I get asked that question a lot. With close friends, I'm able to answer it easy enough. But since I chose not to disclose my lap band surgery at work, it's a little harder. I work in an exceptionally gossipy office. I fly under the radar whenever possible, and don't have a ton of close friends here anymore (my two closest ones quit years ago). I choose to keep most aspects of my life private, and this one wasn't any different. I absolutely didn't want to be gossiped about, scrutinized, or speculated about in any way. This was an extremely personal decision I made, and I have no interest in discussing it with people I consider merely aquaintances.
So when I get asked "How have you lost so much weight?", I have fairly vague stock answers. "I've made a lot of changes in the last year. I eat a lot less than I used to, I track everything I eat. And I try to be more active." All of those are true, but putting it that way still makes me feel dishonest.