The exact question was as follows, from someone who is exploring Judaism but is not Jewish:
I am a celiac with MCAS, and cannot participate in the bread and wine. Who do you have that conversation with? Especially if you haven’t converted yet and are “just exploring.”@pfanderson on Twitter
My experience of Ashkenazi Jewish communities is that in general, we love to talk about medical issues. From a hospitality point of view, that is sometimes a bit of a problem when guests (or members!) feel that others’ questions are a bit too probing.
My suggestion is that you say, “No, thank you.” The offer-er will often urge you to have some (we can’t let you go hungry!) and then you have a choice. Either you can say, “No, really, thanks” and preserve your privacy or you can say, “Do you have gluten free?” They can likely give you grape or apple juice instead of wine if you like, and some synagogues will offer a gluten-free substitute for the bread.
If you choose to disclose your dietary needs, they will assume that you have a medically-limited diet, and that’s when the chatty-about-medical-stuff thing will kick in. Feel free not to play, or to play if you enjoy it. It is perfectly OK to say, “I only discuss my medical situation with my doctor.”
Now, as to bread and wine: In some Christian churches, bread and wine/grape juice are served as a ritual for members only, called “communion.” This is not the case at synagogue: the bread is just bread and it is for everyone. The blessing before the bread acknowledges that bread is a gift of God. The blessing before the wine may be a short blessing acknowledging that God brought wine from the fruit or, if it’s long and musical, it’s a toast to Shabbat. Either way, you are welcome to participate, if you are able to have the juice option, which should always be available.