On the subject in the title: The headache that turns out to be dehydration — the long version Working notes rather than a conclusion.
A narrow question about headache, deliberately narrow, because the broad version has been asked here four times and produced four long threads and no answer.
One question, stated units, stated method, and what I have already ruled out.
Practical note on headache: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.
That is clearer than the version I had in my head. Thank you.
Answering the question post #4 raises rather than the one it answers.
Speaking only to headache as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
The most useful posts here describe what was measured, what was changed, and what happened, in that order.
Happy to be corrected if someone holds better data than mine.
On headache, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
Everything in post #8 holds. The case it does not cover is the one I have.
Two claims get bundled together under headache and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
Adding salt to food is the intervention most consistently reported as helping and is also the one least likely to cause harm.
Reading it back, the second half matters more than the first.
This follows post #11 rather than contradicting it.
For anyone finding this later: the short answer on headache is that it depends on one thing, and the rest of the thread is people identifying which thing.
Coming back to post #11, because the follow-up matters more than the original answer.
Constipation and hydration: dehydration makes constipation worse. Increasing fluid intake is the first intervention for constipation alongside other measures.
Where there is a genuine question, a measurement answers it and a supplement does not. Blood tests for the common electrolytes are routine and cheap.
That matches what I have seen, for whatever a single anecdote is worth.
Picking up post #14: that is the part I would want checked first.
Source for the headache figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.
Commercial rehydration formulations are designed for a specific purpose and are not equivalent to a sports drink, whatever the marketing suggests.
The arithmetic in post #20 is right; the assumption feeding it is the part to check.
Cramping is the symptom most confidently attributed to electrolytes and has the weakest evidence linking it to them.
Nothing to add, except that this is the answer I would give if asked.
One more thing on headache that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.
Post #23 put the caveat in the right place and I want to underline it.
Summarising the headache thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.
Adding the measurement that post #24 says would settle it.
Vomiting and diarrhoea lose fluid and electrolytes together, and replacing only fluid is what turns a short illness into a longer problem.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Post #23 describes the usual case. This is about the unusual one.
Rapid correction of a genuine abnormality can be more dangerous than the abnormality, which is another reason this belongs in clinical hands.
The most useful reply I ever got about headache was a request to state my units. It sounds like pedantry and it has saved me twice.
Where a laboratory result exists, quoting it with the reference interval makes the discussion tractable. Without it the thread is guesswork.
This has been discussed before and I could not find the thread, so, again.
I read post #26 twice before replying, because I had assumed the opposite.
Genuine question rather than a rhetorical one: has anyone here actually observed headache, as opposed to read about it? The thread is long and I cannot tell.