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Keto flu: what it is, why it happens and how to overcome it
Published on June 16, 2026
Headaches, fatigue and irritability in the first days of the ketogenic diet? It's the so-called 'keto flu'. Here's why it happens and how to handle it simply.
Have you started the ketogenic diet a few days ago and feel tired, with a headache and little clarity? You're probably experiencing the so-called "keto flu". It's not a real flu and, above all, it's not a sign that you're doing something wrong: it's a temporary transition phase. Let's look at why it happens and how to overcome it quickly.
What the keto flu is
In the first days of keto the body switches from using glucose to using fats (and ketones) as fuel. This "engine change" can cause temporary symptoms:
- headache
- fatigue and weakness
- irritability and poor concentration
- muscle cramps
- sugar cravings
They usually appear between the 2nd and 4th day and last from a few days to a week.
Why it happens (the real cause)
When you reduce carbs, insulin drops (on why, see glucose and insulin explained simply). With low insulin, the kidneys eliminate more water and, with it, important minerals: sodium, potassium and magnesium. Most of the keto flu symptoms are actually a deficiency of these electrolytes, not "lack of sugar."
How to overcome it
- Replenish electrolytes. This is move number one: sodium (even just more salt in your meals), potassium and magnesium. A good electrolyte supplement makes everything easier.
- Drink enough. More water, but together with the salts (drinking a lot without salts can make things worse).
- Don't be afraid of fats. They're your new fuel: eating enough of them reduces fatigue and hunger.
- Rest and take it easy with training in the first days.
- Give your body time: it's an adaptation phase, it passes on its own.
When it passes
For most people the keto flu disappears within 1 week, giving way to stable energy and less hunger. If the symptoms are intense or last a long time, talk to a doctor.
Article for informative and educational purposes; it does not replace the advice of a doctor or nutritionist. Some links are affiliate links: by buying through them we may receive a small commission, at no cost to you.
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