system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: what exactly is a vitamin?

And eg when I take a vitamin b12 supplement pill, what are its contents and how do they interact with my body at the cellular level?
gpt
The word "vitamin" just means "a specific chemical that your body needs but cannot make itself, so must be acquired through diet".

Vitamins are quite diverse from each other, and all have a variety of properties. Each one does something different - the term "vitamin" doesn't describe their specific use.

Vitamin C, for example - it's chemical name is ascorbic acid or ascorbate. It's a type of sugar-acid, similar to glucose. We need small amounts of ascorbate to assist our natural enzymes to create collagen, a protein used to make bone, skin, connective tissue and other essential structural bits. Without an external supply of ascorbate, your body cannot effectively repair wounds, because it can't create collagen.

This eventually leads to scurvy, which can be fatal - you can look up the effects of scurvy if you're interested.