Why do I feel weird on ketosis? Understanding the 'Keto Flu'

October 25, 2025 •

4 min read

Up to 25% of people starting a ketogenic diet report flu-like symptoms. This common phenomenon, the 'keto flu,' is a primary reason for feeling weird on ketosis during the initial metabolic change.

Quick Summary

The 'keto flu' causes many people to feel unwell at the start of a ketogenic diet. This happens as the body adapts to burning fat instead of carbohydrates, due to electrolyte imbalances, dehydration, and carb withdrawal, resulting in fatigue, headaches, and brain fog. Addressing these factors can provide relief.

Key Points

Metabolic Shift: Feeling weird is a normal side effect of your body's switch from burning glucose (carbohydrates) to burning fat and producing ketones for energy.
Electrolyte Imbalance: The most significant cause of keto flu symptoms is the loss of electrolytes (sodium, potassium, magnesium) due to increased urination during the initial phase.
Common Symptoms: Expect temporary fatigue, headaches, brain fog, irritability, and muscle cramps as your body adapts to the new fuel source.
Remedy with Electrolytes: Combat symptoms by increasing your intake of sodium, potassium, and magnesium through foods like bone broth, avocados, and leafy greens, or with supplements.
Stay Hydrated: Drink plenty of water throughout the day, as dehydration can worsen many keto flu symptoms.
Exercise Moderately: Avoid strenuous workouts during the adaptation period and stick to light activities to conserve energy.
Seek Medical Advice When Necessary: Consult a doctor if symptoms are severe, prolonged, or if you have pre-existing health conditions that could be impacted by the diet.

The Metabolic Shift: From Sugar to Fat

For years, the body has used glucose, from carbohydrates, as its main fuel. Cutting carbs drastically on a ketogenic diet forces the body to use another energy source. This tells the liver to break down fat into ketones, which are used as fuel, a state known as ketosis. This shift does not happen immediately; it takes days to weeks for the body and brain to adapt to using ketones. The uncomfortable feelings often arise while the body adjusts to this new fuel source and experiences withdrawal.

The Critical Role of Electrolytes and Hydration

A significant reason for feeling weird on ketosis is the loss of water and electrolytes, a result of low carb intake. When carbs are restricted, insulin levels decrease. This tells the kidneys to excrete more water and sodium, leading to dehydration. The body also loses other crucial electrolytes: potassium and magnesium. This depletion drives many keto flu symptoms:

Sodium: A lack of sodium causes headaches, fatigue, and weakness.
Potassium: Low potassium results in muscle cramps and heart palpitations.
Magnesium: Insufficient magnesium contributes to muscle aches, headaches, and sleep disturbances.

Staying hydrated is essential, but it must be balanced with sufficient electrolyte intake. Simply drinking more water without replenishing lost minerals can worsen the electrolyte imbalance and the associated symptoms.

Common Symptoms You Might Experience

The 'keto flu' symptoms vary, but many report similar side effects in the first week or two. Recognizing these symptoms helps you manage them:

Fatigue and Lethargy: A general feeling of low energy is common as the body searches for a new energy source.
Headaches: Often linked to dehydration and electrolyte imbalance, headaches are frequent.
Brain Fog: The brain's difficulty switching from glucose to ketones causes a fuzzy feeling.
Irritability and Mood Swings: Carb withdrawal and hormonal shifts can affect mood.
Digestive Issues: Changes in fat and fiber intake cause constipation or diarrhea.
Muscle Cramps: A lack of potassium and magnesium is a common cause of cramping, especially in the legs.
Difficulty Sleeping: Some experience insomnia or disturbed sleep during the transition.

Effective Strategies to Combat Keto Flu Symptoms

These symptoms are temporary and manageable with key strategies.

Replenish Your Electrolytes

Increase Sodium: Use high-quality salt, such as Himalayan pink salt, on food. Drinking bone broth is a good way to get more sodium and other minerals.
Boost Potassium: Eat potassium-rich, keto-friendly foods like avocados, spinach, mushrooms, and salmon.
Consume Magnesium: Eat foods high in magnesium, such as pumpkin seeds, almonds, and dark leafy greens. Magnesium supplements can also help.

Prioritize Hydration

Drink plenty of water throughout the day. Check your urine color; it should be a pale yellow. If you exercise or are in a hot climate, fluid and electrolyte needs increase.

Moderate Your Physical Activity

During adaptation, energy levels for strenuous exercise may be lower. Stick to lighter activities like walking, stretching, or yoga for the first few weeks. As the body becomes fat-adapted, performance and endurance should return.

Optimize Your Fat Intake

The body needs healthy fats to produce ketones. Focus on sources like avocados, nuts, seeds, and healthy oils such as olive and coconut oil. Some report digestive issues with high amounts of MCT oil initially, so start with smaller quantities.

Ensure Adequate Sleep

Fatigue and irritability are often made worse by lack of sleep. Aim for 7-9 hours of quality sleep nightly. A consistent sleep schedule and limiting screen time before bed can help.

Keto Flu vs. Keto-Adaptation: A Comparison


Feature	Keto Flu (Initial Phase)	Keto-Adaptation (Achieved State)
Timing	First few days to weeks	After the initial phase; can take 1-4 weeks or longer
Fuel Source	Body is transitioning from glucose to ketones	Body is efficiently using ketones for fuel
Energy Levels	Low, fatigue, lethargy	Increased, stable energy
Mental State	Brain fog, irritability, poor focus	Improved mental clarity and focus
Physical Performance	Decreased endurance and strength	Improved performance and endurance
Symptoms	Headaches, cramps, nausea, cravings	Symptoms resolve, often leading to better overall feelings
Hunger	Unstable appetite, potential cravings	Suppressed appetite, more stable hunger hormones
Hydration Status	Tendency towards dehydration	Normalizes with proper electrolyte management

When to Consult a Doctor

While the keto flu is generally a temporary inconvenience, be aware of your body's signals. If symptoms are severe or don't improve after a few weeks, or if you have pre-existing health conditions like diabetes, kidney disease, or heart conditions, consult a doctor. Dizziness, fainting, extreme fatigue, or persistent digestive problems can indicate a need for medical advice. You can also explore reputable resources like this guide from Harvard Health for more information.

Conclusion: Patience is Key

Feeling "weird" on ketosis is a normal part of the metabolic adaptation. It is a sign of a successful shift from glucose to fat. By prioritizing hydration, replenishing electrolytes, moderating exercise, and allowing the body time to adjust, you can reduce or eliminate keto flu symptoms. The discomfort is a temporary price for the long-term benefits of ketosis, including increased mental clarity and stable energy levels. Stay patient and consistent, and your body will reward you.

Frequently Asked Questions

The 'keto flu' is temporary, flu-like symptoms that some experience during the initial phase of a ketogenic diet. It happens as the body adapts from using glucose to burning ketones from fat.

Common symptoms include fatigue, headaches, brain fog, irritability, muscle cramps, digestive issues like constipation or diarrhea, and sometimes difficulty sleeping.

To alleviate symptoms, focus on replenishing electrolytes (sodium, potassium, magnesium), staying well-hydrated with plenty of water, getting adequate rest, and eating enough healthy fats to fuel your body.

For most, the keto flu resolves within a few days to a week or two. It may last longer, but symptoms typically fade as the body becomes fully keto-adapted.

While electrolytes can often be obtained through diet by consuming mineral-rich foods, supplements are a convenient way to meet increased needs, particularly if very active.

You may be able to minimize or prevent the keto flu by transitioning into the diet more gradually, rather than cutting carbs completely all at once. Ensuring you stay well-hydrated and consume sufficient electrolytes from day one can also help.

Consult a doctor if you have underlying medical conditions (like diabetes or heart disease) or if you experience severe symptoms like high fever, prolonged vomiting, or confusion.