It usually arrives on the second or third day. You have done everything the plan asked for, the carbohydrates are gone, and instead of the clarity you were promised you have a headache, a short fuse, and legs that feel like they belong to someone else. Stand up too quickly and the room takes a moment to catch up.

This is what people call the keto flu. The name is unhelpful in two ways. It is not a flu — there is no virus, no fever, no infection — and calling it one suggests you simply have to endure it until it passes. For most people, most of the time, you do not. What you are feeling is largely a salt and water problem, and salt and water problems have solutions that work within a day.

What is actually happening

Cutting carbohydrate sets off a chain of events that has nothing to do with detoxification and everything to do with plumbing.

Your body stores carbohydrate as glycogen, in the liver and the muscles. Glycogen is stored wet: each gram holds roughly three grams of water alongside it. When you stop eating carbohydrate, you spend that glycogen over the first day or two, and the water it was holding leaves with it. This is the dramatic first-week weight loss that keto is famous for, and it is also the reason the scales stop being interesting after the first fortnight. It was water.

The second event matters more. As carbohydrate intake falls, so does insulin. Insulin has a job most people never hear about: it tells the kidneys to hold on to sodium. When insulin drops, that instruction stops, and the kidneys begin excreting sodium at a much higher rate — along with more water. You are now losing fluid and the primary electrolyte dissolved in it, at the same time, faster than you are replacing either.

Low sodium, and the low blood volume that goes with it, produces a very recognisable list: headache, fatigue, light-headedness when you stand, irritability, muscle cramps, poor sleep, a heart that occasionally seems to be working harder than the situation warrants. Add the potassium and magnesium you are also losing and you can add constipation, twitching calves, and the particular flavour of brain fog where you read the same paragraph three times.

That is the keto flu. Not withdrawal. Not toxins. Not your body objecting on principle. Mostly sodium.

The fix, in the order that matters

Almost everyone reaches for water first and salt last. Reverse that.

1. Salt, properly

This is the single change that resolves most cases, usually within twelve to twenty-four hours. During the first two or three weeks, most people eating this way need substantially more sodium than the low-fat era taught them to fear: somewhere in the region of three to five grams of sodium a day, which is roughly seven and a half to twelve and a half grams of salt (salt is about 40% sodium). That sounds enormous because we have all been trained to think of salt as something to be minimised. In the context of very low insulin and accelerated sodium excretion, it is replacement, not excess.

The practical version: salt your food more heavily than feels normal, and keep a cup of warm water with half a teaspoon of salt stirred in for the moment the headache starts. It works faster than it has any right to.

The exception that matters. If you take medication for blood pressure, have kidney disease, heart failure, or have been told by a doctor to restrict sodium, none of the above applies to you without their input. This is not a formality. Speak to them first, and let them adjust the plan.

2. Potassium, from food where possible

Potassium losses follow sodium losses. Food handles this well: avocado, leafy greens, mushrooms, salmon, and a decent amount of meat between them cover most of it. Supplemental potassium is worth more caution than sodium — it is easier to overdo, and it interacts with several common medications — so unless a clinician has suggested otherwise, food first.

3. Magnesium, usually in the evening

Magnesium is the one people notice most in the second week: cramps at night, restless legs, sleep that never quite deepens. A supplement in the region of two to four hundred milligrams, taken in the evening, is common practice — glycinate and citrate are the forms most people tolerate; oxide is cheap and poorly absorbed and will mostly reacquaint you with your bathroom.

4. Fluid — but stop drinking to a target

Water is not the hero here. Drinking litres of plain water on top of an already low sodium level dilutes what little you have left and can make every symptom on the list worse. Drink to thirst, salt what you drink or what you eat alongside it, and let thirst do the work it evolved to do.

5. Time, and slightly more food than you think

Two changes at once is one change too many. If you have cut carbohydrate to twenty grams and also cut calories hard and also started training six days a week, the fatigue you are attributing to keto flu may simply be under-fuelling. For the first fortnight, eat enough. Fat loss is not the job of week one; adaptation is.

How long it should last

With sodium handled from day one, many people never get a bad version of it at all — a slightly foggy afternoon, perhaps, and that is the extent of it. With sodium handled from the moment symptoms appear, expect it to settle within one to three days.

Left alone, it typically drags on for a week to ten days, which is precisely the window in which most people conclude that keto does not agree with them and stop. That is the real cost of the keto flu: not the discomfort, but the perfectly reasonable decision it provokes.

What it is not

A few things get filed under keto flu that do not belong there, and it is worth being able to tell them apart.

Why the structural version works better

Most keto advice treats the first week as an ordeal to be survived, then moves on. That framing is the problem. If the opening days of a way of eating require willpower to endure, the whole thing is resting on willpower — and willpower is the one resource that reliably runs out during a hard week at work.

A better first fortnight is boring by design: adequate salt from day one, enough food, two structured meals rather than constant grazing, and no attempt to fix everything at once. Done that way, there is nothing to endure. People who begin like this tend to describe week one as unremarkable, and unremarkable is exactly the goal. The interesting part — steadier energy, quieter hunger, the long afternoons that stop collapsing — arrives in weeks two to six, once fat adaptation is underway.

Structure isn't the opposite of freedom. It's the path that makes freedom sustainable.

The short version

Where this fits

In Keto Dive this is the Reset phase — the first of five — and it exists precisely because so many people never get past it. The book's version includes the electrolyte baseline, the two-meal rhythm that keeps hunger predictable while your body switches fuels, and the specific mistakes that turn a two-day adjustment into a two-week ordeal.

If you want the detail on the minerals themselves — what each one does, where to get it, and when to take it — that is the electrolytes guide.

And if you would rather have the first fortnight on a single page you can pin to a cupboard door, the Reset Card is free: it lives at the bottom of ketodive.life.

The first week is not supposed to hurt. If it does, it is usually telling you something specific, and the answer is usually salt.

This guide is general information, written to accompany a book, and is not medical advice. Everyone's health context is different — medication, pregnancy, diabetes, kidney or heart conditions, and a history of disordered eating all change what is sensible. Before making significant changes to how you eat, particularly if any of those apply, talk to your GP or a registered dietitian.