Sometimes I get really tired of defending what I choose to eat too and I agree that it's really nobody else's business
what I choose to put in my mouth. As I see it, there are several options:
1) If someone is rude enough to comment unasked about what you are eating, you could always respond with an equally personal question such as "are you wearing cotton panties or nylon? Nylon is very unhealthy, you know". Another variation of this would be, "Thank you for your concern. Where were you when I was stuffing myself with Twinkies and ice cream?" This approach is not for the faint hearted.
2) If someone asks how you lost weight, say something like, "I'm cutting out sugars and starches and eating more veggies". This approach is usually met with the comment of, "oh..that sounds very healthy!"
3) Tell people that you have an allergy to wheat/grains and sugar gives you a migraine.
4) Tell people directly that you are low carbing and if they start in on you about how unhealthy it is tell them that you'd be more than happy to discuss the topic with them once they've read a book on the subject and actually understand what it's all about, but until then you'd rather not discuss it.
5) Ignore the naysayers and outlive them all. After all, living well and in good health is the best argument in favor of what you're doing there is.