I'd acknowledge to them that weight regain is very possible, but that you have a strategy. I'd also let them know that you "aint dead yet", so they need not worry. Then I'd keep the convo moving to other things. I'd acknowedge that I had surgery, address whatever question they had briefly, and then keep the discussion moving even if it's obvious that you are trying to change the conversation.
Andddddd if the person asking is someone who doesn't care for your best interest, I'd go with what the poster earlier said ....."I burn more calories than I consume".