What do you say when a co-worker wants to talk about uncomfortable topics?

Asked by hug_of_war (10735) January 10th, 2014 from iPhone

I work in a team of five. Person A is obsessed with talking about her weight loss. Person B is overweight and has some medical issues relating to it, and other unrelated, serious conditions. B was absent today and A kept talking to C and me about how B would feel so much better if she lost x number of pounds. I am really not okay with talking about how my coworkers need to lose weight, except if you already are close and talk to them privately. I’m not going to tell my coworkers how to live their lives and I’m really not going to talk to other coworkers about how overweight someone is.

I can’t ignore A because we work a lot of hours together and they are my supervisor. How do you redirect a conversation in a professional way?