Will I get sick if I don't eat for 2 days?: Understanding the Body's Response to Short-Term Fasting

October 16, 2025 •

4 min read

After about 12 hours without food, your body typically depletes its stored glucose, leading to a metabolic shift. So, will I get sick if I don't eat for 2 days? While serious illness is unlikely for most healthy adults, you should expect to experience some side effects as your body adapts to using alternative fuel sources.

Quick Summary

The body switches from burning glucose to fat (ketosis) during a 48-hour fast, potentially causing temporary side effects like fatigue, headaches, and nausea. This metabolic change can offer benefits like improved insulin sensitivity and cellular repair but is not suitable for everyone.

Key Points

Metabolic Shift: After about 24 hours without food, your body transitions from using glucose to burning stored fat for energy, a process called ketosis.
Common Symptoms: You will likely experience hunger, fatigue, headaches, dizziness, and irritability during a 48-hour fast, especially on the second day.
Safety Depends on Health: For most healthy adults, a 48-hour fast is manageable, but it is dangerous for certain populations, including those with diabetes or eating disorders.
Refeed with Care: Breaking a fast requires slowly reintroducing food to avoid digestive issues and the potentially serious risks of refeeding syndrome.
Hydration is Critical: You must continue to drink non-caloric fluids like water and may need electrolytes to prevent dehydration and manage side effects.

How the Body Responds to a 48-Hour Fast

When you stop eating, your body, an incredibly efficient survival machine, begins a predictable metabolic process to find energy. The journey over 48 hours is a shift from burning primarily glucose to utilizing stored fat for fuel.

The first 12-24 hours: Your body's primary fuel source is glucose, which is readily available from carbohydrates. The liver stores a reserve of this glucose, called glycogen. During the initial hours of a fast, your body uses up this stored glycogen to keep its energy levels stable. As these stores run low, typically around the 12 to 24-hour mark, your blood sugar levels drop, which can cause initial symptoms like hunger and irritability.
The 24-48 hour mark: As glucose reserves become critically low, your body initiates a metabolic shift known to ketosis. The liver begins to break down stored body fat into fatty acids, which it then converts into molecules called ketones. These ketones become the body and brain's new primary energy source. While effective, this transition is the source of many of the common side effects associated with a 48-hour fast.

The Common Side Effects of a 48-Hour Fast

While fasting for 48 hours is generally considered safe for healthy individuals, it's not without its challenges. Common side effects often appear as the body adapts to ketosis. These typically subside as the body becomes more efficient at using fat for fuel.

Hunger pangs: The most obvious side effect, which many find most challenging. While hunger often peaks around day two, some report it diminishes as the body adapts.
Fatigue and sluggishness: The initial drop in glucose can lead to low energy and a feeling of sluggishness. Staying hydrated is key to managing this.
Headaches and dizziness: These can be caused by the body's transition into ketosis, dehydration, or electrolyte imbalances. Adding electrolytes to water can help alleviate these symptoms.
Irritability and mood swings: Changes in blood sugar and hormone levels can lead to a 'hangry' state, causing irritability and difficulty concentrating.
Digestive issues: An empty stomach can lead to excess acid, potentially causing nausea. Additionally, a lack of food can result in constipation.
Insomnia: Some people report interrupted sleep or insomnia during extended fasts.

Potential Benefits of a Short-Term Fast

Beyond simply resting the digestive system, a controlled, 48-hour fast may offer several health benefits for some individuals, though it should be undertaken with care and, ideally, medical advice.

Cellular repair (Autophagy): Fasting triggers autophagy, a process where the body cleans out and recycles damaged or dysfunctional cell components.
Weight loss and improved insulin sensitivity: By forcing the body to burn fat for energy, fasting can aid in short-term weight loss. It can also improve insulin sensitivity, which is beneficial for managing blood sugar levels.
Reduced inflammation: Some studies suggest that fasting may help lower chronic inflammation, which is linked to various diseases.
Metabolic reset: For some, a 48-hour fast can serve as a metabolic reset, encouraging the body to adapt to using different fuel sources.

Refeeding: Breaking Your Fast Safely

After a prolonged fast, it is crucial to reintroduce food slowly to avoid digestive upset and, in severe cases, the dangerous complication of refeeding syndrome. Your digestive system has been dormant and needs to be reactivated gently.

Recommended refeeding guidelines

Start with easily digestible foods: Begin with small portions of foods like broth-based soups, fermented foods, or steamed vegetables. Avoid heavy, processed, or sugary foods immediately after a fast.
Eat slowly: Take your time and chew your food thoroughly. Listen to your body's hunger cues.
Stay hydrated: Continue to drink plenty of fluids, including water and electrolyte-rich drinks.
Avoid overindulging: Eating a large, heavy meal can overwhelm your system and cause digestive distress.

Who Should Avoid a 48-Hour Fast?

While short-term fasting can be safe for many, certain individuals should avoid it entirely or consult a healthcare provider beforehand. Extended fasting carries significant risks for vulnerable populations.

Pregnant or breastfeeding women: Adequate nutrition is critical for the health of both mother and child.
Individuals with a history of eating disorders: Fasting can be a trigger for disordered eating patterns.
People with diabetes: Fasting can cause dangerous fluctuations in blood sugar levels, especially for those on insulin or blood sugar-lowering medications.
Those who are underweight: Individuals with a low body mass index are at higher risk for complications.
Anyone taking certain medications: Some medications, including blood thinners, NSAIDs, and blood pressure medication, require food for proper absorption.

Short-Term Fasting vs. Prolonged Starvation: A Comparison


Feature	Short-Term Fasting (e.g., 48 hours)	Prolonged Starvation (Weeks/Months)
Energy Source	Shifts from glucose to stored fat (ketosis).	Breaks down fat, then muscle tissue for energy.
Purpose	Often voluntary for perceived health benefits or religious reasons.	Involuntary and life-threatening due to lack of food.
Side Effects	Temporary side effects like hunger, fatigue, headaches.	Severe consequences including muscle wasting and organ failure.
Body's Response	Initiates adaptive metabolic processes like ketosis and autophagy.	Alters normal functions to preserve energy; metabolic rate slows dramatically.
Long-Term Effects	Minimal if done safely; some potential health benefits.	Severe and potentially irreversible health damage.

Conclusion

For a healthy adult, abstaining from food for 48 hours is unlikely to cause a serious illness but will certainly lead to noticeable, and sometimes uncomfortable, side effects like hunger, fatigue, and irritability as the body transitions to ketosis. The experience varies significantly based on individual health, lifestyle, and hydration. While studies show potential benefits like cellular repair and improved insulin sensitivity, the practice is not without risk, and a cautious approach, including proper hydration and careful refeeding, is essential. Always consult a healthcare professional before attempting an extended fast, especially if you have underlying health conditions or take medications.

Note: For more information on intermittent fasting methods and safety, you can visit authoritative sources like Johns Hopkins Medicine.

Frequently Asked Questions

After 48 hours without food, your body has used up most of its stored glucose and enters a state of ketosis, where it begins burning stored fat for energy. This metabolic shift is often accompanied by fatigue, headaches, and increased hunger.

The initial signs of not eating enough can include fatigue, difficulty concentrating, irritability, extreme hunger, and headaches. These symptoms are often caused by low blood sugar levels (hypoglycemia).

For most healthy adults, a controlled 48-hour fast is not necessarily bad and may offer benefits like cellular repair and improved insulin sensitivity. However, it is not suitable for everyone and carries risks, and should only be undertaken after careful consideration and medical advice.

Yes, fasting can cause feelings of sickness, including nausea and dizziness. Nausea can result from a build-up of stomach acid on an empty stomach. These symptoms are usually temporary as the body adjusts to ketosis.

To break a 48-hour fast safely, start with small, easily digestible meals like bone broth or fermented foods. Slowly increase food quantity and complexity over the following days to avoid digestive issues and complications like refeeding syndrome.

A 48-hour fast is not recommended for pregnant or breastfeeding women, individuals with a history of eating disorders or diabetes, those who are underweight, or anyone on certain medications (such as insulin or NSAIDs).

Light physical activity like walking or yoga is generally acceptable during a fast. However, strenuous exercise should be avoided due to lower energy levels and the risk of dizziness. Listen to your body and prioritize rest.