Sometimes I think their concern is genuine and the person is not well educated on lc. In that case, I usually tell them "I've done a lot of research and am confident this is a way of eating I can live on permanently, with many more benefits than health concerns". That usually shut's them up as they typically have only heard the nasty media reports.

Others are not really concerned, just want to be smarter than you and "advise" you on what they know. I just say, "thanks, but I'm really happy with this plan and I'll take my chances" It doesn't give them much room for a comeback, besides "oh" or some other lame response.