I can't say I really care about whether or not orange juice is "natural" - it's all made of chemicals at the end of the day, so the only thing that really matte

I can't say I really care about whether or not orange juice is "natural" - it's all made of chemicals at the end of the day, so the only thing that really matters to me is which ones, and how much - but the degree to which truth-in-advertising has essentially ceased to exist does bother me. Exactly how much do I have to modify something before I can't advertise it as "100% Natural!" anyway? Speaking of which, can anyone provide a good (comprehensive) source for information on the subject from a legal perspective?