Then they really freak out when they see you eating, say, a chicken breast and plain oatmeal for breakfast instead of the Krispy Kremes everyone else is eating. Or taking a protein shake to a meeting.
<insert Homer Simpson voice> UUUmmmmm, Krispy Kremes....
Or you go out with some cow-orkers after work and either have a Diet Coke or perhaps one Amstel Light while they start pounding beers. Then you pass on the chicken wings.
I try not to make a big deal out of this stuff (just say "hey, I'm on a diet..."), but it does get noticed and remarked on.