system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why I can't eat just multivitamins and something filling like bread and stay healthy?

Thank you for the very informative read, especially to FreakyBloodHound... that's a pretty scary awesome concept that we don't know why but we kinda of know what works.

Thanks ELI5 I think I learned something.
gpt
There are a couple of reasons:

1)  Nutrient bioavailability matters.  If your body can't absorb the nutrition in the pill, it doesn't do you any good.  In this sense, a lot of multivitamins just give you very expensive pee. 

2)  You need macronutrients as well as micronutrients.  So carbs, fats, and proteins (especially the latter two) are important for a source of energy and for your body to build and repair things.  You'd have to swallow a ton of pills to get your 2000 calories a day, defeating the purpose.

3)  We still don't really know how nutrition works.  You require a fairly delicate synthesis of vitamins, minerals, antioxidants, and coenzymes.  There is a high probability that a multivitamin is missing something essential that hasn't even been discovered yet, or that isn't recognized as essential.