Note that I specifically wrote "Of course one should work with a health professional when considering taking dietary supplements".
That's what I do - I have multiple practitioners, and have my levels tested periodically (blood testing is of limited value with magnesium, whilst hair testing can give a better indication, though it has its own pitfalls regarding the testing procedure and the interpretation of the data, which need to be well understood by the practitioner).
You've also focused on the risk of "megadosing", which I don't practice or advocate for any nutrient.
Indeed, the real point of my comment is that supplementing any single nutrient by itself is likely to have consequences that you'll be unaware of without really doing your research. This article focuses on the risks of "megadosing" vitamin D. I'm not sure how you interpreted my pointing out that vitamin D supplementation can lead to magnesium deficiency as a recommendation to "megadose" magnesium. I've certainly never taken magnesium in any quantity that gets close to toxicity and I'd never advocate anyone else do that.
(Incidentally, I've felt before that Gwern's approach of experimenting with supplementation of a single nutrient and looking for a single change is of limited value, as it doesn't take into account the complexity effects of that supplementation. His list of self-experiments includes a magnesium/sleep trial and a vitamin D/sleep trial, with negative results in both cases. But for the reasons discussed above, a trial involving both magnesium and vit D, and perhaps other nutrients as well would likely yield very different results, and probably ones that are far more useful).
Your comments about the efficacy/cost of higher bioavailability supplements are worthy of consideration. Though it seems contradictory to warn against "megadosing" in one paragraph but then recommending increasing the dose in another.
The particular benefit I'm interested in is passing the blood brain barrier, as the biggest issues for me are mental - e.g., anxiety and memory, for which magnesium l-threonate is said to be particularly beneficial. I would expect that increasing the dosage of a different form, which doesn't cross the BBB so effectively, would potentially lead to an excess elsewhere in the body.
To be clear, I'm not primarily focusing on magnesium; the main target of my approach to healing is currently the thyroid, and I'm cautiously taking tyrosine and iodine for that, as well as working on gut/microbiome health to improve uptake so that ultimately all nutrients can be attained from the diet so and no supplementation is needed, which is ideal as you point out. But I'm not there yet; several years of testing and work has confirmed that.
Excess magnesium is eliminated very slowly, meaning overdose can happen after months of steady supplementation.
Anecdotally, gwern tracked some results after consistent magnesium supplementation and found a trend toward worsening after an initial boost: https://www.gwern.net/nootropics/Magnesium
I think we’re going to look back at this period of high-dose vitamin exuberance with a lot of regret. It’s relatively easy to hit optimal levels of all vitamins with basic attention to one’s diet. We already have several studies showing that multivitamin supplementation is slightly negatively correlated with longevity in elderly populations (or at least statistically insignificant). These people taking unnaturally large amounts of vitamins are in uncharted territory.
> As I’ve had evidence of Mg deficiency, I’ve been taking magnesium malate for a while, but am now looking at the more absorbable forms magnesium l-threonate and magnesium bisglycinate.
Two suggestions:
1) Don’t guess. Get a blood test to check for low magnesium levels. There are too many bad sources of information on the internet that attribute vague symptoms to magnesium deficiency with little supporting evidence. Getting a blood test is cheap.
2) Don’t bother with exotic higher bioavailability supplements. Just take more of the cheap stuff. If the cheap stuff has (example) 20% bioavailability and the expensive stuff has 40%, you’re almost always better off just taking twice as much of the cheap stuff.
Another complication is that the L-threonate and glycine in the more exotic formulations isn’t entirely inert. Glycine, for example, is well studied to improve sleep when taken on an empty stomach. A lot of those glowing reviews of “magnesium” for sleep are from people taking magnesium glycinate and then mis-attributing the effects of the glycine to the magnesium because they read glowing reviews of magnesium on the Internet. It’s a mess out there.