Is it Normal to Get Sick on Keto? What the 'Keto Flu' Really Means

January 5, 2026 •

4 min read

It is a fact that up to half of all people starting a ketogenic diet experience flu-like symptoms, a phenomenon commonly known as the 'keto flu'. While disconcerting, getting sick on keto is a normal and temporary side effect of your body transitioning from burning glucose for energy to burning fat.

Quick Summary

The 'keto flu' refers to the temporary, flu-like symptoms that can occur when starting a ketogenic diet, caused by the body's metabolic adjustment. This shift, combined with electrolyte imbalances and carbohydrate withdrawal, triggers common side effects like headaches, fatigue, and nausea, which are typically short-lived and manageable with proper care.

Key Points

Keto Flu is Normal: Experiencing sickness on keto, known as the 'keto flu,' is a normal and temporary side effect of your body adapting to ketosis.
Electrolytes are Key: Dehydration and electrolyte imbalances are the primary causes of keto flu symptoms like headaches, fatigue, and muscle cramps.
Manage Symptoms with Hydration and Salt: Combat keto flu by drinking plenty of water, consuming electrolytes (salt, potassium, magnesium), and enjoying bone broth.
Ease the Transition: Consider a gradual reduction in carbs instead of an abrupt change to lessen the shock to your system.
Listen to Your Body: Rest, engage in light exercise, and consult a doctor if symptoms are severe or prolonged, especially if you have a fever.
Prepare for Side Effects: Acknowledge that short-term symptoms are possible but manageable, and that long-term monitoring for nutrient deficiencies and lipid changes is wise.

Understanding the 'Keto Flu'

When you drastically reduce carbohydrate intake, your body is forced to find a new primary fuel source. It begins breaking down fat for energy, a metabolic state known as ketosis. This transition is a significant shock to the system, causing a variety of physiological changes that can manifest as unpleasant, temporary symptoms. The term 'keto flu' is a misnomer, as it's not a viral illness but rather a period of metabolic adaptation. The severity and duration of symptoms vary widely among individuals; some may experience no issues at all, while others feel quite unwell for several days or weeks.

What Causes Keto Flu Symptoms?

Several factors contribute to the keto flu experience:

Carbohydrate Withdrawal: For those accustomed to a high-carb diet, removing simple sugars and starches can cause withdrawal-like symptoms. Your body and brain are used to a quick, constant supply of glucose, and the adjustment period can lead to fatigue, brain fog, and irritability.
Electrolyte Imbalance: As insulin levels drop on a low-carb diet, the kidneys excrete more sodium and water from the body. This rapid fluid loss can lead to imbalances of key electrolytes like sodium, potassium, and magnesium, which are essential for nerve and muscle function. This deficiency can cause muscle cramps, headaches, and dizziness.
Dehydration: The initial depletion of glycogen stores (stored carbohydrates), which bind to water, causes the body to shed a significant amount of water weight. This, combined with increased urination, can lead to dehydration if fluid intake is not prioritized.
Adjustment to Ketones: It takes time for the brain and muscles to efficiently use ketones as a fuel source. During this adaptation phase, you may experience temporary lethargy and reduced physical performance until your body becomes 'fat-adapted'.

How to Manage and Alleviate Keto Flu Symptoms

Fortunately, the keto flu is temporary, and there are several strategies to help you manage and reduce its effects:

Stay Hydrated: Drink plenty of water throughout the day. Since the keto diet can have a diuretic effect, you will need more fluids than usual to compensate for the water and electrolyte loss.
Increase Electrolyte Intake: Actively replenish lost electrolytes. This can be done by:
- Adding more salt to your food.
- Consuming electrolyte supplements or sugar-free electrolyte drinks.
- Incorporating potassium-rich, keto-friendly foods like avocados, leafy greens, and mushrooms.
- Drinking bone broth, which provides both sodium and other minerals.
Don't Skimp on Healthy Fats: Ensuring you consume enough healthy fats provides your body with the necessary fuel to transition to fat-burning more smoothly. Healthy fat sources include avocados, nuts, seeds, and fatty fish.
Ease Into It: If you find the transition too abrupt, consider gradually reducing your carbohydrate intake over a week or two instead of going 'cold turkey.' This can give your body more time to adapt without severe symptoms.
Get Adequate Rest: The initial fatigue and irritability can be demanding. Prioritize getting enough sleep (7-9 hours) to support your body's adaptation process.
Incorporate Light Exercise: Avoid strenuous workouts during the first week. Instead, opt for light activities like walking, yoga, or stretching, which can help boost mood and energy without overtaxing your system.

Keto Flu vs. Actual Illness: A Comparison

It is important to differentiate between keto flu symptoms and a real illness. Here is a comparison to help clarify:


Symptom	Keto Flu	Viral Illness (e.g., Cold/Flu)
Fever	Rare or Absent	Common
Nausea & Vomiting	Possible, especially early on	Common
Fatigue	Common, due to metabolic shift	Common, due to immune response
Headache	Common, often due to dehydration	Common
Body Aches	Possible (muscle cramps)	Common, widespread
Sore Throat	Rare	Common
Runny/Stuffy Nose	Rare	Common
Duration	Typically a few days to a week	Varies, can last a similar duration
Cause	Metabolic adaptation, electrolyte imbalance	Viral infection

If your symptoms are severe, persistent beyond a few weeks, or include a fever, it is wise to consult a healthcare provider to rule out other medical conditions.

Are There Long-Term Side Effects of the Keto Diet?

While the keto flu is a short-term issue, some individuals may experience other side effects, especially if the diet is not managed correctly. Potential long-term concerns include:

Nutrient Deficiencies: The restrictive nature of the diet can lead to low intake of essential vitamins and minerals found in fruits and certain vegetables. Supplementation or careful meal planning is often required.
Kidney Stones: Increased ketone production can alter urine pH, which in some cases, can increase the risk of kidney stone formation.
High Cholesterol: Some people may experience an increase in LDL ('bad') cholesterol, particularly those consuming large amounts of saturated fat. Monitoring lipid panels with a doctor is important.
Digestive Issues: Constipation or diarrhea can persist in some individuals, often related to inadequate fiber and changes in gut bacteria.

The Role of Health Professionals

Before starting a ketogenic diet, especially if you have pre-existing health conditions like diabetes, kidney disease, or heart issues, it is crucial to consult a healthcare provider or a registered dietitian. A professional can help ensure the diet is safe for you and can guide you on managing potential side effects. They can also help create a personalized plan to prevent nutrient deficiencies.

Conclusion

In conclusion, feeling unwell or getting 'sick' on keto is a common and normal part of the body's metabolic transition, known as the 'keto flu.' It is not a true illness but rather a temporary period of adjustment marked by symptoms like fatigue, headaches, and nausea. By prioritizing hydration, replenishing electrolytes, consuming enough healthy fats, and getting adequate rest, you can significantly mitigate these effects. Most people find that symptoms subside within a week or two as their body adapts. However, if symptoms are severe or prolonged, consulting a healthcare professional is advisable. A smooth transition and positive long-term outcome depend on understanding these normal side effects and managing them effectively. For further reading, consult the National Institutes of Health.

National Institutes of Health (NIH)

Frequently Asked Questions

The keto flu usually lasts a few days to a week for most people, though it can persist for a few weeks in some cases. Symptoms generally peak within the first 2 to 4 days and then gradually subside as your body becomes fat-adapted.

The main cause is metabolic adaptation. When you cut carbs, your body shifts from burning glucose to burning fat for fuel. This transition, combined with water and electrolyte loss, triggers flu-like symptoms.

Common symptoms include headaches, fatigue, brain fog, nausea, irritability, muscle cramps, and difficulty sleeping. Some people may also experience digestive issues like constipation or diarrhea.

While not always preventable, you can significantly minimize symptoms by staying well-hydrated, replenishing electrolytes, and easing into the diet gradually rather than all at once. Some people with higher 'metabolic flexibility' may not experience it at all.

Focus on electrolyte-rich foods and supplements. Options include bone broth, avocado, leafy greens, salted foods, and nuts. Ensure you are consuming enough healthy fats to fuel your body and reduce cravings.

Yes, they are completely different. Keto flu is a mild, temporary side effect of dietary change. Ketoacidosis is a life-threatening medical emergency involving dangerously high ketone and blood sugar levels, most common in people with uncontrolled Type 1 diabetes.

You should see a doctor if your symptoms are severe, include a fever, last longer than a few weeks, or if you have pre-existing conditions like diabetes, kidney, or heart disease. It’s important to rule out other medical issues.

Yes, light exercise like walking or yoga can help improve symptoms. It can boost mood and energy levels without placing excessive stress on your body during the adaptation phase. Avoid intense workouts until you feel better.