hydrated and eat nutritiously on non-fasting days. However, it may not be more effective for weight loss than daily calorie restriction and can carry risks for some individuals.

These symptoms can occur due to low blood sugar or dehydration. It is important to drink plenty of water. If the symptoms persist or worsen, you should break your fast and consider if this method is right for you, consulting a healthcare professional if necessary.

Drinking plenty of water is essential and can help reduce hunger pangs. Unsweetened black coffee or herbal tea can also be consumed. Keeping busy with distractions can also help take your mind off food.

Yes, you can typically drink zero-calorie beverages such as water, black coffee, or plain herbal tea. However, it is best to avoid any beverages with added sugar or sweeteners that could break the fast.

It is best to break a fast with a small, easily digestible meal to avoid overwhelming your digestive system. Recommended foods include fruits, steamed vegetables, or lean protein. Avoid heavy, fatty, or highly processed foods immediately after fasting.

Performing a 24-hour fast once or twice a week is considered relatively safe for healthy individuals. Any more frequent fasting should only be done with medical supervision, as it could increase risks like heart arrhythmias or nutrient deficiencies.