Natvia is Stevia.  David talks about Stevia in his book, and while I'm not blindly following his advice without thinking about it for myself, I tend to agree with him in general ( which is why I am here ) and I agree with him about stevia specifically, that is, buy some every now and again but don't eat a lot, and in generall, use dextrose when you want a sweetener whose effects on your body are well known.

I was able to buy stevia sweetened soft drink in the US.  I couldn't drink it, I found it had an odd taste.