I don't know if this test is exhaustive or not, in other words, it could be possible to be dehydrated without cracking on the tongue, but if there is cracking on the tongue, you are guaranteed to be extremely dehydrated.
I think you're referring to what's called the "median sulcus" of the tongue. Searching the net seems to uncover the idea that it exists in all tongues and is more or less pronounced by individual variation. There does seem to be correlation between dehydration and having a more visible sulcus, but I can't find anything that clearly confirms this.
(What you mean by "exhaustive" is probably "
sensitive".)
So I think that this is not a good test for dehydration.
The best indicator I've found is my lips. Are they full, red, and not chapped? I'm hydrated.
Dehydration occurs for a number of reasons, including lack of hydration. With "Adrenal Fatigue" sufferers, the main cause seems to be electrolyte imbalance, specifically, as you say, low sodium (relative to potassium). Underproduction of aldosterone means you don't hold on to sodium very well.
What you say about the body giving preference to keeping your blood balanced (and thus making it look like your sodium levels are fine when measured by blood test, despite having low sodium in your tissues) sounds entirely plausible to me. Thanks for sharing that.
I think even healthy people should be careful about drinking too much water without proper electrolytes in it. People who are said to have died of ecstasy are often victims of severe
low sodium. They washed it all out by drinking too much water and not replenishing salts ("water poisoning"). For healthy people, I recommend hydrating with an
"isotonic" solution, something with similar
Sugar and salt concentrations as human blood.
Bear in mind that the body needs to tightly maintain a particular ratio between sodium and potassium. AF sufferers lose sodium because of hormonal problems, but not potassium. So getting sodium helps, but getting potassium hurts. Oranges and bananas are high in potassium. Try eating one and see if it makes you feel worse. Try eating a second one. Take it slow and be careful. Be ready to eat a bunch of (sodium) salt after.
For AF sufferers, I don't recommend an isotonic solution that includes potassium. You might be able to get away with it, but I bet it's better to have a solution that's mostly isotonic, but heavy on sodium and light or absent potassium.
Sugar does indeed help with hydration.
You might also consider supplementing with zinc as that's also supposed to help (see the oral rehydration therapy article again), and zinc's supposed to help combat copper build-up, which many here believe is associated with AF. It was my experience that zinc helped.