system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do multivitamins and Omega-3 pills actually do anything? or is it more of a placebo-type thing?
gpt
**TL;DR**: *Depends on whether you have a terrible diet or a good (or even average) diet. They only really help if you have a complete absence of some nutrients; you'll get enough to get by if you eat a variety of non-junk foods like most of us do.*

With a terrible diet, such as if you're a senior citizen that can't cook and eats very small portions or you live off of low-end ramen noodles or something, you're simply missing out on a ton of nutrients, particularly vitamins but also protein and fibre. 

Since you're not eating any greens or organ meats at all and stick to inexpensive pre-prepared foods with soft drinks on the side only, you're not going to get any Vitamin A, C or K, for example. The first is important for a lot of stuff like vision and immune system, the second helps keep your blood systems healthy, and the last is critical to avoid a condition known as scurvy (but doesn't actually do much else despite what the marketers tell you). 

(Omega-3's are needed in there too, but it's kind of overstated as to how much they help unless you have cardiovascular issues. Bad if you don't eat any fish at all ever, usually okay otherwise if you get a trace of 'em from that Tuna Helper casserole or have anchovies on that pizza.)

And there's lots of similar vitamins that can be missed if you go the other route and stay completely uniformly vegan too.

So if you have a really shitty diet, that once-per-day vitamin pill gives you SOME of those vitamins when otherwise you don't get any at all. That's good and helpful...

...but most people don't need them if they have a mixed salad once in a while or eat "fortified" foods which have them added. In that case, they get arguably limited value out of multivitamins.