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?