What is keto flu? Learn about symptoms and prevention

Flu-like symptoms are a common side effect of the keto diet. Find out why, what it feels like and whether there’s anything that works to prevent it.
Published 18 December, 2019

No one gets a kick out of feeling under the weather, right? So, if you’re considering trying the keto diet, it’s important to know that there’s a good chance you will feel a bit ‘off’ for at least the first few days, but maybe weeks, after you begin following it.

In fact, feeling ill after embarking on keto is so common there’s even a name for it – ‘keto flu’.

Unlike the actual flu, which is caused by the influenza virus, the side effects or symptoms that keto flu consists of are triggered by switching to the keto diet, thanks largely to the fact that it’s incredibly low in carbohydrates. The typical keto ratio demands that 70-80 per cent of your daily calories come from fat, about 10-20 per cent from protein, and just 5-10 per cent from carbohydrates.

While the exact combination and amount of different foods to eat will vary from person to person depending on daily energy requirements, as a guide, it would mean limiting your carbohydrate intake to between 20 and 50 grams a day. In reality that can mean that once you’ve eaten one-and-a-half slices of wholegrain bread, you’ve used your carbohydrate allowance for the day.

When you consider that carbohydrate-rich foods are held in high regard by your body because they provide glucose, its main energy source, it’s not surprising that it’d need time to adjust – and that it will probably complain a bit in the process.


Keto flu symptoms


The list of flu-like symptoms that transitioning to a keto diet can cause is long and varied, but the most common ones include…

  • Headaches

While the dehydration and electrolyte imbalances that a keto diet can cause play a role here, keto flu headaches also occur as a result of the drastic reduction in the number of carbohydrates – especially in the form of natural sugars – that a keto diet demands.

  • Fatigue

You might feel tired, weak or lethargic, and more irritable than usual. It’s a side effect of forcing your body to switch from burning carbohydrates for energy, to burning fat for energy.

  • Dizziness

Feeling light-headed is common on very low-carbohydrate diets, and can be linked back to the dehydration that they go hand in hand with, something that causes a drop in both blood pressure and oxygen supply to the brain.

  • Brain fog

You might feel confused, disorganised, forgetful and just generally ‘off your game’– hardly surprising when you consider carbohydrates are the brain’s main fuel source.

  • Nausea

While simply feeling sick can be bad enough, for some people this can progress to vomiting and other digestive problems, like constipation.

  • Insomnia

Initially, you might experience difficulties either falling or staying asleep. One explanation can be found in the results of a University of Sydney study, which confirmed a link between carbohydrates and better sleep, perhaps thanks to the way certain carbohydrate-rich foods boost levels of brain chemicals that play a key role in the sleep cycle.


How to prevent keto flu


For most people, keto flu symptoms will eventually subside on their own, but it can take weeks rather than days before you’ll feel back to normal. It’s one of the reasons, in addition to the severe dietary restriction ‘going keto’ involves, why many people who embark on the diet report that sticking to it, even in the short term, can be challenging.

Unlike the actual flu, keto flu doesn’t cause fever or upper respiratory congestion, so taking go-to over-the-counter flu medications, like paracetamol and decongestants, won’t help to prevent or relieve symptoms. That said, some of the same strategies you’d use when you’re fighting the real flu may be useful against keto flu.

For example, thanks to the fact that a lot of keto flu symptoms can be linked back to the dehydration that a very low-carbohydrate diet can cause, some research suggests that drinking more fluid than usual may help to reduce the onset and severity of symptoms.

A loss of electrolytes has also been identified as a key factor that contributes to keto flu, so topping up levels of minerals like sodium, potassium and magnesium, may help.

Likewise, it’s a good idea to put any form of physical activity that’s more lively than just a gentle walk on hold for a while. That can be frustrating, but vigorous exercise intensifies and contributes to the dehydration that triggers many symptoms of keto flu.