You decided to start keto. You cut back on carbs, cleaned up your meals, and maybe the scale has already moved. You're feeling good about it and ready to keep going.
Then a few days in, you have a headache. You're tired. Your workout feels harder than usual, it's tough to concentrate, or you just don't feel like yourself.
You may be going through what's commonly called the "keto flu." It's common, it's usually temporary, and there's a lot you can do to make it easier.
What is the keto flu?
Keto flu isn't actually the flu. It's an informal name for a group of temporary symptoms some people get while their body adjusts to eating far fewer carbohydrates and moving toward nutritional ketosis.
Research has reported these symptoms starting within the first few days of a ketogenic diet. For many people they improve as the body adapts, though how long they last varies a lot from person to person.
What does keto flu feel like?
Everyone's experience is different. Some people move into keto without noticing much at all. Others may have:
- Headaches
- Fatigue or low energy
- Lightheadedness or dizziness
- Brain fog or trouble concentrating
- Muscle cramps
- Weakness
- Mood changes
- Lower exercise performance
- Constipation
- Nausea or other digestive changes
- Increased thirst
- Bad breath
A 2025 scoping review of the research listed headache, lightheadedness, fatigue, lethargy, brain fog, decreased exercise capacity, mood changes, constipation, muscle cramps, diarrhea, and bad breath among the symptoms reported when people start keto.
Why does keto flu happen?
There probably isn't one single cause. Starting keto changes several things at once: your body's main fuel, your glycogen stores, how much water and how many electrolytes you hold onto, and often the amount and type of food you eat.
Researchers believe several of these changes contribute to keto flu, though the exact mechanism hasn't been pinned down for every symptom.
1. Your body is changing its fuel source
If you've been eating plenty of carbohydrates, your body has had a steady supply of glucose for energy.
When carb intake drops sharply, less glucose is available, and your body starts relying more on fat. (Here's more on how lower glucose helps you reach ketosis.) Your liver also makes more ketone bodies, including beta-hydroxybutyrate (BHB), which your body can use as fuel.
That switch doesn't happen overnight. While your body adapts, the change in available fuel may contribute to fatigue, weakness, lower exercise performance, or trouble concentrating. Researchers have proposed this as one possible contributor, but the evidence tying it to specific symptoms is still limited.
2. You're using up your glycogen stores
Your body stores carbohydrates as glycogen, mainly in your liver and muscles. When you cut carbs, your body draws on those stores. Glycogen is stored with water, so using it up releases that water too.
That helps explain why you may have lost a few pounds in your first days of keto. Seeing the scale move can be motivating, but some of that early loss is usually water rather than body fat.
3. You may lose more water and sodium
This is one of the most important changes to understand when you start keto.
Eating fewer carbohydrates generally lowers insulin. Insulin does more than manage blood glucose. It also affects how your kidneys handle sodium. When insulin drops, your kidneys can excrete more sodium, a process called natriuresis. Water follows sodium, so you may urinate more and lose extra fluid.
Research on fasting and keto initiation has found that sodium and potassium losses can be especially high in the first several days and tend to ease as the body adapts. That shift in fluid balance is a plausible explanation for headaches, dizziness, fatigue, weakness, and muscle cramps.
Researchers are careful to note that studies haven't yet shown that electrolyte loss directly causes every symptom grouped under keto flu.
Why do electrolytes matter during the keto flu?
Because when you lose extra fluid, you're not only losing water. Your body also relies on electrolytes like sodium, potassium, and magnesium for fluid balance, nerve and muscle function, and many other processes.
That's why staying hydrated on keto isn't only about drinking more plain water. Foods that provide electrolytes and fit most keto plans include:
- Avocado
- Leafy green vegetables
- Nuts and seeds
- Meat and fish
- Eggs
- Broth
- Appropriately salted food
There's a strong physiological reason to pay attention to electrolytes when you start keto. That said, research hasn't settled on one universal electrolyte plan for preventing keto flu, so your own needs and any medical conditions should guide what you do.
Can keto affect your digestion?
Yes. Not every keto flu symptom comes from electrolytes. Switching to keto can change a lot about what you eat. You may be eating more fat and fewer grains, fruits, legumes, and other carbohydrate foods.
That can change how much fiber and overall food volume you're getting, which may contribute to constipation or other digestive changes while you adjust. Instead of focusing only on carb counts, pay attention to the quality and variety of what you're eating.
How long does keto flu last?
For many people, a few days to a few weeks. There isn't one timeline that fits everyone.
A 2025 scoping review found that symptoms commonly appeared about 2–3 days after starting a ketogenic diet and that most reported symptoms resolved within 2–4 weeks.
An earlier study that analyzed people's descriptions of keto flu in online forums found that reports peaked in the first week and dropped off considerably by about four weeks. Because it was based on self-reported online posts rather than a controlled trial, it's worth reading cautiously.
Some people have a day or two of mild symptoms. Others take longer to feel fully adapted. Both are normal.
What can you do to ease the keto flu?
Give your body time to adjust, and support it while it does.
Stay hydrated.
Your body may be releasing more water than you're used to, especially early on.
Pay attention to electrolytes.
Make sure your meals include good sources of sodium, potassium, magnesium, and other essential minerals.
Try a float tank or Epsom salt bath to relax.
A float session or warm bath with Epsom salts (magnesium sulfate) can be a relaxing way to unwind if you're tired or your muscles are sore. It won't replace the electrolytes you've lost, though. Evidence that meaningful amounts of magnesium are absorbed through the skin is limited, so keep getting fluids and electrolytes through food or other appropriate sources.
Eat enough.
Keto doesn't mean going hungry. You're already asking your body to adapt to a big change in fuel, so give it enough nourishing food to work with.
Include fiber-rich, low-carb foods.
Vegetables, nuts, seeds, and avocados provide nutrients and fiber and fit most keto plans.
Ease off workout intensity for a bit.
Exercise may feel different while your body adapts. Lower exercise capacity has been reported during keto initiation, and it usually improves.
Listen to your body.
How you feel, along with objective measurements like blood ketone testing, helps you understand how your body is responding.
When is it not the keto flu?
Feeling unwell after starting keto doesn't automatically mean it's keto flu. Persistent or severe vomiting, fainting, confusion, significant weakness, an irregular heartbeat, severe dehydration, or other worrying symptoms shouldn't be written off as part of adapting. Get medical care.
If you have diabetes, be especially careful not to confuse nutritional ketosis with diabetic ketoacidosis (DKA). They are very different states. DKA is a potentially life-threatening emergency that needs immediate medical attention.
If you take medications for diabetes or blood pressure, or you have kidney, heart, or other medical conditions, talk with your healthcare provider before making significant changes to how you eat. Cutting carbs, and the fluid and electrolyte shifts that come with it, can affect your treatment needs.
Is keto flu a sign that keto is working?
No. Feeling terrible isn't proof that keto is "working." Keto flu is a transition, not the goal.
There's no need to push through severe symptoms or chase the highest ketone reading you can get. Your body is adapting to a different way of eating, and that takes time. Stay hydrated, pay attention to electrolytes and nutrition, and listen to your body.
Everyone responds to cutting carbs differently. Understanding your own body's response is more useful than comparing your numbers, or your keto journey, to anyone else's.
Common questions about the keto flu
How long does the keto flu last?
A 2025 scoping review found symptoms commonly start about 2–3 days after beginning keto and most resolve within 2–4 weeks. Some people have only a day or two of mild symptoms, while others take longer to feel fully adapted.
How do you get rid of the keto flu fast?
There's no instant fix, but you can make it easier. Drink enough fluids, get sodium, potassium, and magnesium from food like broth, leafy greens, avocado, and salted meals, eat enough overall, include fiber-rich low-carb foods, and ease off hard workouts for a week or two.
Does keto flu mean I'm in ketosis?
Not necessarily. Keto flu symptoms come from the transition, like fluid and electrolyte shifts and changes in what you eat, not from ketosis itself. The reliable way to know whether you're in ketosis is to measure your blood ketones.
Is the keto flu dangerous?
For most people it's uncomfortable but temporary. Severe or persistent vomiting, fainting, confusion, significant weakness, or an irregular heartbeat are not normal parts of adapting and need medical attention. People with diabetes should know the difference between nutritional ketosis and diabetic ketoacidosis (DKA), which is a medical emergency.
Sources
- Skartun O, Smith CR, Laupsa-Borge J, Dankel SN. Symptoms during initiation of a ketogenic diet: a scoping review of occurrence rates, mechanisms and relief strategies. Frontiers in Nutrition. 2025;12:1538266. doi.org/10.3389/fnut.2025.1538266
- Bostock ECS, Kirkby KC, Taylor BV, Hawrelak JA. Consumer Reports of "Keto Flu" Associated With the Ketogenic Diet. Frontiers in Nutrition. 2020;7:20. doi.org/10.3389/fnut.2020.00020
- Harvey CJDC, Schofield GM, Williden M. The use of nutritional supplements to induce ketosis and reduce symptoms associated with keto-induction: a narrative review. PeerJ. 2018;6:e4488. doi.org/10.7717/peerj.4488
This article is for educational purposes only and is not intended to diagnose, treat, or replace professional medical advice. Talk with a qualified healthcare professional before making significant dietary changes, especially if you have a medical condition or take prescription medications.
