What Is the Keto Flu? Symptoms, Timing and When to Get Help
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What is the keto flu? It's an informal name for symptoms some people report when they start a keto diet. A British Dietetic Association (BDA) article by a hospital dietitian says some patients use the term for their tiredness, nausea, headaches and irritability. BBC Food says there is very little medical research on it. The NHS lists some of the same symptoms for dehydration, low blood sugar and real flu, so if you feel very unwell, get medical advice.
At a glance
- 2–7 dayswhen symptoms can kick in, says a dietitian quoted by BBC Food
- 1–2 weekshow long symptoms typically occur, says a Cleveland Clinic dietitian (US)
- 6 to 8cups or glasses of fluid a day, as the Eatwell Guide recommends (NHS)
- 6gthe most salt adults should have in a day (NHS)
On this page
What is the keto flu, what does it feel like, and when should you get help? This guide sets out what UK dietitians and the NHS say about the term, the symptoms people report, how it differs from real flu, and other conditions with similar symptoms. For a week-by-week view of the first weeks on keto, see our guide to the stages of keto adaptation.
What is the keto flu?
"Keto flu" is a name people use for symptoms they get when they start a keto diet. Hannah Ellis, a paediatric dietitian (neurology and ketogenic diet) at Bristol Royal Hospital for Children, wrote for the BDA about spending 12 days on a keto diet, to experience it from her patients' families' perspective. Her patients are children who follow a medical ketogenic diet for epilepsy, and for her own trial she followed the modified Atkins diet. She writes:
"Some patients refer to the term 'keto flu' when starting the diet as a description of their symptoms of tiredness, nausea, headaches and irritability."
BBC Food's article on the keto diet says:
"People who restrict their carbohydrates and increase fatty foods may develop 'keto flu'. There is very little medical research on this, but there are thousands of personal testimonies. Though it sounds unpleasant, it isn't necessarily unsafe and many reports indicate the discomfort subsides. It is also not clear whether it only occurs on the keto diet or can be caused by other restrictive diets."
Despite the name, it isn't influenza. Cleveland Clinic, a US non-profit academic medical centre, says "keto flu isn't the real flu — or an actual illness at all". We look at how to tell the two apart further down.
What does keto flu feel like? Reported symptoms
There's no official list of keto flu symptoms. These are the symptoms three UK sources name when they describe it:
| Symptom | BDA | BBC Food | Good Food |
|---|---|---|---|
| Tiredness | Yes | Yes | Yes |
| Headaches | Yes | Yes | Yes |
| Nausea | Yes | Yes | – |
| Irritability | Yes | – | – |
| Constipation | – | Yes | – |
| Difficulty sleeping | – | Yes | – |
| Brain fog | – | – | Yes |
BDA: Hannah Ellis's description of what patients report. BBC Food: dietitian Sophie Medlin. Good Food: the "keto flu" item in BBC Good Food's list of main risks of a keto diet.
Hannah Ellis also describes her own first days. She writes that "the first 48 hours on the diet were very challenging", and that "getting out of bed in the morning had never been so difficult and my brain felt very foggy". She adds: "I was very thirsty and experienced increased urination."
Cleveland Clinic's list of symptoms also includes muscle cramps, constipation or diarrhoea, and an upset stomach. If those are what's bothering you, see our guides to leg cramps on keto and constipation on keto, which set out the NHS advice for each.
When does keto flu start, and how long does it last?
We haven't found an NHS or BDA figure for this. Two sources give one, counting from different points:
- When it starts. BBC Food reports that dietitian Sophie Medlin cites keto flu among the potential reasons why people might struggle to stick to the diet, with symptoms that "can kick in after two to seven days". Cleveland Clinic's dietitian, Beth Czerwony, counts from a different point: "Keto flu is a group of symptoms that can appear anywhere from two to seven days after someone is in ketosis."
- How long it lasts. Beth Czerwony says: "Symptoms typically occur for one to two weeks." Cleveland Clinic says symptoms "last for as little as two days or as long as a month, but usually somewhere in between".
- Not everyone gets it. Cleveland Clinic says not everyone who follows the keto diet experiences keto flu.
Why do people get keto flu?
There isn't a settled answer. BBC Food says there is very little medical research on keto flu, and that it isn't clear whether it only happens on keto. Beth Czerwony at Cleveland Clinic says: "There's no explanation of why some people have these symptoms while others don't."
BBC Good Food's beginner's guide to the keto diet links the early symptoms to water loss. In its nutritionist's view, it says:
"The symptoms associated with ketosis are often temporary and relate predominantly to dehydration, because of the water loss experienced in the early stages of the diet. These may include headache, dry mouth, bad breath, fatigue and nausea."
That's one explanation, not a diagnosis. The same symptoms can have other causes, as the next two sections show.
Keto flu or real flu?
Real flu is an infection. The NHS says "antibiotics do not work for viral infections such as flu", and that flu "will often get better on its own, but it can make some people seriously ill". It lists these symptoms, which it says come on very quickly:
- a sudden high temperature
- an aching body
- feeling tired or exhausted
- a dry cough
- a sore throat
- a headache
- difficulty sleeping
- loss of appetite
The NHS adds: "The symptoms are similar for children, but they can also get pain in their ear, sore eyes, swollen glands and appear less active."
Tiredness, a headache and difficulty sleeping appear on both lists. A sudden high temperature, an aching body, a dry cough and a sore throat are on the NHS flu list, but none of the keto flu descriptions we quote here include them. If you have those, the NHS flu advice is the one to follow, whatever you're eating.
Ask for an urgent GP appointment or get help from NHS 111 if you or your child have symptoms of flu and:
- you're worried about your baby's or child's symptoms
- you're 65 or over
- you're pregnant or have recently given birth
- you have a long-term medical condition – for example, diabetes or a condition that affects your heart, lungs, liver, kidneys, brain or nerves
- you have a weakened immune system – for example, because of chemotherapy or HIV
- you feel very unwell
- you feel short of breath
- your symptoms do not improve after 7 days
Call 999 or go to A&E if:
- you get sudden chest pain
- you have severe difficulty breathing – you're gasping, choking or not able to get words out
- you start coughing up blood
Could it be something else?
Several symptoms linked to keto flu, such as tiredness, headaches and nausea, are also listed by the NHS for other conditions. Here are three to know about.
Dehydration
The NHS says "dehydration means your body loses more fluids than you take in", and that babies, children and older adults are more at risk. It lists these symptoms of dehydration in adults and children:
- feeling thirsty
- headache and feeling light-headed
- dark yellow, strong-smelling pee
- peeing less often than usual
- feeling dizzy or lightheaded
- feeling tired
- a dry mouth, lips and tongue
- sunken eyes
The NHS says you should drink fluids if you have symptoms of dehydration. It says to ask for an urgent GP appointment or get help from NHS 111 if:
- you're feeling unusually tired (or your child seems drowsy)
- you feel dizzy when you stand up and it does not go away
- you have dark yellow pee or you're peeing less than normal (or your baby has fewer wet nappies)
- you or your child are breathing quickly or have a fast heart rate
- your baby or child have few or no tears when they cry
- your baby has a soft spot on their head that sinks inwards (sunken fontanelle)
It says to call 999 or go to A&E if you or somebody else has signs of dehydration and:
- blue, grey, pale or blotchy skin, lips or tongue – on brown or black skin, this may be easier to see on the palms of the hands or soles of the feet
- skin that feels cold
- difficulty breathing, or is taking lots of quick breaths
- are confused
- are sleepier than normal or difficult to wake
Low blood sugar (hypoglycaemia)
In her BDA diary, Hannah Ellis writes: "On day five, I experienced symptoms of dizziness, nausea and lethargy. I tested my blood glucose levels and realised I was hypoglycaemic." She writes that, as per her team's patient protocol, she knew she needed to treat this with some carbohydrate.
The NHS says: "Low blood sugar usually affects people with diabetes who take insulin or some other diabetes medicines. It's rare in people without diabetes." The BDA's low carb diets fact sheet says: "If you have type 2 diabetes, you may need to reduce some medication such as insulin or sulfonylureas to avoid hypoglycaemia, and this should be done under medical supervision."
The NHS page on low blood sugar (hypoglycaemia) sets out how to treat a hypo, when to see a GP and when to call 999. It says to see a GP if "you do not have diabetes and you get symptoms of low blood sugar such as shaking, sweating, a fast heartbeat and feeling confused". If you have diabetes, it says you can also speak to your diabetes care team for advice.
Diabetic ketoacidosis (DKA)
If you have diabetes, don't assume that feeling unwell on keto is keto flu. The NHS says diabetic ketoacidosis (DKA) "is a serious condition that can happen in people with diabetes. It's where a lack of insulin causes harmful substances called ketones to build up in the blood. It can be life threatening and needs urgent treatment in hospital." Its symptoms include feeling thirsty, needing to pee more often, feeling sick and feeling tired, sleepy or confused, and the NHS page lists the rest. It says the symptoms usually develop over 24 hours, but it can be faster, and that "DKA usually affects people with type 1 diabetes, but it can also happen in people with type 2 diabetes who need insulin."
Call 999 or go to A&E if:
- you have a high level of ketones – over 3mmol/L in your blood, or over 2+ in your pee
- you do not know your ketone levels but have symptoms of diabetic ketoacidosis, such as feeling thirsty and needing to pee more often, feeling sleepy or confused, and breath that smells fruity (like pear drop sweets or nail polish remover)
- you have symptoms of diabetic ketoacidosis and have not been diagnosed with diabetes
Call your diabetes care team now or get help from NHS 111 if you have diabetes and:
- your blood glucose is high and your insulin treatment is not working to reduce it, even if your ketones are normal
- your ketones are slightly high (0.6 to 1.5mmol/L) and you feel unwell
- your ketones are 1.6 to 3mmol/L, even if you do not feel unwell
Fluids and salt: what the NHS says
We haven't found an NHS or BDA page that recommends a treatment for keto flu, including salt, electrolyte drinks or supplements. BBC Good Food's list of things to do before starting a keto diet includes: "manage your hydration levels and electrolyte intake to minimise the risk of 'keto flu'". Here's the general NHS guidance on fluids and salt, which applies whatever you eat.
Fluids. The NHS says most people should aim to drink enough during the day so their pee is a clear pale yellow colour. In the Eatwell Guide, the government recommends 6 to 8 cups or glasses of fluid a day. The NHS says you may need to drink more fluids if you're:
- pregnant or breastfeeding
- in a hot environment
- physically active for long periods
- ill or recovering from illness
Salt. The NHS says adults should have no more than 6g of salt a day (around 1 level teaspoon), and that this includes the salt that's already in our food and the salt added during and after cooking. It also says eating too much salt can cause high blood pressure, which increases the risk of heart attacks and strokes. Our guide to sodium on keto shows how much salt common keto foods contain, using UK food data.
Who should get advice before starting keto
BBC Food says: "If you take any medication or have any medical issues, consult a doctor or dietitian before starting a keto diet. For some people, a keto diet may cause harm, even with careful monitoring."
BBC Good Food says that diabetics and anyone with a blood sugar management issue should discuss the potential implications with their GP and healthcare team before embarking on such a regime, and so should anyone who meets one or more of these criteria:
- have kidney and/or liver disease or a family history of such
- is under 18 years old or elderly
- has a pre-existing medical condition
- are pregnant or breastfeeding
- are on prescribed medication
The BDA's low carb diets fact sheet says: "Ketogenic diets should not be followed by women who are pregnant." For children, the ketogenic diet Hannah Ellis's team uses for epilepsy "is highly restrictive and requires close supervision and monitoring by ketogenic dietitians".
On medicines, Beth Czerwony at Cleveland Clinic says: "Some medications, like those for blood pressure and diabetes, can actually make symptoms worse."
Speak to your GP, diabetes nurse or a registered dietitian before you make big changes to your diet. This is especially important if you have diabetes and take insulin or other medicines that can cause hypos, or take SGLT2 inhibitors, as your medication may need to change. The same goes if you're pregnant, breastfeeding or have a history of eating disorders.
Key takeaways
- It's an informal name. People use "keto flu" for symptoms such as tiredness, headaches, nausea and irritability when they start keto.
- There's little research. BBC Food says there is very little medical research on it.
- It isn't real flu. A sudden high temperature, aching body, dry cough or sore throat points to the NHS flu advice.
- Know the other causes. Dehydration, low blood sugar and DKA share some symptoms. If you feel very unwell, get help.
Frequently asked questions
What is the keto flu?
It's an informal name for symptoms some people report when they start a keto diet. A British Dietetic Association article by a hospital dietitian says some patients use the term for their symptoms of tiredness, nausea, headaches and irritability. BBC Food says there is very little medical research on it, but there are thousands of personal testimonies.
What does keto flu feel like?
People describe tiredness, headaches, nausea, irritability, brain fog, constipation and difficulty sleeping, according to the BDA, BBC Food and BBC Good Food. Cleveland Clinic says not everyone on keto gets them. Some symptoms overlap with dehydration, low blood sugar and real flu, so if you feel very unwell, or you're worried, get medical advice rather than assuming it's keto flu.
How long does keto flu last?
There's no UK figure. A dietitian quoted by BBC Food says symptoms can kick in after two to seven days. A dietitian at Cleveland Clinic, a US medical centre, says symptoms typically occur for one to two weeks, and the clinic says they can last from two days to a month. BBC Food says many reports indicate the discomfort subsides.
Is keto flu the same as the flu?
No. The NHS describes flu as a viral infection, and Cleveland Clinic says keto flu isn't the real flu or an actual illness at all. The NHS lists a sudden high temperature, an aching body and a dry cough among flu symptoms, which none of the keto flu descriptions we quote include. If it could be flu, follow NHS advice.
How do you get rid of keto flu?
We haven't found an NHS or BDA page recommending a treatment for keto flu, and BBC Food says there's very little medical research on it. As general advice for everyone, the NHS says to drink enough fluid for clear pale yellow pee. Get medical advice if you feel very unwell, you're worried or you take diabetes or blood pressure medicines.
Sources
- British Dietetic Association. "My 12 days on the keto diet" (Hannah Ellis, paediatric dietitian, Bristol Royal Hospital for Children).
- British Dietetic Association. Low carb diets food fact sheet.
- BBC Food. Is a keto diet a good way to lose weight? (dietitian Sophie Medlin).
- BBC Good Food. What is a keto diet? A beginner's guide (Sarah Lienard, reviewed by Kerry Torrens).
- NHS. Flu
- NHS. Dehydration
- NHS. Low blood sugar (hypoglycaemia)
- NHS. Diabetic ketoacidosis
- NHS. Water, drinks and hydration
- NHS. Salt in your diet
- Cleveland Clinic (US). 'Keto Flu': What It Is and How To Manage It (Beth Czerwony, RD, LD).
