Quick answer: Keto flu is a set of temporary symptoms some people feel in the first days to weeks of a ketogenic diet, such as fatigue, headache, dizziness, irritability, brain fog, nausea and muscle cramps. It isn’t an infection. It’s thought to be linked to fluid and electrolyte losses and the body adjusting to burning fat. Hydration, electrolyte-rich foods and easing into keto usually help.
Key takeaways
Keto flu can bring fatigue, headache, dizziness, irritability, brain fog, nausea and muscle cramps in the first days.
It often comes from fluid and electrolyte shifts as glycogen is used up and the kidneys release more sodium.
Steady water, electrolyte-rich foods and lowering carbohydrates gradually can ease it, while severe or lasting symptoms need medical attention.
Symptoms
Symptoms can include fatigue, headache, dizziness, irritability, brain fog, nausea and muscle cramps.
Why it happens
Fluid and electrolyte shifts
As glycogen is used up, water is released, and lower insulin makes the kidneys excrete more sodium. Potassium and magnesium can drop too. See balancing electrolytes.
Switching fuel sources
There can be a lag while your body ramps up making and using ketones. See what is ketosis.
Carbohydrate withdrawal
Cutting sugar and refined carbs abruptly can bring cravings and irritability.
How to ease it
Drink water steadily through the day, and eat electrolyte-rich foods such as leafy greens, avocado, nuts and broth. Ease in by lowering carbs over a week or two, rest, sleep and keep exercise gentle at first, and avoid common keto mistakes.
When symptoms aren’t keto flu
Symptoms that are severe, last more than a couple of weeks, or include vomiting, confusion or a racing heart need medical attention, especially if you have diabetes.
Healthy aging lens
Older adults and people on blood pressure medication or diuretics are more prone to electrolyte problems, so build up slowly and check with your provider.
What the research shows
“Keto flu” has been described mostly through personal reports, with few formal studies.
What “keto flu” looks like (Bostock 2020, analysis of online reports). Researchers reviewed 448 posts from 300 people in 43 online forums. The most common complaints were flu-like feelings, headache, fatigue, nausea, dizziness, brain fog and stomach upset, mostly in the first week and fading by about 4 weeks; the few people who reported when symptoms ended said it took a median of 4.5 days. A limitation is that these were self-reports on the internet, not a clinical study, so true frequency is unknown.
Six weeks of keto in healthy adults (Urbain 2017, before-and-after study). 42 healthy adults followed a ketogenic diet (about 72% fat, 21% protein and 8% carbohydrate) without calorie limits for 6 weeks. They lost about 2 kg, split equally between fat and lean mass. Peak power dropped 4%, LDL cholesterol rose about 11%, and insulin fell 22%. A limitation is that there was no control group, so changes can’t be fully separated from other factors.
Strength of the evidence: Evidence is limited: most information comes from self-reports, and controlled studies rarely track these symptoms.
When to see a naturopathic doctor
It may be time to see a naturopathic doctor if you have diabetes or take insulin, sulfonylureas or SGLT2 inhibitors (risk of low blood sugar or ketoacidosis), or if you take blood pressure medication or diuretics. The same is true if you have kidney, liver, gallbladder or pancreas disease, or high cholesterol, if you are pregnant, breastfeeding, or have a history of disordered eating, or if you want a keto plan tailored to your labs and goals.
See a medical provider promptly for nausea and vomiting, abdominal pain, confusion, fruity-smelling breath with rapid breathing, or very high blood sugar or ketone readings, which can signal ketoacidosis.
Want a keto plan that’s safe for your health?
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Questions to bring to your clinician
These questions can help you get the most out of your next visit.
Am I more prone to electrolyte problems on keto because of my medications?
How can I ease keto flu symptoms in my first week?
Could electrolytes help me feel better during the transition?
Which symptoms would mean I should check in with you rather than push through?
Frequently asked questions
What is the keto flu?
Temporary symptoms while the body adapts to keto.
How long does keto flu last?
Usually a few days to a couple of weeks.
How do I get rid of keto flu?
Hydrate, get electrolytes from food and ease in.
Is keto flu dangerous?
Usually not, but severe or lasting symptoms need medical care.
Sources
- Bostock ECS, et al. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet. Front Nutr 2020. PMID: 32232045. https://pmc.ncbi.nlm.nih.gov/articles/PMC7082414/
- Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr 1992. PMID: 1615908. https://pubmed.ncbi.nlm.nih.gov/1615908/
- Brands MW, Manhiani MM. Sodium-retaining effect of insulin in diabetes. Am J Physiol Regul Integr Comp Physiol 2012. PMID: 23034715. https://pmc.ncbi.nlm.nih.gov/articles/PMC3533616/
- Filippatos TD, et al. Hyponatremia in the elderly: challenges and solutions. Clin Interv Aging 2017. PMID: 29180859. https://pmc.ncbi.nlm.nih.gov/articles/PMC5694198/
- MedlinePlus: Fluid and Electrolyte Balance
- MedlinePlus: Sodium
- Bostock ECS, et al. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet. Front Nutr 2020. PMID: 32232045. https://pubmed.ncbi.nlm.nih.gov/32232045/
- Urbain P, et al. Impact of a 6-week non-energy-restricted ketogenic diet on physical fitness, body composition and biochemical parameters in healthy adults. Nutr Metab (Lond) 2017. PMID: 28239404. https://pubmed.ncbi.nlm.nih.gov/28239404/
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before starting supplements or changing medications, especially if you are pregnant, nursing, or managing a health condition.
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