As with anything gut-related, it’s more about “what you’re accustomed to” than any kind of upper limit. Titrate over time — same as adjusting to changes in fiber intake.
The type (taurate, threonate, citrate, glycinate, or even carbonate) matters less. There are differences in uptake and where the salts get broken up (which is why some people take a mix). Mostly, though, the salts will be broken up and reused elsewhere. Taurine is maybe another thing worth considering in your stack, so magnesium taurate is one way to get both.
Some (knowledgeable) people (Peter Attia) take a gram a day of magnesium (in the form of several grams of mixed supplements). If you do that all at once, on your first attempt, then you’re gonna spend some time on the throne(!).
Four hundred grams per day is closer to your requirement, although there are varying degrees of uptake (which is why some people take more). You’ll get some in your food, as well, especially green leafy plants with chlorophyll. That’s why some people take less. Most people don’t die if they take none, but they may have a wide variety of other mysterious issues that they cannot trace to anything else.
If you investigate this stuff you’ll see that a common estimate is that most North Americans are deficient in magnesium due to soil depletion, but recommendations for replacement amounts vary due to problems with measuring and differences in uptake. Routine blood plasma tests are very nearly useless because your body stores and retrieves magnesium to/from your bones, which keeps your blood levels pretty steady unless you’re seriously dysfunctional.
That doesn’t mean that you don’t need to supplement to stay healthy for your lifetime — it only means that you’re not going to die if you don’t.
BTW, it isn’t necessary to take magnesium or calcium in your electrolyte mix. Daily intake is more important, and in much greater than amounts than you sweat out. Sodium and potassium are the main things that you deplete by sweating. Gear Skeptic isn’t wrong about the makeup of sweat, but he didn’t consider overall daily requirements in his discussion of electrolytes.
TLDR: Ryan’s (and Alan’s) tip(s) for a single tablet/capsule/dose for sleep is about right as a starting ballpark, but it is worthwhile to try it at home for a few days before testing it in the field.
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