Hi, Andrea
I totally understand what you're saying. When I first started in RR, I would get sucked into those kind of frustrating conversations.
Long ago, I stopped explaining anything. Life is so much easier that way! I'm doing this for me -- I don't have to explain a thing. Even though I may be really excited about PnP and want to tell the world, most people don't care. So I don't waste my energy.
My conversations would go something like this:
He: Here, have some cake.
I: Nah, thanks, I'll pass.
He: Why is that? This is good stuff.
I: Maybe so, but I feel so much better when I don't eat that stuff.
He: I think it's all in your head.
I: Could be, but I like my head this way. I think I'll have a cup of tea while you're having your cake. Want some?
Then while making or ordering tea, change the subject.
I only explain if people comment positively on changes in me and are really interested, not just trying to talk me into eating something or wanting to discuss how weird I am. Otherwise, I just have several simple lines that I use and avoid all explanations.
That gets me out of the rolled eyes, the discussion of willpower, and the need to do involved debates about the scientific validity of what I'm doing. It certainly works for me! 
Jo
: I have trouble explaining sugar
: addiction to some people. This is
: a thing of mine that I do not
: quite understand, but I shut down
: in discussions because I will all
: too soon hit a wall anyway and
: will not know how to argue on. So
: whilst KNOWING that I have a good
: point, I will feel like I am
: "losing" the argument as
: the other person just has the
: better arguments. Very
: frustrating. Can anybody help me?
: This is how a recent discussion
: went:
: He: Here, have some cake.
: I: Nah, thanks, I ´ll pass.
: He: Why is that? This is good stuff.
: I: I don ´t eat sugar.
: (exclamations of surprise on is side,
: then my explaining that this is
: not a diet)
: He: I think it´s all in your head.
: I: That´s the point. And also it´s
: biochemical.
: He: What happens when you eat sugar?
: I: It re-enforces my anxiety. And
: also, I can ´t stop.
: He: Is there science to back this up?
: I: Experimental science, yes. It´s
: like with alcoholism.
: He: So there ´s no science to back it
: up?
: I: It´s experience values. (I know, I
: need to get the BE and serotonin
: stuff right and learn how to
: represent it.)
: He (huffs): It´s a question of
: willpower.
: I: No, it´s not. It´s like with
: alcoholism.
: He: But that´s just it, it´s a
: question of willpower. I know many
: friends who just quit drinking
: from one day to the next.
: Willpower. They had a drinking
: problem in college, and then they
: just quit.
: I: Well....
: He: So is this like a diet, then?
: And so on. At some point the eating
: disorder question will pop up. But
: more often than not, I will have
: just aborted discussion before it
: gets to this, confining myself to
: being perceived as weird,
: neurotical, or both.
: Any ideas?