I've seen a patient's depression (hx multiple admissions, though to be fair also hx cluster B traits) clear up and stay in remission with iron supplementation, with a starting ferritin in the low normal range.

I wouldn't be surprised to find that there are a lot of problems related to "subclinical" nutrient and hormone deficiencies that have not been fully clarified yet. Iron is probably one of the more egregious examples - when the normal range spans well over an order of magnitude, you have to wonder just how "normal" that range is. Not too many MDs are familiar with the literature on iron and, for example, athletic performance. If a 22M athlete comes in c/o fatigue on working out, most people will be satisfied with a normal Hgb and MCV. And the minority who go on to check an iron panel will be satisfied if there are no abnormal values. But there are studies showing improvements in athletic performance and subjective complaints with iron supplementation, with starting ferritins as high as 50. I've had patients (some healthy young men with no lab abnormalities) report massive PRs after starting iron, not to mention the subjective improvements.

MD's are not aware of this and neither are athletic trainer types. Only at the highest level are trainers sophisticated enough to make such recommendations for athletes.

I can think of a couple other examples. The upper limit of normal for TSH is lowered periodically. IIRC it used to be 10, over time lowered to ~4, and now some talk of it being lowered to 2.5. Thyroid extract is considered and inferior treatment for hypothyroidism, and it has no effect on hard outcomes - but patients do feel better on it, and this has been shown with a randomized protocol.

What's the lower limit for B12? 150? Now some people are recommending supplementation for older adults with B12 under 500.

Then I'm sure everyone is familiar with vitamin D, which is a bit of a different issue but similar in a way.

This forces the question - how many similar problems are we currently missing?

I know that the article is addressing a different issue, but my point is that titles like this are kind of ridiculous. The problem is not that a practitioner tried to clean up nutritional deficiencies. People see "vitamins" and immediately jump to "pseudoscience, quackery!", etc., which is unwarranted.