Quick Answer: Cutting carbohydrate lowers insulin, and lower insulin tells the kidneys to excrete sodium and water. That sodium loss, not the absence of carbohydrate itself, causes most of what people call keto flu: headache, fatigue, lightheadedness on standing, muscle cramps, and constipation. The practical response is deliberately adding sodium, roughly 3 to 5 grams of sodium daily in the first weeks for most people, plus adequate potassium and magnesium from food. It usually resolves symptoms within a day or two.

The first week of low carb has a reputation, and the symptoms are real. What is misleading is the name, because keto flu implies something you have to wait out.

Most of it is a fluid and electrolyte shift with a specific, fast fix.

Why does low carb cause sodium loss?

Through insulin’s effect on the kidney, which is direct and well described.

Insulin promotes sodium reabsorption in the renal tubules. When you cut carbohydrate, insulin falls, that reabsorption signal weakens, and the kidneys excrete more sodium in urine. Water follows sodium.

Three consequences follow immediately:

• Rapid weight loss in the first week. Commonly 1 to 3kg, and predominantly water, not fat. This is why the scale moves dramatically then slows, which people misread as a plateau.
• Reduced blood volume, which produces lightheadedness on standing, reduced exercise capacity, and headache.
• Loss of other electrolytes alongside sodium, particularly potassium and magnesium.

Glycogen adds to it. Each gram of stored glycogen holds roughly 3 grams of water. Depleting muscle and liver glycogen releases a substantial amount of water, which is excreted with the sodium.

Which explains the symptom timing. Symptoms typically start on day two or three, peak around day three to five, and resolve as either you replace the sodium or your body adapts over one to two weeks. Replacing it is faster and more comfortable.

And it explains why the standard advice fails. Someone starting low carb who is also reducing salt, because reducing salt is generally framed as healthy, gets the worst version of this.

How much sodium do you actually need?

More than standard guidance, specifically because you are excreting more.

Commonly recommended in low-carb practice: 3 to 5 grams of sodium per day during adaptation, which is roughly 7.5 to 12.5 grams of salt. General population guidance is around 2.3 grams of sodium, so this is a deliberate increase, not an oversight.

How to get it:

• Salt your food generously. The simplest route, and enough for many people.
• A cup of broth or bouillon, which provides roughly 0.5 to 1 gram of sodium and is the most palatable single intervention.
• Salt in water, around a quarter teaspoon in a large glass, taken once or twice daily. Effective and unpleasant for some.
• Electrolyte products, convenient and usually more expensive than salt.

Potassium, around 3 to 4 grams daily from avocado, leafy greens, salmon, mushrooms, and nuts. Note that many potassium-rich foods are also carbohydrate sources, so this takes a little planning on a low-carb diet. Potassium supplements above small doses warrant medical guidance.

Magnesium, 300 to 400mg daily, from nuts, seeds, leafy greens, and dark chocolate, or via supplement. Magnesium glycinate and citrate are well absorbed; oxide is poorly absorbed and more likely to cause loose stools.

The timeline you should expect: if your symptoms are electrolyte-driven, adding sodium usually improves them within hours to a day. If two days of adequate sodium changes nothing, the cause is probably something else, such as inadequate total calories, insufficient sleep, or simply too abrupt a change.

Important exception: if you have hypertension, kidney disease, heart failure, or are on diuretics or blood pressure medication, do not increase sodium substantially without talking to your clinician. Low-carb diets can also lower blood pressure independently, which sometimes means medication needs adjusting. This is a genuine reason to have the conversation before you start rather than after.

What else causes week-one symptoms?

Three things worth separating out, because they have different fixes.

Under-eating, which is extremely common. Removing carbohydrate removes a large share of most people’s calories, and if fat and protein do not increase to compensate, the result is a substantial unintended deficit. Fatigue, irritability, poor sleep, and feeling cold all follow.

The fix is eating more fat and protein, not more discipline. Track for a few days if you are unsure.

Insufficient fiber and fluid, causing constipation. Cutting grains, fruit, and legumes removes most dietary fiber. Add non-starchy vegetables in volume, plus seeds, nuts, and avocado. Fluid needs are higher, not lower, during adaptation.

Fat adaptation, which is the genuine metabolic part. Upregulating the enzymes for fat oxidation takes time, commonly two to four weeks and sometimes longer for high-intensity performance. This part cannot be shortcut with electrolytes, and it shows up mainly as reduced capacity for hard efforts rather than as feeling unwell.

Expected performance pattern:

• Endurance at easy to moderate intensity recovers within a few weeks and can be maintained well long-term
• High-intensity work, above roughly 80 percent of maximum, is typically impaired and may remain somewhat impaired, since glycolysis needs glucose
• Strength is generally maintained, and top-end power output can drop somewhat

If you train hard, targeted carbohydrate around sessions is a legitimate option rather than a failure, and is the basis of carb-cycling approaches.

A practical first-two-weeks protocol

Set this up on day one rather than reacting on day three.

Daily, from day one:

• One to two cups of salted broth, morning and afternoon
• Salt food generously at every meal
• 2.5 to 3.5 litres of total fluid, more in heat or with training
• A magnesium source or supplement, 300 to 400mg, ideally in the evening
• Potassium-rich low-carb foods: avocado, spinach, salmon, mushrooms

Around training:

• Extra sodium before longer or hotter sessions
• Expect reduced high-intensity capacity for two to four weeks and plan easier sessions during that window
• Do not begin low carb in the same week as a hard training block or a race

Warning signs to act on:

• Heart palpitations, often magnesium or potassium related and worth medical attention if persistent
• Fainting or near-fainting, which warrants stopping and seeking advice
• Severe or worsening cramps despite adequate electrolytes
• Symptoms that persist beyond three weeks, which suggests the approach needs revisiting rather than more patience

On timing your start: avoid beginning during a heatwave, since sweat sodium losses stack on top of renal losses. Late August in much of the US is still hot enough for this to matter, and the first cool weeks of September are an easier window. Also avoid starting the week of a big work deadline, since the adaptation period genuinely affects cognitive comfort for some people.

And one reassurance: the electrolyte piece is the part people most often skip and the part that most reliably changes how the first two weeks feel. Sorting it out is cheap, fast, and does not require any special product.

This is general information, not medical advice. Do not substantially increase sodium if you have hypertension, kidney disease, or heart failure, or take diuretics or blood pressure medication, without speaking to your clinician first.

FAQ: Low Carb Electrolyte Questions, Answered

What causes keto flu?

Mostly sodium and fluid loss. Lower carbohydrate means lower insulin, which reduces sodium reabsorption in the kidneys, so sodium and water are excreted. That produces headache, fatigue, lightheadedness, and cramps.

How much salt should I have on a low-carb diet?

Commonly 3 to 5 grams of sodium daily during the first weeks, which is more than general guidance because you are excreting more. Do not do this without medical advice if you have hypertension, kidney disease, or take diuretics.

How long does it take to adapt to low carb?

Electrolyte symptoms resolve within a day or two of adequate sodium. Fat adaptation for exercise performance typically takes two to four weeks, and high-intensity capacity may stay somewhat reduced.

TL;DR:

• Lower insulin means the kidneys excrete more sodium and water. That, not carbohydrate absence, drives most week-one symptoms.
• Aim for 3 to 5 grams of sodium daily during adaptation, plus potassium from food and 300 to 400mg magnesium.
• Broth is the easiest delivery. Symptoms usually improve within hours to a day if electrolytes were the cause.
• Under-eating, low fiber, and genuine fat adaptation are the other three causes, and each has a different fix.

If you have hypertension, kidney disease, or take diuretics or blood pressure medication, have the sodium conversation with your clinician before you start, not after.