A keto headache usually shows up on day two or three of cutting carbs, with fatigue and a fog that sleep does not clear. It is the most reported symptom of what people call keto flu, and it has a specific cause you can act on: the sodium your body flushes when you drop the carbs.
Why cutting carbs gives you a headache
Carbohydrate drives insulin, and insulin tells your kidneys to hold on to sodium. That effect has been measured directly in people and reviewed in Diabetologia. Cut the carbs and insulin falls, the kidneys stop holding, and sodium leaves in your urine with water following it out. That flush is why the scale drops several pounds in the first week before any fat is involved, and it is why the headache lands around the same time.
The flush itself is well documented. Which signals drive it is still argued: insulin is the main suspect, and a 2020 review in Frontiers in Endocrinology makes a case that glucose handling in the kidney plays a part too.
Keto flu symptoms
The clearest picture of what people actually experience comes from a 2020 study in Frontiers in Nutrition, which read 448 posts across 43 keto forums and coded 256 symptom descriptions from 101 people. By share of those descriptions:
- Flu, used as a catch-all: 44.6%
- Headache: 24.8%
- Fatigue: 17.8%
- Nausea: 15.8%
- Dizziness: 14.9%
- Brain fog: 10.9%
- Stomach and gut discomfort: 10.9%
Of the 60 people who described severity, 15 called it mild, 23 moderate, and 22 severe. Forum posts are self-selected and people who feel fine do not write in, so this describes the shape of the complaint rather than how common it is.
How long does keto flu last
In that same study, symptoms began a median of 9.5 days into the diet, with reports concentrated in the first week and thinning out after four. Eight people said when theirs ended: the median was 4.5 days, most between 3 and 15. Eight is a small number to build anything on, so treat a few days to a couple of weeks as the working range. Symptoms that are severe, or still going strong past that, belong with a doctor rather than with more salt.
What helps
Sodium leads. In the forum data, the most mentioned remedy was more sodium with 58 mentions, then electrolyte supplements at 38, broth at 27, magnesium at 25, and potassium at 24. That order matches the mechanism, since you are replacing what the carb cut pushed out.
How people do it, cheapest first:
- Salt your food on purpose. Low-carb eating removes bread, pasta, and packaged food, which is where most people were getting their sodium without thinking about it.
- A cup of broth. It carries real sodium, it is warm, and it goes down when food does not appeal.
- Salt in water. It works and costs nothing, though it is hard to dose and gives you only sodium.
- An electrolyte mix. Sodium alongside potassium and magnesium in one glass, with no sugar to work against the diet.
Potassium and magnesium are worth covering from food too, and the low-carb-friendly sources are easy: avocado, leafy greens, nuts, and seeds.
Two things people get wrong. Drinking more plain water on its own tends to backfire, because it dilutes the sodium you have left instead of replacing it. And starting keto with a fast makes the first days harder, which the 2025 review below flags as one of the few changes with evidence behind it.
What the evidence actually supports
A 2025 scoping review in Frontiers in Nutrition went through the keto-induction literature and put it plainly: there is a physiological rationale for supplementing electrolytes, and a lack of clinical studies documenting the effect. Nobody has run the trial where one group replaces sodium and the other does not. What exists is a well-measured mechanism, near-universal agreement among people doing the diet, and no controlled test. Anyone selling you certainty here, us included, is out ahead of the science.
The same review found better support for easing in rather than starting the diet with a fast.
When it is not keto flu
Headache, fatigue, and dizziness are unspecific, and the first low-carb week is not the only thing that causes them. If you have kidney disease or high blood pressure, or take medication that affects sodium or fluid balance, how much sodium you add is a question for your doctor.
Where Saltivate fits
Saltivate carries 800mg of sodium per serving, with 240mg potassium and 60mg magnesium, zero sugar and zero carbs. Raw Unflavored has no sweetener or flavoring, so it goes into water, broth, or food, and its roughly 2g of salt seasons whatever it lands in. Start with one serving and adjust to how you feel.
For the diet side of this, see electrolytes for keto flu and electrolytes for keto.
