Can keto make you feel unwell? Understanding the 'Keto Flu' and Other Side Effects

October 26, 2025 •

4 min read

Reports show that up to 25% of people starting a ketogenic diet experience flu-like symptoms. This phenomenon, often called the 'keto flu,' is a primary reason for concern and directly answers the question, can keto make you feel unwell?

Quick Summary

Many people experience temporary flu-like symptoms when starting a ketogenic diet, including fatigue, headaches, and nausea, as their bodies adapt to burning fat for fuel.

Key Points

Initial Discomfort is Normal: Many people experience a temporary 'keto flu' with symptoms like fatigue, headaches, and nausea as their body shifts from using glucose to fat for energy.
Electrolyte Balance is Crucial: Water and key electrolytes like sodium, potassium, and magnesium are lost during the initial stages of ketosis, and replenishing them is essential for symptom relief.
Hydration is Key: Increased fluid loss means you must actively increase your water intake to prevent dehydration, which can worsen 'keto flu' symptoms.
Manage Digestive Issues: Both constipation and diarrhea can occur. Addressing fiber intake, staying hydrated, and monitoring fat sources can help resolve these problems.
Consider Long-Term Risks: A ketogenic diet can present potential long-term risks such as nutrient deficiencies, kidney stones, and negative impacts on heart health if not managed properly.
Seek Professional Guidance: It is highly recommended to consult a doctor or registered dietitian before and during a ketogenic diet to ensure safety and prevent serious complications.

The 'Keto Flu': An Unpleasant Introduction to Ketosis

The most common reason people feel unwell on a keto diet is the initial adaptation period, widely known as the 'keto flu'. This is not a viral infection but a collection of symptoms triggered by the body’s sudden switch from its primary fuel source, glucose from carbohydrates, to burning fat for energy. As your body enters a state of ketosis, it must learn to produce and utilize ketones efficiently, a process that can take a few days to a few weeks.

During this metabolic transition, the body experiences several shifts that can cause discomfort:

Carbohydrate Withdrawal: For decades, your body may have relied on a steady stream of glucose. Restricting carbs can cause withdrawal-like symptoms as your brain's primary fuel source is temporarily reduced, leading to mental fogginess and irritability.
Electrolyte Imbalance: The rapid depletion of glycogen (stored carbs) causes the body to excrete excess water. Crucial electrolytes like sodium, potassium, and magnesium are flushed out along with this water, contributing to fatigue, muscle cramps, and headaches.
Dehydration: Increased urination and flushing out water stores means you can become dehydrated quickly if you don't actively increase your fluid intake. Dehydration exacerbates fatigue, dizziness, and headaches.

Common Symptoms of the Keto Flu

The symptoms experienced during the keto flu can range from mild to severe, and not everyone experiences all of them. The most frequently reported issues include:

Fatigue and low energy
Headaches and brain fog
Nausea, sometimes with vomiting
Constipation or diarrhea
Muscle cramps and weakness
Dizziness and lightheadedness
Irritability
Sleep disturbances, such as insomnia

Managing Initial Keto Side Effects

The good news is that for most people, the keto flu is temporary and manageable. Taking proactive steps can significantly reduce the severity and duration of symptoms. Many of the remedies focus on addressing the underlying causes of dehydration and electrolyte loss.

Here are practical steps to manage and prevent initial discomfort:

Replenish Electrolytes: Increase your intake of sodium, potassium, and magnesium. You can do this by salting your food generously, drinking bone broth, and consuming keto-friendly, electrolyte-rich foods like avocado, leafy greens, and nuts.
Stay Hydrated: Drink plenty of water throughout the day. Consider adding sugar-free electrolyte powder to your water, especially if you exercise.
Eat Enough Fat: Ensure you are consuming adequate calories from healthy fats. Undereating can worsen fatigue. Healthy fats include avocado oil, olive oil, and fatty fish like salmon.
Take it Easy: Avoid intense workouts during the first one to two weeks. Opt for gentle activities like walking or yoga to allow your body to adapt without added stress.
Transition Gradually: Instead of drastically cutting carbs, some people find success by slowly reducing their intake over a week or two. This can make the metabolic shift less jarring.

Potential Long-Term Health Concerns and Considerations

While the initial keto flu is temporary, there are other potential health risks associated with a restrictive ketogenic diet, particularly if followed for an extended period without medical supervision.

Nutrient Deficiencies: The elimination of many fruits, starchy vegetables, and whole grains can lead to deficiencies in essential micronutrients, including B vitamins, vitamin C, calcium, and magnesium.
Kidney Stones: A high-fat, low-carbohydrate diet has been linked to an increased risk of developing kidney stones.
Liver Issues: With so much fat to metabolize, the diet can potentially exacerbate existing liver conditions or cause a buildup of fat in the liver over time.
Constipation: The lack of fiber from grains and many plant-based foods can cause persistent constipation for some individuals.
Heart Health: A diet high in saturated fat can raise 'bad' LDL cholesterol levels, which is a risk factor for heart disease. It is essential to focus on heart-healthy fats and monitor cholesterol with a healthcare provider.

Comparison: Initial Adaptation vs. Long-Term Health


Aspect	Initial Adaptation (Keto Flu)	Long-Term Health Risks
Cause	Metabolic shift from glucose to fat, electrolyte loss	Nutrient deficiencies, dietary restrictions, saturated fat intake
Duration	Temporary (days to a few weeks)	Can persist indefinitely or develop over time
Symptoms	Fatigue, headache, nausea, brain fog, cramping	Kidney stones, micronutrient deficiencies, high LDL cholesterol, liver issues
Management	Hydration, electrolyte intake, reduced exercise	Medical supervision, nutrient planning, careful fat source selection
Outlook	Symptoms resolve as the body adapts to ketosis	Requires careful monitoring and consideration of overall health impacts

Conclusion: Navigating the Challenges of a Keto Diet

So, can keto make you feel unwell? The answer is a clear yes, especially during the initial phase as your body adapts to ketosis. For many, this phase, known as the keto flu, is a temporary but unpleasant side effect marked by fatigue, headaches, and digestive issues. However, by prioritizing proper hydration, electrolyte replenishment, and gentle physical activity, you can significantly mitigate these initial symptoms.

Looking beyond the initial phase, long-term adherence to a restrictive diet like keto introduces additional considerations, including potential nutrient deficiencies, kidney and liver stress, and impacts on heart health. This is why consulting a healthcare professional is crucial before and during any significant dietary change. For most people, a ketogenic diet is not the only path to health and weight management, and less restrictive, more sustainable approaches may offer similar benefits with fewer risks.

For a deeper look into the risks and potential complications, consult a reputable source like the article on ketogenic diet side effects from Medical News Today. Ultimately, a well-informed and cautious approach is key to determining if keto is the right and safest choice for your body.

Frequently Asked Questions

The 'keto flu' is the common term for the collection of flu-like symptoms, such as fatigue, headaches, and irritability, that some people experience as their body transitions to using fat for fuel instead of glucose.

For most people, the symptoms are temporary and should improve within a few days to a couple of weeks. As your body becomes more efficient at using ketones for energy, the symptoms typically subside.

Headaches are often a result of dehydration and electrolyte imbalances, particularly low sodium and potassium, which are common as the body sheds excess water during the initial phase of ketosis.

Yes, both are possible. Constipation can result from a lack of dietary fiber, while diarrhea can be caused by the body struggling to adapt to a high intake of dietary fat.

No, the 'keto flu' is a temporary and mild condition that occurs during metabolic adaptation. Diabetic ketoacidosis is a life-threatening medical emergency involving dangerously high ketone levels and is typically associated with type 1 diabetes.

To replenish electrolytes, increase your intake of sodium (salt your food), potassium (avocado, leafy greens), and magnesium (nuts, seeds). Bone broth and sugar-free electrolyte drinks are also effective.

Potential long-term risks can include nutrient deficiencies, kidney stones, liver issues, and increased LDL cholesterol, especially if not well-managed with a variety of healthy fats and medical supervision.