Why is what you eat any of their business? Just keep reminding them of that. You might want to remind them that if what you eat is up for discussion, so is theirs.

I've done vegan, didn't work out for me, am on a ketogenic diet (for my depressive bipolar), and the first week I told a coworker, feeling self conscious after my snacking habits changed, he brought cookies in and left them where I could see them all night. Lol. Hoping just coincidence. I don't preach my diet, I just tell them it makes me feel better. Then I change the subject to them. People learn quickly not to bug me on the subject, when and if it gets raised.