I don't go around announcing I'm fasting but I don't lie about it.
If I'm at an engagement and I'm asked why I'm not eating, I tell them I'm fasting. After the surprised "Seriously?" It usually opens a door for conversation -- there I am, many days into a fast yet I'm healthy, bright eyed, involved, etc. Kind of takes the wind out of the argument of "Oh my God you're starving yourself."
Those that know me and see that I'm not eating simply ask, "Are you fasting again?" It's all quite normal for them that I do this. And in fact, by being open about the fasting, I have found many others that also fast or have seriously wanted to but didn't know anyone that did.