What Is the Keto Flu? Symptoms, Causes & Relief

Quick Answer

“Keto flu” is a cluster of temporary symptoms—fatigue, headache, brain fog, irritability, lightheadedness, nausea, constipation, muscle cramps, and poor exercise tolerance—that some people experience in the first 2–7 days after starting a very low-carb or ketogenic diet. It’s linked to shifts in fluids and electrolytes and the metabolic switch to using ketones. Symptoms usually ease within several days to a few weeks with smart hydration, electrolytes, sleep, and a gradual carb taper.

Key Takeaways (AEO)

• Typical timing: Onset in days 2–7; peaks in week 1; improves over 1–4 weeks for most.
• Why it happens: Lower insulin → glucose stores drop, and your body adapts by burning fat. Ketones are produced during this process and used for energy.
• Relief basics: Prioritize electrolytes (sodium, potassium, and magnesium), fluids, adequate calories/protein, and sleep. Consider tapering carbs over 1–2 weeks instead of going “cold turkey.”
• When to call a clinician: Severe or persistent vomiting, diarrhea, dizziness/fainting, and heart palpitations. Anyone with a chronic health condition should not attempt a keto diet without first discussing it with their doctor.

Common Keto Flu Symptoms

• Fatigue, reduced exercise tolerance 
• Dizziness/lightheadedness, brain fog, headache
• Irritability, insomnia
• Nausea, constipation or diarrhea, abdominal discomfort
• Muscle cramps/aches

What Causes Keto Flu?

Early keto induction lowers insulin and drains glycogen—a stored form of carbohydrate that binds water. As glycogen falls, you can lose some water weight quickly. Along with this, you can lose sodium and other electrolytes (like potassium and magnesium). The electrolyte depletion, plus the metabolic switchover to fat/ketones, are potential factors that can contribute to keto-flu symptoms. Replacing fluids and electrolytes and giving your body time to adapt usually solves it.

How to Prevent & Ease Keto Flu

• Sodium & fluids: Drink adequate fluids and add broth or salted foods, especially in the first 1–2 weeks. Make sure to ask your clinician how much sodium is right for you. As the keto flu eases up, it's best to cut back on the salt!
• Potassium & magnesium from foods: Make sure to include leafy greens, avocado, nuts/seeds, and beans (beans are higher in carbs, so they should be included in limited amounts).
• Protein & calories: Don’t undereat. Aim for a protein source at every meal to support satiety and muscle.
• Taper, don’t crash: Reduce carbs gradually over 1–2 weeks and/or keep some carbs around workouts while you adapt.
• Train smart: Keep high-intensity efforts short the first week; favor walking, mobility, and easy rides/runs until energy normalizes.
• Sleep & caffeine: Prioritize 7–9 hours; keep caffeine moderate.

FAQs

How long does keto flu last?

Most people improve within 3–7 days. Some may experience symptoms for up to 2–4 weeks while fully adapting. If symptoms persist or are severe, check in with your clinician.

Is keto flu dangerous?

Usually no—but it can be uncomfortable. However, severe dehydration/electrolyte imbalance can be risky. Make sure to discuss your symptoms with your physician if needed.

Do electrolytes really help?

They can—make sure to include whole foods that are sources of important electrolytes.

Do I have to be strict keto to lose weight?

No! Plenty of people lose weight with a more moderate carb reduction and a higher protein/fiber intake. If strict keto doesn’t feel good to you, consider a sustainable middle ground.

Make It Easier (Food & Electrolytes)

Want protein-forward meals while you adjust your carb intake? Explore the Clean Eatz rotating High-Protein Meal Plans or customize your plate with the Build-a-Meal Plan. Check out the Nutrition Info for all Clean Eatz options to help guide your choices.

Related Reads

• How to Replenish Electrolytes Naturally After Exercise
• 7 Keto-Approved Recipes for the Whole Family
• What Alcohol Can You Drink on Keto?
• What Is Keto Bread?
• Best & Worst Snack Foods

Additional Resources

• JAMA Patient Page — Ketogenic Diets jamanetwork.com
• Harvard Health Publishing — What is keto flu? health.harvard.edu
• Cleveland Clinic's Health Library — Ketosis my.clevelandclinic.org