I have celiac. Most people know that that means "no wheat", but it also means having a difficulty absorbing nutrition in general, and I've gotten up close and personal with a variety of nutritional deficiencies, even in cases where I nominally had enough in my diet. For me personally, I seem to have the most trouble with the metals we're supposed to absorb; I can't speak to how common this is for celiacs (and I haven't found any particular research on that exact question; it may very well be out there but I haven't seen it). It turns out that in the last year I've discovered this also encompasses magnesium deficiency. What fun.

So, I'd add: I'm really not convinced by the bioavailability of magnesium oxide. YMMV because maybe there's a threshold effect and I couldn't get anything from it and your normal digestive system will be able to, but I would suggest that if you are interested in magnesium, the actually bioavailable stuff isn't that much more expensive. I've been using: https://www.amazon.com/gp/product/B000BD0RT0/ref=oh_aui_deta... and it very likely is having a positive effect on me where magnesium oxide was not having any effect. (For various complicated reasons it is difficult for me to be completely confident in that, but this claim is very likely.)

In fact this seems to be a general pattern; what you can buy in your local grocery stores is often quite crap in terms of bioavailability, and even what you can get from dedicated health stores is often only marginally better. So you might think the "good stuff" must be likely to be more expensive... but it often isn't actually that much more expensive if you order it online. It's sort of weird.

The other thing the article at best tangentially alludes to is that magnesium supplementation is likely to be a relatively slow process. In my experience, for some deficiencies I take a pill or two and within 24 hours I know yeah, I was missing that; iron I knew about after about a week (takes time to make blood). Magnesium takes a lot longer; I've read from others in similar positions to me that six months to a year in you can still be seeing improvements, which is matching my experience. If you want to try this out, be aware that it may be a subtle improvement over a long period of time, but it could also be a really important one. And that sucks, because the low amplitude of the signal relative to the day-to-day noise makes it really hard to tell if it's doing anything for you. However, that is simply what it is.

Another thing that's relevant is that there are some nutrients that need to be in some balance, and it is possible to raise up your magnesium only to find out that need more potassium, too. There's also some interaction with calcium and sodium. I suspect that if you're normally healthy and eat reasonably healthily that you won't encounter these issues, though. (I have substantial evidence, but again, no proof, that my body does a particularly poor job absorbing potassium. Hard to imagine what proof would look like, honestly. Fortunately and/or amusingly, potassium supplements have the opposite problem most supplements have... they're actually dangerously bioavailable. So that one turns out to be somewhat easy to cover, except that I have to remember to take it in small doses throughout the day, and it shouldn't be mixed with magnesium at the same time because it turns out they like to get together in your stomach and react to form a compound that has neither the magnesium nor the potassium bioavailable. Isn't this just such fun? Ugh.)

(Also, before someone smacks reply and starts trying to read me the riot act, please do review the careful caveating I've written into this post. I'm not an expert by any means; it is just that due to circumstances beyond my control and that I would not choose, I've been thrust into the deep end of this world and forced to go beyond what science can provide me completely rigorous answers for. And I am working with the conventional medical system here... however, there are just some ways that I'm having to improvise because when it really comes down to it, we're all unique whether we like it or not, and the doctors don't have all the answers for me. I'll cop to not being as careful as gwern, but, well, I'm generally dealing with much larger effects and much greater urgency than he is... I'm generally thinking in a multi-armed bandit style moreso than a conventional null-hypothesis p-value style.)