Keto flu and how to get through it

In the first days of keto plenty of people feel rotten: the head aches, the legs feel heavy, the mind goes foggy. This gets called "keto flu", although it has nothing to do with the flu; nobody catches it from you, and it usually clears up by itself within a few days.

If you are only just starting, the basics of the diet are in the guide The keto diet for beginners.

The symptoms

There is not much research on keto flu. One study, from Australia, had researchers read through what 300 people had written on keto forums; 101 of them (roughly one in three) described symptoms. The ones that came up most often were:

• feeling as if you have the flu

• headache

• tiredness and low energy

• nausea

• dizziness and feeling faint

• brain fog

• stomach and bowel discomfort

• changes in heartbeat

How bad it gets varies a great deal from person to person. Of the 60 who said how rough they felt, 15 described their symptoms as mild, 23 as moderate and 22 as severe. The study is small and built on online posts, so the figures give only a first picture.

Why it happens

Your body is changing two things abruptly, at the same time.

One is the fuel. Until yesterday it ran mostly on glucose and now it has to get through the day on fat and ketones, which the brain needs a few days to get used to.

The other is water. Glycogen is stored together with three to four times its own weight in water, so as it empties the water goes too. And with insulin low, the kidneys hold on to less sodium. Along with the water, then, you lose salts. That accounts for much of the headache, the dizziness and the tiredness.

How long it lasts

In the Australian study most complaints were posted in the first week; after four weeks they had almost stopped. Eight users also said when their symptoms went away: anywhere from 3 to 30 days, with half of them fine within four or five.

What helps

Since much of the trouble is water and salts leaving the body, that is where you start. The remedies the forum users suggested most often pointed the same way: more salt, electrolytes, broth. These are people's own experiences, though they fit what is going on in the body.

Drink water steadily through the day (more when it is hot or you are exercising) and leave the salt in your food. The easiest way to get enough is a hot, salty broth once or twice a day.

For potassium there is avocado and leafy greens such as spinach; for magnesium, nuts, seeds and green vegetables. If constipation sets in, have vegetables with every meal, a little flaxseed and, of course, water.

Don't forget the fat either. Anyone who cuts the carbs without putting something in their place ends up hungry and tired at once. Leave hard training for a few days; keep the walking.

To go through your symptoms one by one there is the Keto flu checklist, while the Electrolyte calculator works out how much sodium, potassium and magnesium you need. If you have high blood pressure or a kidney problem, talk to your doctor before you add salt or take supplements.

When it is not keto flu

Some symptoms are not ones you wait out. Chest pain, fainting, vomiting that will not stop, confusion, or fast and deep breathing need help straight away.

People with diabetes who take an SGLT2 inhibitor, such as dapagliflozin or empagliflozin, are a separate case. The European Medicines Agency has warned that with these medicines ketoacidosis can develop even when blood sugar is not especially high. Nausea, vomiting, stomach pain, strong thirst and unusual tiredness look like keto flu, but here they mean seeing a doctor the same day.

Frequently asked questions

Should I stop keto?

As a rule, no. It passes on its own; water and salt ease most of the symptoms. If you feel really unwell, though, raise your carbs a little and come down more slowly.

Do I need supplements?

Not necessarily. Salt in your food, broth, vegetables, avocado and nuts are usually enough.

Will it come back if I take a break?

It might. A few days of plenty of carbs refill your glycogen, so getting back into ketosis can mean a few rough days again.