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 rapid 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. Sources

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 → you excrete more sodium/fluid; glycogen drops (and with it stored water), and your body adapts to burning fat/ketones.
• Relief basics: Prioritize electrolytes (sodium, potassium, magnesium), fluids, adequate calories/protein, and sleep. Consider tapering carbs over 1–2 weeks instead of “cold turkey.”
• When to call a clinician: Severe or persistent vomiting, diarrhea, dizziness/fainting, heart palpitations, or if you have chronic conditions (especially if using insulin or SGLT2 meds).

Common Keto Flu Symptoms

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

What Causes Keto Flu?

Early keto induction lowers insulin and depletes glycogen—a stored form of carbohydrate that binds water. As glycogen falls, you can lose several pounds of water weight quickly, along with sodium and other electrolytes. That “natriuresis” plus the metabolic switchover to fat/ketones explains most keto-flu symptoms. Replacing fluids and electrolytes and giving your body time to adapt usually solves it. See mechanistic & clinical sources

How to Prevent & Ease Keto Flu

• Salt & fluids: Add broth or mineral water and salt foods to taste, especially the first 1–2 weeks. If you have hypertension, kidney, or heart issues, ask your clinician how much sodium is right for you.
• Potassium & magnesium from foods: Leafy greens, avocado, nuts/seeds, beans (if you include them), and low-sugar electrolyte mixes (watch added sugars).
• 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; don’t lean solely on caffeine for “fixes.”

FAQs

How long does keto flu last?

Most people improve within 3–7 days. A minority report symptoms up to 2–4 weeks while fully adapting. If symptoms persist or are severe, check in with your clinician. Evidence

Is keto flu dangerous?

Usually no—it’s uncomfortable but self-limited. However, severe dehydration/electrolyte imbalance can be risky. People with diabetes (especially on insulin) or on SGLT2 medications should involve their care team to avoid complications.

Do electrolytes really help?

Yes—many symptoms stem from sodium/fluid losses. Moderate electrolyte repletion (with low-sugar options) often helps. Avoid megadoses; too many electrolytes can also be harmful. See guidance

Do I have to be strict keto to lose weight?

No. Plenty of people lose fat with moderate carb reduction and higher protein/fiber. If strict keto doesn’t feel good after a fair trial, consider a sustainable middle ground.

Make It Easier (Food & Electrolytes)

Want protein-forward meals while you adjust carbs? Explore our rotating High-Protein Meal Plans, customize plates with Build-a-Meal Plan, and check Nutrition Info. For hydration tips, see How to Replenish Electrolytes Naturally After Exercise.

Related Reads

• 7 Keto-Approved Recipes for the Whole Family
• What Alcohol Can You Drink on Keto?
• What Is Keto Bread?
• Best & Worst Snack Foods

References

• StatPearls — Ketogenic Diet: typical short-term adverse effects (“keto flu”), hydration/electrolytes help, and expected resolution. ncbi.nlm.nih.gov
• Harvard Health — What is keto flu? Onset in 2–7 days; common symptoms. health.harvard.edu
• Cleveland Clinic — Ketosis overview and keto-flu symptoms. my.clevelandclinic.org
• Peer-reviewed analysis of consumer reports — Symptoms peak in week 1 and decline by week 4. Bostock et al., 2020
• Glycogen & water loss mechanism (why the scale drops fast): ~3 g water per gram glycogen. Murray, 2018
• Cleveland Clinic — Practical electrolyte sources & low-sugar options; avoid overdoing electrolytes. health.clevelandclinic.org