My first attempt at keto lasted four days. I felt fine on Monday, slightly off on Tuesday, and by Thursday I had a dull headache, legs that felt hollow on the stairs, and the sort of foggy irritability where you snap at people for no reason. I decided the diet did not suit me and went back to bread. It was a full year before I tried again, and the second time I understood what had actually gone wrong. It was not the diet. It was salt.
That first horrible week has a name, the keto flu, and it put me off for twelve months when the fix would have taken about ten minutes and cost the price of a bag of table salt. So this is the piece I wish someone had handed me at the start. It is what the keto flu actually is, the electrolyte routine that made my second start almost boring by comparison, and the handful of symptoms that mean you should stop reading blogs and ring a doctor instead.
Why the first week hits you
The name is misleading because there is no virus and nothing you can catch. What people call the keto flu is a cluster of symptoms that turns up in the first days of cutting carbohydrate hard, and underneath it is a fairly simple bit of physiology.
When you eat plenty of carbohydrate, your body stores it as glycogen in the muscles and liver, and every gram of glycogen is held alongside roughly three grams of water. Cut the carbs and you burn through that stored glycogen over the first day or two, and all the water bound up with it goes as well. That is the sudden weight drop everyone reports in the first week, and almost none of it is fat. It is water.
The catch is what leaves with the water. Sodium goes with it in large amounts, and there is a second mechanism stacked on top. Carbohydrate drives insulin, and insulin tells the kidneys to hold on to sodium. When you drop the carbs your insulin falls, the kidneys stop clinging to sodium, and they flush it out. So a keto eater in week one is losing salt through two doors at once, far faster than someone living on pasta and toast.
Sodium sits at the centre of a lot of everyday functions, so when it drops you feel it everywhere. The headache, the fatigue, the light-headedness when you stand up, the muscle cramps, the brain fog, the short temper. That is not your body objecting to the absence of bread. It is a mineral and fluid dip wearing the costume of an illness. And because it is a shortfall rather than a disease, you can top it back up.
Sodium is the one that matters most
If you fix nothing else, fix your salt. This is the single change that turned my second start around, and I noticed the difference within a day.
On a normal carbohydrate diet the general public health message is to keep salt down, and for most people eating processed food that is sound advice. On keto the maths flips. You are not retaining sodium the way you used to, so you have to put it back deliberately. The figure commonly cited for a well-formulated ketogenic diet is somewhere around 3,000 to 5,000 mg of sodium a day, which is more than most people are used to aiming for. For reference, a level teaspoon of table salt holds roughly 2,300 mg of sodium, so you are looking at something like one and a half to two teaspoons’ worth across the day, most of it from salting your food properly and some of it added on purpose.
The way I do it now is simple. I salt my meals more heavily than I used to, without apology. And on the days I feel that early flu creeping in, I have a mug of salted broth or a stock cube dissolved in hot water, which gives you a clean gram or two of sodium in one go and, oddly, feels like a treat when you are flagging. A pinch of salt in a glass of water does the same job if broth is not to hand. Within twenty minutes or so the headache tends to lift. The first time that happened I actually laughed, because a year earlier I had given the whole thing up rather than reach for the salt cellar.
Potassium and magnesium, the supporting cast
Sodium does most of the heavy lifting in the first week, but the other two minerals matter, especially once you are past the initial slump and into the long haul.
Potassium works in a delicate balance with sodium, and when you lose a lot of one the other can drift too. A commonly cited target on keto is around 3,000 to 4,000 mg a day. The good news is that you can get most of this from food rather than pills, and the foods are all firmly keto. Avocado is the obvious one, with a whole fruit carrying a serious dose. Cooked spinach is excellent, as is salmon, and mushrooms chip in too. I eat an avocado most days and a good pile of leafy greens, and I do not fret about the exact number.
A word of genuine caution here. Potassium has a narrow safe window, meaning too much is as much of a problem as too little, and it can affect the heart. This is the one electrolyte I would not go loading up on through high-dose supplements without a doctor’s say-so, particularly if you have any kidney issue or take blood pressure medication. Food is the safe way to get your potassium, and it is very hard to overdo it through avocados and spinach.
Magnesium is the one people most often miss, and it is the one behind the night-time leg cramps, the twitchy eyelid and the poor sleep in the early weeks. Most of us run a bit short on magnesium even before we start keto, and the diet tends to expose it. Here a supplement makes sense, because it is genuinely hard to hit the target from food. Something in the region of 300 to 400 mg a day of supplemental magnesium is a reasonable and safe amount for most people with healthy kidneys. I take mine in the evening, partly because that is when cramps strike and partly because it seems to help me settle. If it loosens your stomach, drop the dose or switch form, since some types are gentler than others.
Hydration, and why more water alone backfires
The instinct when you feel rough is to drink gallons of plain water. On keto that can actually make things worse, because you flush out what little sodium you have left and dilute yourself further. I did exactly this on my first attempt, drank litres of water thinking I was being virtuous, and wondered why I felt worse.
Drink to thirst, and make sure the water you drink comes with salt somewhere in the day, not on its own. Water plus salt is the combination that rehydrates you properly. Water on its own, in large volumes, is closer to the problem than the cure. This was the single most counter-intuitive thing I learned, and it is why the salted broth trick works so well. You are not just adding salt, you are adding salt and fluid together, which is exactly what the body is short of.
Timing, and how long it lasts
You do not need a rigid schedule, but a loose rhythm helps. I get some salt in early, with breakfast or a morning broth, because that is when the standing-up dizziness used to hit me. I keep the potassium coming through meals across the day. And I take magnesium in the evening. That is the whole routine. It is not clever and it does not need an app.
As for duration, the keto flu is a short-lived thing. For most people the worst of it lands in the first few days and clears within a week to three weeks, with the salt fixes shortening it considerably. A small number of people take up to about six weeks to feel fully settled, but that is the tail, not the norm. If you get your electrolytes right from day one, there is a fair chance you barely notice a keto flu at all. My second start had a single slightly foggy afternoon, sorted by a mug of broth, and that was it.
When it is not the keto flu
This is the part I take seriously, because the whole point of understanding the keto flu is knowing when what you have is something else.
Ordinary keto flu is uncomfortable but mild, it responds to salt and rest, and it fades over days. It should not be dramatic. If your symptoms are severe, getting worse rather than better after a week, or simply do not fit the pattern above, treat that as a signal to get checked rather than to push through.
There is one condition worth naming plainly, because the names sound alike and cause confusion. Nutritional ketosis, the ordinary state of a keto diet, is not the same as diabetic ketoacidosis, or DKA. DKA is a medical emergency that overwhelmingly affects people with diabetes, particularly type 1, when a lack of insulin lets blood ketones climb to dangerous levels and turn the blood acidic. It is a genuinely different thing from the mild ketosis of a low-carb diet in someone without diabetes. Its warning signs come on fast and include intense thirst, frequent urination, nausea and vomiting, stomach pain, a fruity smell on the breath, rapid breathing and confusion. If you have diabetes and you are considering keto, do it with your doctor’s involvement and monitoring, not off the back of an article.
For anyone, diabetic or not, the sensible red flags are the same: persistent vomiting where you cannot keep fluids down, a racing or irregular heartbeat, chest pain, fainting, severe or worsening symptoms that salt does not touch, or symptoms dragging on well beyond the first couple of weeks. Any of those means stop guessing and see a doctor. The keto flu is a salt problem you fix at home. Those are not, and it is worth knowing the difference before you need to.
Get the electrolytes right and the first week goes from the thing that nearly ended keto for me to a minor footnote. It really is mostly salt.
This is general information about the ketogenic diet, not medical advice. If you have diabetes, kidney disease, heart problems, high blood pressure, or take regular medication, speak to a doctor before starting keto or supplementing electrolytes, as some of these amounts will not be right for you.
Sources: Virta Health on keto electrolyte targets, Diet Doctor on keto supplements, Mayo Clinic on diabetic ketoacidosis, Cleveland Clinic on ketosis vs ketoacidosis.