If someone is on an Atkin’s-like diet, South Beach etc., with the basic principle that eating bread is a problem for the diet, how should one deal with Shabbat/Tom Tov with Kiddush and challah if the diet forbids eating bread or other mezonot?
First, let’s consider whether there might be a possibility to make HaMotzi. You can eat a very small amount – the actual size of an olive. Also, you can make the bread from oats, which don’t have gluten and I think are a lot lower in carbs. Finally, you can make bread from rice flour, as long as 1/8 (or 1/9 = כזית בכדי אכילת פרס) is from one of the five grains – and make HaMotzi and Birkat HaMazon. See Mishnah Challah 3:7 and Shulchan Arukh OC 208:9. So that means 1 kezayit, which is 7 parts rice and 1 part oats. That should be doable.
If it is not – then I would say that if the diet is for health reasons, which I assume is true in 95% of the cases, and we know that being overweight/obesity is one of the biggest health dangers, then, yes, that could override the need for lechem. As to Kiddush – you could rely on the Rishonim that fruit or the wine counts as the makom seudah, or on Rav Moshe (Iggrot Moshe YD 2:163 and OC 4:63) who says that you only need makom seudah for it to exempt you from saying Kiddush again when you want to eat more, but even without makom seudah you can eat what you want with this Kiddush.
My gut, though, is that a person is probably “cheating” a lot more than 1 minimum kezayit that is 7/8 rice, and that unless s/he is perfect in all other areas, this is not the place to be making compromises.
OK – re bread from rice flour plus one of the 5 grains, someone correctly pointed out to to me that the Mishnah Berurah on the Shulchan Arukh I quoted states that in such a 7/8th – 1/8th mixture, a person can only say ברכת המזון after eating 8 kezaytim – in other words, there is no real din of tziruf, you are just looking at a mixture that has a little bit of bread (the stuff made from the 5 grains) mixed up in a lot of rice. You would still say HaMotzi, however, since even though it is only a tiny bit of bread (1/8 of a kezayit), if the total mixture is a kezayit, you still say HaMotzi on even a small amount. If this is the explanation of this halakha, then it does not help our case.
However, this simple sense of the Mishnah, and the Yerushalmi which invokes the principle of גוררין, is that rice has properties to combine with the grains, and it is all considered HaMotzi. Thus, the Mishnah rules that it is חייב בחלה and that you would fulfill your matzah obligation with it, presumably even just a kezayit of it. This is the way the Rambam, Magid Mishnah, the Ramban and the Rosh explain. That’s a pretty chashuv lineup.
So, following those poskim, EVERYTHING is considered HaMotzi in such a mixture, and you can bentch (and fulfill matzah – relevant for those with Celiac diseases!) with just a kezayit of this. I think you can definitely follow that position when necessary, as in my mind it is the peshat of the sources and strongly backed by the Rishonim.
By the way, that position probably doesn’t even required a kezayit bikhdei achilat pras, but it doesn’t hurt.
I should also say that the Magen Avraham 208:15, quotes Ramban, Rambam, etc., explicitly and makes this point, that for them, even one kezayit of this counts as a full kezayit of HaMotzi.
And thus we learn the lesson that not everything can be found in the Mishnah Berurah.