The nutrition study the supplement industry doesn't want you to see

It's one thing to say the evidence that most supplements do any good just has not been produced, despite endlessly repeated, strongly motivated efforts.

It's quite another thing to discover, over and over again, that they are actively harmful. This has happened with vitamins A, C, D, E, iron, calcium, fish-oil extracts, and various anti-oxidants. The most common effect is increased cancer risk. Actually being outside, and eating actual fish, fruits, and vegetables really does provide the benefits people hope for from supplements. Evidently the good stuff isn't really the vitamins or the oils.

If there is a good story for multivitamin supplements, it would have to be (something like) that many people have increased need for certain nutrients because of various impairments and minor mutations. Discovering exactly who all these people are, and exactly what their metabolic quirks are, would be far more expensive than just giving everyone multivitamin / mineral supplements, and hoping to cover most of the problems without knowing precisely who is being helped.

But when we find supplements causing active harm in some cases, and no measurable gain despite looking very hard for any, it gets less defensible.

I have a need for 5000x as much vitamin B12 as most people, and it makes an easily perceptible difference for me. The typical supplement with 100 or 500 ug wouldn't help me enough. Fortunately, there is a simple test for this deficiency, so it is easy to tell if the supplement is working. (Also, the shooting pains in my forearms and numbness in thumb and fingers are pretty noticeable when they come and when they go.)

I would like for supplements, in general, to be safe and to help some people, even if most people get no benefit, but the increased cancer risk seems like too high a price.

There might be some supplements that are both safe and helpful for many, but I don't know if we know of any. Zinc?