Q. It’s up to You. I came across this title. The question is whether to capitalize up. My gut says do it, because it’s an idiom of sorts, and because the typical grammar rules about prepositions and title caps don’t quite seem to address this case.
A. Up in your phrase is actually an adverb, so under Chicago’s headline rules it would be capped. When you’re confused about a part of speech, a good dictionary is a terrific resource, because it provides example sentences for each part of speech. At Merriam-Webster Unabridged under the word up, at adverb definition 5(b), you can see the example “put the problem . . . up to the states,” which is similar to your “it’s up to you.” The examples in the preposition definition of up are different (“up a tree”; “up river”).