How much fluid is actually needed, and how you would know

The question in the title: How much fluid is actually needed, and how you would know I will give what I have already checked below so nobody repeats it.

Reporting what I changed and what happened, with the caveat that I changed two things at once and therefore know less than I would like.

Over 15 weeks: fluid up, salt up, both by an amount I recorded. Symptoms improved. I cannot separate the two changes and I cannot rule out that it would have settled anyway.

How would you design this so the next person gets an answer rather than an anecdote?

The opening post answers the question as asked. The question underneath it is different.

Cramping is the symptom most confidently attributed to electrolytes and has the weakest evidence linking it to them.

Fluid intake and electrolyte intake are separate questions and drinking more water alone can make a dilutional problem worse.

Not the answer, but possibly the question that gets there.

Adding salt to food is the intervention most consistently reported as helping and is also the one least likely to cause harm.

Adding a source would improve this post and I do not have one to hand.

Noted, and thank you for writing it out rather than summarising it.

Commercial rehydration formulations are designed for a specific purpose and are not equivalent to a sports drink, whatever the marketing suggests.

Somebody will have a better source than mine, and I hope they post it.

The most useful posts here describe what was measured, what was changed, and what happened, in that order.

Symptoms that resolve quickly with fluid and salt are consistent with a simple explanation, which is worth noting without treating it as proof.

This has been discussed before and I could not find the thread, so, again.

Vomiting and diarrhoea lose fluid and electrolytes together, and replacing only fluid is what turns a short illness into a longer problem.

It is worth checking rather than assuming, which costs nothing.

Picking up post #8: that is the part I would want checked first.

Rapid correction of a genuine abnormality can be more dangerous than the abnormality, which is another reason this belongs in clinical hands.

I am describing what is, rather than arguing for what should be.

On post #10 — agreed on the reasoning, with one qualification.

Magnesium: deficiency during weight loss is common and symptoms (fatigue, muscle cramps) are nonspecific. Supplementation is inexpensive and low-risk if needed.

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.

Commercial rehydration formulations are designed for a specific purpose and are not equivalent to a sports drink, whatever the marketing suggests.