Therefore what to do? I have seen these Hacker News vitamin D ads appear every few months for the past 15 years, or so. I always seem to have a vitamin D deficiency, so it reminds me to take supplements. I take them for a few months, hoping to see a change, but I don't feel any benefit. Then, I forget to take the supplements until the next time I see an ad. How to know if they're actually doing something useful?

Until you see an article like this which calls for '5000 mg' of supplementation, decide that you didn't take enough and overdose...

HN and dubious self-medication advice go hand-in-hand. Please consult a medical professional instead of a bunch of ad-tech devs.

you could make a decision informed by actual information, i.e. your blood levels