What is the keto flu?
The keto flu refers to the cluster of symptoms that show up in the first 2-7 days of starting a ketogenic diet. It's not an actual flu: no virus, no infection. It's your body adapting to using ketones instead of glucose as its primary fuel.
The 7 most common symptoms
- Intense fatigue — muscle glycogen is depleted, ketones aren't being produced in volume yet
- Headaches — usually starting day 2, a sign of electrolyte loss
- Muscle cramps — magnesium and potassium deficiency
- Brain fog — your brain is asking for its usual glucose
- Irritability — drop in serotonin tied to lower carbs
- Nausea — digestive adjustment to higher fat intake
- Trouble sleeping — cortisol fluctuations
How long does it last?
For most people, the keto flu starts 24-48 hours after dropping below 20g net carbs and lasts 3 to 5 days. With the remedies below, you can shorten it to 24-48 hours. Without doing anything, it can drag on for a full week.
Why these symptoms happen
When you cut carbs, your body releases a lot of water (1g of glycogen holds 3g of water). With that water, you also lose sodium, potassium, and magnesium through urine. This electrolyte loss causes 90% of symptoms — not ketosis itself.
The 5 proven remedies
1. Salt your food generously
Add 2 to 4g of salt per day to your meals. Not refined table salt: pink Himalayan, sea salt, or kosher salt. Salted bone broth in the morning is a magic recipe.
2. Magnesium at bedtime
Supplement with 300-400mg of magnesium bisglycinate before bed. Reduces nighttime cramps and improves sleep. Avoid magnesium oxide (poorly absorbed).
3. Potassium through food
Avocado (700mg), cooked spinach (840mg), salmon (500mg), mushrooms. Avoid OTC potassium supplements without medical advice (cardiac risk at high doses).
4. Double your hydration
Drink 2.5 to 3.5L of water per day. In the morning, add a pinch of salt and a squeeze of lemon: it's the homemade equivalent of a sugar-free electrolyte drink.
5. Eat enough fat
Many beginners fall into an overly aggressive caloric deficit at the start. Hit your maintenance calories the first week. Your body needs fuel to produce ketones.
When to see a doctor
The keto flu is benign. But consult if: symptoms last beyond 10 days, severe dizziness, heart palpitations, or if you take medications (diabetes, blood pressure, antidepressants). Keto changes the absorption of several treatments.
How to avoid it next time
If you've done keto before and you're restarting after a break, the flu comes back every time (unless the break was less than 2 weeks). To minimize: ramp down carbs gradually over 5 days instead of a hard cut, and start electrolytes from day one.
A good keto plan prevents the keto flu
For the full picture on this topic, check our complete keto diet guide: the science, macros, food list, 7-day plan and FAQ in one place.
A well-balanced plan naturally includes the right electrolyte sources. The Cetona app includes avocado, spinach, salmon, and bone broth in your first days to minimize symptoms. You focus on adapting, not on calculations.