Keto flu and electrolytes: the cause, and the fix

"Keto flu" is mostly not a flu and mostly not about ketones. It is an electrolyte shortfall. When carbohydrate intake drops, insulin drops, and lower insulin makes the kidneys excrete more sodium. Potassium and magnesium follow. The result — headache, fatigue, light-headedness when you stand up, cramps, poor concentration — is what people feel in the first week of keto, carnivore, or any very-low-carb diet. The fix is to replace what is being lost, starting on day one.

The mechanism

Insulin does more than move glucose into cells. It also signals the kidneys to hold on to sodium. On a normal-carb diet, insulin is elevated after most meals and the kidneys retain sodium accordingly. Cut carbohydrate to under about 50g a day and insulin falls to a low baseline. The kidneys respond by letting more sodium go out in urine. Water leaves with it — that is a large part of the fast weight drop in week one, covered in the first-week guide.

Sodium does not leave alone. As sodium falls, the body shifts potassium to compensate, and potassium is lost too. Magnesium intake is often already low on a Western diet, and a low-carb diet removes some of its common sources (grains, legumes, most fruit). Three electrolytes go down at once, in the same week, in a body that is also adapting to a new fuel.

Two other things add to the first week. Glycogen — the body's stored carbohydrate — is depleted, and until the liver ramps up ketone production there is a short window where both fuels are low. And caffeine, which many people cut at the same time, has its own withdrawal headache. Neither of those is fixed by salt. The electrolyte part is, and it is the largest part.

What it feels like

• Headache, often behind the eyes or across the forehead
• Fatigue and low energy, especially in the afternoon
• Dizziness or light-headedness on standing (a classic low-sodium sign)
• Muscle cramps, particularly calves and feet at night (the magnesium signature)
• Brain fog, irritability, trouble concentrating
• Heart palpitations or a fluttering feeling (usually potassium or magnesium)
• Constipation, from less water in the gut and, on carnivore, less fiber

These symptoms are the most common reason people quit in week one. They are usually attributed to the diet itself — "low carb doesn't agree with me" — when the diet is doing exactly what it should and the salt intake simply has not caught up.

The fix

Sodium: about one extra teaspoon of salt a day

Beyond whatever you already use in cooking, add roughly one teaspoon of salt per day, which is about 2,000mg of sodium. The simplest delivery is a cup of salted broth (bone broth, stock, or bouillon) once or twice a day. Salting food more generously and drinking a glass of water with a good pinch of salt in it also works. This sounds like a lot to anyone raised on low-sodium advice; it is roughly the amount a normal-carb diet retains automatically through insulin. On low carb you replace by mouth what the kidneys are no longer keeping.

Magnesium: in the evening, mainly for cramps

If cramping is the symptom, magnesium is usually the missing piece. A magnesium supplement taken in the evening resolves it in most people. Magnesium glycinate or citrate are the common forms; oxide is poorly absorbed. Food sources that fit a low-carb diet — pumpkin seeds, spinach, avocado, dark chocolate at high cocoa — help on keto and ketovore but are out on carnivore, which is why carnivores in particular tend to need the supplement.

Potassium: mostly from food, carefully from supplements

Meat, fish, avocado, spinach, and mushrooms all carry potassium. Salt substitutes (potassium chloride, sold as "lite salt") are an easy top-up, and a mix of regular salt and lite salt is a common approach. Large potassium supplements are a bad idea without medical supervision, particularly for anyone with kidney problems or on blood pressure medication, because too much potassium is more dangerous than too little.

Start on day one

The most useful single change is timing. Add the salt from the first day of the diet, before any symptom appears. Waiting until the headache arrives means spending two or three days feeling terrible while the deficit reverses. Most people who salt from day one either skip keto flu entirely or get a mild, short version.

More on training days and in heat

Sweat is salty. Anyone training hard, working outdoors, or living somewhere hot loses more sodium and needs to replace more. A cup of broth thirty minutes before a session prevents the mid-workout dizziness that low-carb beginners often hit. The training guide covers the rest of the exercise-specific adjustments.

When it is not keto flu

Keto flu comes on in the first few days, stays mild to moderate, responds to salt within a day or two, and is gone inside a week or two at most. Anything outside that pattern deserves a proper look rather than more salt:

• Symptoms that persist beyond two weeks or get worse instead of better
• A fever — keto flu does not cause a fever; an actual infection does
• Vomiting, severe abdominal pain, or an inability to keep fluids down
• Palpitations that are frequent, prolonged, or come with chest discomfort or fainting
• Anyone on diuretics, blood pressure medication, insulin, or other blood-sugar medication — a sudden carbohydrate drop can change what dose is safe, and that is a clinician's call, not a diet blog's

The short version

1. Low carbs mean low insulin; low insulin means the kidneys dump sodium; potassium and magnesium follow.
2. Add about a teaspoon of salt a day beyond cooking — broth is easiest — starting on day one.
3. Magnesium in the evening if you cramp.
4. More of all of it on training days and in hot weather.
5. If it lasts past two weeks, involves a fever, or you are on relevant medication, get it checked.

Handled this way, the first week of a low-carb diet is mostly uneventful, and the electrolyte question becomes a habit rather than a crisis. It applies equally to keto, ketovore, and carnivore, and carnivore needs it most.

The app's water tracker and daily checklist make the salted-broth habit a box to tick each day, so the electrolyte fix happens on schedule instead of after the headache starts.