What does the keto flu feel like?

December 18, 2025 •

4 min read

Approximately 24 to 48 hours after severely restricting carbohydrates, many individuals begin to experience a cluster of side effects commonly known as the 'keto flu'. This uncomfortable period is marked by symptoms resembling a viral illness, leaving many to wonder exactly what does the keto flu feel like during this metabolic transition.

Quick Summary

The keto flu describes flu-like symptoms, including fatigue, headaches, brain fog, and nausea, that occur as the body adapts to burning fat instead of glucose for energy. It is primarily caused by carbohydrate withdrawal, dehydration, and electrolyte imbalances during the initial phase of a ketogenic diet.

Key Points

Symptom Variety: The keto flu manifests with flu-like symptoms such as headaches, fatigue, brain fog, nausea, and irritability, though not everyone experiences it.
Root Cause: The primary reasons for the discomfort are the body's withdrawal from its glucose fuel source, dehydration, and a subsequent imbalance of essential electrolytes.
Temporary Phase: For most people, the symptoms are temporary, lasting from a few days to a week, after which the body becomes more efficient at using fat for energy.
Electrolyte Replenishment: The most effective way to combat symptoms is by staying hydrated and actively replacing lost electrolytes like sodium, potassium, and magnesium, often through supplements, bone broth, or salting food.
Gradual Approach: For those highly sensitive to change, a slower, more gradual reduction of carbohydrates can help minimize or prevent the shock to the system and alleviate severe symptoms.
Rest and Hydrate: Rest and adequate hydration are crucial for managing fatigue and muscle cramps during the adaptation period, allowing the body to adjust more smoothly.
Distinguish from Illness: Unlike a viral flu, the keto flu does not typically include a fever, cough, or sore throat; if these symptoms appear, it may indicate a separate illness.

What is the Keto Flu and What Causes It?

The term "keto flu" refers to a temporary collection of symptoms that some people experience in the first few days to weeks of starting a ketogenic diet. This is not a real illness caused by a virus, but rather a short-term side effect of the body's major metabolic shift. Your body is accustomed to running on glucose, its primary and most easily accessible fuel source, derived from carbohydrates. When you drastically cut carb intake, forcing your body to switch to burning fat for energy—a state called ketosis—this transition can cause withdrawal-like symptoms.

The primary reasons behind this feeling of malaise are a combination of factors:

Carbohydrate Withdrawal: Your body and brain are temporarily deprived of their usual energy source, leading to fatigue and cravings.
Electrolyte Imbalance: A sharp reduction in carbs causes your body to flush out excess water and, along with it, vital electrolytes like sodium, potassium, and magnesium.
Dehydration: The initial rapid loss of water weight can lead to dehydration, which exacerbates many flu-like symptoms.

A Detailed Look at Keto Flu Symptoms

The symptoms experienced during the keto flu vary in intensity from person to person, but many feel a general sense of being unwell.

Physical Symptoms

Headache: One of the most common complaints, often linked to dehydration and electrolyte imbalance.
Fatigue and Weakness: A significant drop in energy levels is common as your body adjusts to its new fuel source.
Nausea and Upset Stomach: Many report stomach pain, nausea, and digestive issues like constipation or diarrhea as the gut adapts to a different diet composition.
Muscle Cramps: A classic sign of low magnesium and potassium, often affecting the legs.
Dizziness and Irritability: Feelings of lightheadedness or being on edge are common during the initial adaptation period.
Intense Cravings: The brain can crave its old glucose energy source, leading to powerful sugar and carb cravings.

Mental and Other Symptoms

Brain Fog: A feeling of mental sluggishness, poor concentration, and difficulty focusing is a frequent side effect.
Difficulty Sleeping (Insomnia): Some individuals experience trouble falling or staying asleep during the first week.
Bad Breath: As the body produces ketones for energy, one of them, acetone, is released through the breath, causing a distinct fruity or metallic odor known as 'keto breath'.

How the Keto Flu Compares to a Regular Flu

It is important to differentiate between the temporary keto flu and a viral infection, as they have some overlapping, but also some key, distinguishing features. A true viral flu will typically include a fever, cough, or sore throat, which are not characteristic symptoms of the keto flu.


Symptom	Keto Flu	Regular Flu (Viral)
Onset	Occurs within 2-7 days of starting a low-carb diet.	Occurs suddenly, often without a dietary trigger.
Key Features	Headaches, fatigue, cravings, digestive issues, irritability.	Fever, body aches, chills, cough, sore throat, runny nose.
Duration	Generally lasts a few days, up to one week for most, though some may experience symptoms for longer.	Typically lasts 3-7 days, with symptoms like cough possibly lingering.
Relief	Improved by addressing hydration and electrolytes; resolves as body adapts.	Manages with rest, fluids, and medications; resolves as the body fights the virus.
Trigger	Metabolic shift from burning glucose to fat.	Infection with an influenza virus.

Remedies and Management Strategies

Managing keto flu symptoms involves supporting your body as it adapts to its new metabolic state. The key is to address the underlying causes of dehydration and electrolyte imbalance.

Best practices for relief include:

Increase Electrolytes: Replenish lost sodium, potassium, and magnesium. You can do this by adding more salt to your food, drinking bone broth, or using a quality electrolyte supplement. Many people find that adding half a teaspoon of salt to a glass of water can provide quick relief from headaches and lethargy.
Stay Hydrated: Drink plenty of water throughout the day. The loss of glycogen stores releases a significant amount of water, making proper hydration essential to combat fatigue and muscle cramps.
Prioritize Rest: Give your body time to adapt. Avoid intense, strenuous exercise during the first week and opt for light activities like walking or gentle yoga. Lack of sleep can also increase stress hormones and worsen symptoms.
Increase Healthy Fats: Ensuring you consume enough healthy fats can help satisfy your appetite and provide ample fuel for your body during the transition. Consider adding avocados, MCT oil, or extra virgin coconut oil to your meals.
Gradual Transition: If the symptoms are too severe, a slower transition might be better. Gradually reduce your carb intake over a week or two instead of cutting them all at once. This gives your body more time to adjust.

Keto-friendly electrolyte sources:

Sodium: Broth, salted nuts and seeds, leafy greens, and salting your food.
Potassium: Avocados, spinach, mushrooms, and salmon.
Magnesium: Spinach, almonds, pumpkin seeds, and fatty fish.

Conclusion

While the keto flu is an unpleasant hurdle for many beginning a ketogenic diet, it is a temporary and manageable condition. Understanding that the symptoms stem from a metabolic transition, rather than an illness, is the first step toward relief. By focusing on hydration, replacing lost electrolytes, and giving your body adequate rest, you can significantly reduce the severity and duration of the symptoms. For most, the discomfort passes within a week, paving the way for the potential benefits of sustained ketosis. For persistent or severe symptoms, consulting a healthcare provider is always recommended to rule out other causes.

For more detailed information on managing the keto diet transition, consult authoritative health guides like this one from Healthline: The Keto Flu: Symptoms and How to Get Rid of It.

Frequently Asked Questions

For most people, the keto flu lasts between a few days and a week. In some cases, the symptoms can persist for a few weeks, but they typically decrease gradually as your body adapts to ketosis.

The most commonly reported symptoms include headaches, fatigue, brain fog, irritability, nausea, and muscle cramps.

The keto flu is caused by the body's metabolic adjustment when it switches from burning glucose (from carbs) to burning fat for fuel. This transition causes temporary dehydration and an imbalance of electrolytes.

Keto flu headaches are often caused by dehydration and low sodium. Increasing your intake of water and salt, for example by drinking bone broth, can often provide quick relief.

Yes, muscle cramps are a very common symptom and are often a sign of low potassium and magnesium levels. Replenishing these electrolytes is the key to stopping them.

No, the keto flu is not a viral illness. It is a temporary side effect of your body transitioning into a state of ketosis, and unlike a viral flu, it does not involve a virus or cause symptoms like fever or cough.

You can minimize or potentially avoid the keto flu by transitioning into the diet more slowly, staying well-hydrated, and focusing on replenishing electrolytes from the very beginning.