Why do I feel unwell when fasting? Explaining common symptoms

November 2, 2025 •

6 min read

According to a 2023 study surveying participants practicing intermittent fasting, around 61% experienced headaches and 68% felt lethargy in the first month. For many, these and other uncomfortable sensations can make you wonder, why do I feel unwell when fasting?. This is often your body's normal, albeit unpleasant, adjustment to a new eating pattern and is rarely a sign of serious danger.

Quick Summary

Fasting can trigger side effects like headaches, fatigue, and nausea as the body adapts to using fat for fuel. Causes include dehydration, electrolyte imbalances, low blood sugar, and caffeine withdrawal. These symptoms are usually temporary and can be managed with proper hydration and nutrition planning.

Key Points

Metabolic Transition: Feeling unwell during fasting is often due to your body switching from burning glucose to burning stored fat for energy.
Dehydration and Electrolytes: Fluid and electrolyte imbalances, caused by dropping insulin levels, are a major source of headaches, fatigue, and muscle cramps.
Hypoglycemia: Low blood sugar can cause weakness, dizziness, and irritability, particularly during the initial phase of fasting.
Manage Caffeine Withdrawal: Headaches can be triggered by suddenly stopping caffeine intake, a common side effect for regular consumers.
Start Slowly and Stay Hydrated: For new fasters, beginning with shorter durations and drinking plenty of water throughout the day can ease the transition and minimize discomfort.
Break Your Fast Gently: Avoid heavy or sugary meals after fasting, as this can overwhelm your digestive system and cause a blood sugar spike.
Listen to Your Body: While minor symptoms are normal, persistent or severe unwell feelings should prompt consultation with a healthcare professional.
Not for Everyone: Fasting is not suitable for everyone, including those who are pregnant, breastfeeding, or have certain medical conditions like diabetes.

Your body's metabolic switch explains initial discomfort

When you stop eating, your body undergoes a metabolic switch from burning glucose (sugar) for energy to burning stored fat. This transition is the root cause of most uncomfortable side effects, sometimes dubbed the "keto flu". For many people, this is a dramatic change from relying on a constant supply of carbohydrates for fuel. It's like changing the primary fuel source for a factory that has run on the same fuel for years; the switch won't be perfectly smooth.

The physiological culprits behind fasting discomfort

Several specific physiological changes happen in your body that lead to the classic symptoms of feeling unwell. Understanding these processes is the first step toward managing them.

Dehydration and electrolyte imbalance: During the initial stage of fasting, dropping insulin levels signal your kidneys to release excess water and sodium. If you don't actively replace these lost fluids and electrolytes, dehydration and a mineral imbalance can occur. This can lead to headaches, dizziness, fatigue, and muscle cramps.
Low blood sugar (hypoglycemia): For many, particularly in the early days of fasting, your body's glucose stores are used up. This dip in blood sugar can cause feelings of weakness, fatigue, shakiness, and irritability. The brain relies heavily on glucose, so low levels can lead to mental fog and headaches.
Caffeine withdrawal: If you are a regular coffee or tea drinker, abstaining from caffeine during a fast can cause a painful withdrawal headache. This is a common and predictable side effect that many people experience.
Changes in digestion: Restricting food intake can affect your digestive system. Some people experience constipation due to a lack of fiber, while others may experience diarrhea as their body adjusts. Acid reflux can also become a problem for those with a pre-existing history.
Fatigue and sleep disturbances: As your body adapts to burning fat for energy, you may experience fatigue or low energy. This can be exacerbated by sleep disturbances, which are also common, as hormonal changes can affect sleep-wake cycles.

Comparison of fasting and refeeding symptoms

It's important to distinguish between the symptoms experienced during the fast and those that can arise when you break it. Refeeding syndrome, while rare, is a severe condition that can occur when reintroducing food too quickly after a prolonged fast.


Symptom	During the fast	During refeeding (if done improperly)
Energy Levels	Often feel low, fatigue is common.	Can feel a sudden energy rush, but may be followed by a crash.
Digestive Issues	Constipation or mild diarrhea; acid reflux.	Bloating, severe nausea, fluid retention.
Electrolyte Balance	Depleted sodium, potassium, and magnesium lead to muscle cramps and headaches.	Potentially life-threatening drops in blood phosphate, potassium, and magnesium.
Blood Sugar	Low blood sugar (hypoglycemia) is a common cause of dizziness and weakness.	Rapid spike in blood sugar, insulin response can be extreme.
Mental State	Mood swings, irritability, and brain fog are frequent.	Can experience mental clarity or, in severe cases, confusion from rapid electrolyte shifts.

How to minimize discomfort while fasting

While some discomfort is normal during the initial adaptation period, you can take steps to manage and alleviate the worst symptoms. Listening to your body is key.

Preparation and hydration are crucial

Start slowly: If you are new to fasting, start with shorter fasting windows (e.g., 12-16 hours) and gradually increase the duration. This allows your body to adapt more gently.
Drink plenty of water: Hydration is vital. Drinking ample water throughout your eating window and fast is crucial to prevent headaches, fatigue, and dizziness. Keep a water bottle with you and sip consistently.
Replenish electrolytes: Since your body loses electrolytes like sodium, potassium, and magnesium during a fast, it's wise to replenish them. You can add a pinch of salt to your water or consider a sugar-free electrolyte supplement.

Mindful eating and lifestyle adjustments

Prioritize nutrient-dense foods: When you do eat, focus on balanced, nutritious meals rich in complex carbohydrates, protein, healthy fats, and fiber. This helps stabilize blood sugar and provides sustained energy.
Avoid heavy, processed foods: Break your fast gently with a light meal rather than a large, heavy, or sugary one, which can cause discomfort and a rapid blood sugar spike.
Manage caffeine intake: If caffeine withdrawal is an issue, consider reducing your intake in the days leading up to a fast to mitigate headaches.
Get enough rest: Aim for consistent sleep. Fatigue can worsen the effects of fasting, so prioritizing sleep and managing stress is important.
Listen to your body: While minor discomfort is normal, severe or persistent symptoms are not. Seek medical advice if you experience extreme fatigue, severe headaches, or dizziness.

Conclusion: The path to comfortable fasting

Feeling unwell when fasting is a common experience, but it is not a sign of failure. It is simply your body's natural reaction to the metabolic shift from burning sugar to burning fat. By understanding the root causes—such as dehydration, electrolyte imbalances, and low blood sugar—you can take proactive steps to minimize discomfort. Proper hydration, gradual adaptation, and mindful eating are your most powerful tools. With these strategies, you can navigate the initial challenges and achieve a safer, more comfortable fasting experience. Remember to consult a healthcare professional, especially if you have underlying health conditions, to ensure fasting is right for you.

The "Keto Flu" vs. the flu

Many people confuse the symptoms of transitioning into ketosis with a real sickness. Here's a brief breakdown:

Duration: The "keto flu" typically lasts for a few days to a couple of weeks, while a viral flu can persist longer.
Fever: The keto flu does not cause a fever, which is a key symptom of a viral flu.
Symptoms: Keto flu symptoms (headaches, fatigue, irritability) are tied to metabolic changes, whereas a true flu involves respiratory symptoms and fever.

Who should avoid fasting?

While fasting can be beneficial for many, it's not suitable for everyone. Certain individuals should avoid fasting or do so only under strict medical supervision.

Pregnant or breastfeeding individuals: Increased energy needs make calorie restriction unsafe.
Those with a history of eating disorders: Fasting behaviors can trigger or exacerbate disordered eating patterns.
People with diabetes or blood sugar issues: Fasting can cause dangerous fluctuations in blood sugar levels.
Children and adolescents: Fasting can interfere with proper growth and development.
Individuals on certain medications: Some medications require consistent food intake or can be affected by electrolyte changes.

Breaking your fast safely

Breaking a fast, especially a longer one, requires care. Introducing food slowly prevents shocking your system and minimizes digestive discomfort.

Start with simple, easily digestible foods: Begin with a small portion of soup, broth, or fermented foods like yogurt to reintroduce nutrients gently.
Avoid overeating: It's tempting to feast, but a large, heavy meal can overwhelm your digestive system. Eat mindfully and stop when you feel satisfied, not stuffed.
Prioritize protein and healthy fats: These macronutrients are easier on the digestive system than a large load of carbohydrates.
Drink water and rehydrate: Continue to focus on replenishing fluids and electrolytes.

The role of hormones in fasting symptoms

Beyond immediate glucose and electrolyte shifts, fasting also triggers a complex cascade of hormonal changes that can influence how you feel.

Cortisol and adrenaline: These stress hormones increase during early fasting, which can heighten alertness but also cause jitteriness and anxiety in some individuals.
Ghrelin: Known as the "hunger hormone," ghrelin levels can increase, leading to intensified hunger pangs.
Leptin: As a fast progresses, levels of the "satiety hormone" leptin drop.
Growth hormone: Fasting can stimulate the release of growth hormone, which helps conserve muscle mass and promote fat breakdown.
Gut microbiome: Fasting can lead to changes in your gut bacteria, which can influence everything from digestion to mood.

Navigating these hormonal and metabolic shifts with preparation and awareness can make your fasting experience more manageable and comfortable.

The potential for nutrient deficiencies

While most fasting-related symptoms are temporary, prolonged or poorly planned fasting can lead to nutritional deficiencies.

Replenish electrolytes: As mentioned, maintaining adequate levels of sodium, potassium, and magnesium is vital.
Eat nutrient-rich meals during eating windows: To avoid deficiencies, ensure your non-fasting meals are packed with a wide variety of fruits, vegetables, whole grains, and lean proteins.
Consider supplements: For longer fasts, or if you have specific dietary restrictions, a healthcare provider might recommend certain supplements.

Long-term adaptation and symptom resolution

Most of the unpleasant side effects of fasting, including fatigue and headaches, tend to subside as your body adapts to its new metabolic state. For most people, this takes a few weeks of consistent practice. After this initial adjustment period, many fasters report improved energy levels, mental clarity, and focus. Regularity is key to adapting smoothly.

Consult a professional for personalized guidance

If you have concerns about fasting or experience persistent severe symptoms, it is always recommended to consult a healthcare professional. They can provide personalized advice based on your health history and help you determine if fasting is an appropriate choice for you.

Frequently Asked Questions

The main cause is the body's switch from burning glucose (sugar) to fat for energy, a process called ketosis. This metabolic shift can cause temporary symptoms like fatigue and headaches as the body adapts.

Yes, dehydration is a very common cause of headaches while fasting. Reduced fluid intake, combined with changes in electrolyte balance, can affect blood volume and lead to headaches.

During fasting, your body uses up its stored glucose (glycogen). As these reserves deplete, blood sugar levels drop, which can cause symptoms like weakness, dizziness, and irritability.

Keto flu is the term for the flu-like symptoms, such as headaches and fatigue, that can occur when the body first enters a state of ketosis during fasting. These symptoms are part of the body's normal adaptation process.

To prevent fatigue, ensure you get enough sleep, stay properly hydrated by drinking plenty of water, and ensure your eating-window meals are rich in nutrients, protein, and healthy fats for sustained energy.

During fasting, lower insulin levels cause the kidneys to excrete more water and electrolytes like sodium and potassium. This imbalance can lead to muscle cramps, fatigue, and headaches. Replenishing them with salt or supplements can help.

Yes, mood swings and irritability are common, especially in the initial stages of fasting. This can be caused by fluctuating blood sugar and hormone levels, as well as general discomfort.