Plenty of people start keto, feel grim by day three, and quietly decide the diet does not agree with them. Most of the time what they are feeling is the so-called keto flu, a short and largely avoidable patch of feeling off. It has very little to do with the diet failing and almost everything to do with salt and water. Knowing what it is, roughly how long it lasts, and the handful of things that fix it turns the worst week of keto into a non-event.

What the keto flu actually is

The name is misleading, because there is no virus and nothing contagious about it. The keto flu is the collection of symptoms some people get in the first days of cutting carbohydrate hard, and it is essentially a mineral and fluid dip dressed up to feel like illness.

Here is the mechanism. Carbohydrate is stored in your muscles and liver as glycogen, and every gram of glycogen holds roughly three grams of water alongside it. When you stop eating carbs, your body burns through that stored glycogen and releases all the water it was carrying. As the water leaves, so does a good deal of sodium, because the two travel together and your kidneys start dumping salt once insulin falls. That double loss of water and sodium is what leaves you feeling rough. It is not your body rejecting fat; it is running low on the minerals that keep your nerves, muscles and blood pressure steady.

This is also why the scales drop so dramatically in the first week. Several pounds gone in a few days is almost entirely water, not fat, which is worth remembering before you get too excited or too discouraged by an early number.

Symptoms and how long they last

The usual line-up is headache and fatigue. On top of those, people commonly report:

You will not get all of these, and many people get almost none. Severity varies enormously from one person to the next, partly down to how abruptly they cut carbs and partly down to how salty their previous diet was.

On timing: the keto flu typically shows up one to two days after you start, peaks around days three and four, and fades within a few days to a week. If you manage your salt and fluids well from the start, you can blunt it so much that you barely notice it. The brain fog in particular tends to lift as your body gets better at making and using ketones, and the steadier focus that many people prize on keto sits on the other side of this dip. There is more on that shift in the piece on mental clarity and focus.

Why salt is the lever that matters most

If you only do one thing about the keto flu, make it salt. The instinct most of us have absorbed is that salt is something to cut, so it feels wrong to add it deliberately. On a low-carbohydrate diet that advice is reversed. With insulin low, your kidneys excrete sodium freely, and without carbs prompting you to retain it you can slide into a mild sodium deficiency surprisingly quickly. Most of the classic symptoms, the headaches, the dizziness on standing, the fatigue and cramps, are textbook signs of running low on sodium and the minerals that partner with it.

So salt your food generously, more than you think you need. A cup of warm salted broth, stock or bouillon once or twice a day during the first week is the simplest fix going, and it does more than any supplement. Some people add a quarter teaspoon of salt to a glass of water and drink it down in the morning, which is not glamorous but works.

Sodium is the headline, but it does not act alone. Potassium and magnesium both fall as well, and low magnesium is behind a lot of the night cramps and poor sleep. Top up potassium through food where you can: avocado, leafy greens, salmon and a sprinkle of certain salt substitutes all help. Magnesium is harder to get enough of from food during the changeover, which is why a magnesium supplement taken in the evening is one of the few that genuinely earns its place. It tends to ease the cramps and improve sleep at the same time. The full breakdown of how much of each you actually need is in the guide to electrolytes on keto.

Fluid: drink to thirst, not to drown

There is a popular idea that you should flood yourself with water on keto. Be careful with it. Drinking is fine and you should not go thirsty, but pouring litres of plain water through a body that is already short on sodium dilutes things further and can make the headache and fog worse, not better. The goal is to replace salt and fluid together, not to chase water for its own sake. Drink to thirst, salt your food and broth properly, and the balance looks after itself.

Ease in rather than slamming the door

You do not have to drive carbs to zero on day one. Cutting them down over the course of a week, rather than overnight, gives your body time to adjust the way it handles sodium and softens the whole dip. If you are the cautious type, this gentler on-ramp is well worth it, and it pairs naturally with the wider plan in the first thirty days guide.

Eating enough matters just as much. The early days are the wrong moment to stack a steep calorie cut on top of the carb cut. Your body is learning to run on fat, and starving it while it does so guarantees you feel terrible. Eat to genuine satisfaction, keep your protein up, and let the appetite suppression that keto brings do the calorie work later, once you are adapted.

The small levers people forget

A few extra things quietly help. Do not slash your caffeine at the same time as your carbs, or you stack a caffeine-withdrawal headache on top of the keto flu and cannot tell them apart; change one thing at a time. Go easy on hard exercise for the first week, since heavy training while you are mineral-depleted is a reliable way to trigger cramps and dizziness. And protect your sleep, because being run down makes every symptom feel worse than it is.

When it is not the keto flu

The keto flu is mild, predictable and short. Treat it as a signal to top up salt and minerals, not as a medical event. That said, keep some perspective. If you feel genuinely unwell rather than merely flat, if symptoms drag on well past a week despite getting your salt and fluids right, or if you have a condition such as diabetes or a heart problem, or take medication that affects fluid and salt balance, do not simply push through. Those situations deserve a proper look rather than another cup of broth.

This is general information about the ketogenic diet, not medical advice. Keto does not suit everyone; if you are pregnant, on medication, or managing a condition such as diabetes, speak to a doctor or dietitian first.