Magnesium is essential for hundreds of cellular reactions, and the body stores about 25 grams of it, mostly in bones. But according to a WIRED report, that doesn't mean swallowing more magnesium will boost brain function. The article explains that magnesium in supplements is bound in salts, and only the dissolved ionic form—Mg²⁺—can be used by the body. How much gets absorbed depends on dose, stomach contents, and how much magnesium the body already has, with higher doses actually absorbed less efficiently.
The form of magnesium matters too. Cheap supplements often use magnesium oxide, which dissolves poorly and instead draws water into the intestines, acting mostly as a laxative. Magnesium citrate dissolves better and has higher bioavailability in lab tests. But even well-absorbed forms do not direct themselves to specific tissues, despite label claims about targeting muscles or the brain.
Brain-focused marketing, particularly around magnesium L-threonate, is often based on rodent experiments using high, long-term doses. Human studies are small, short, and sometimes funded by supplement makers, and they have not measured magnesium levels in the brain itself. As the WIRED article concludes, a brain-targeting magnesium supplement is, in all likelihood, just an expensive laxative.