Q39) How much bread does someone on a low carb diet need to eat for a (Shabbos) meal?
A39)There are several factors involved in eating a bread meal and another factor if it's a Shabbos or Yom Tov meal. Because of the number of issues and the controversy about the size of a kezayit it would be preferable to ask one's orthodox Rabbi to get a precise size. However, I put together the different issues that I could to explain the basics of this halacha.
For the actual Bracha of Hamotzei on bread there is no minimum requirement and one must make Hamotzei even if one is eating less than a kezayit of bread. (S"A 168:10 and 210:1)
II. Birkat HaMazon
There is no requirement to bentch (make Birkat Hamazon) unless one ate a minimum of a kzayit of bread. (S”A 168:9, 210:1)
According to Sephardim, if one ate less than a kezayit of bread there's no obligation to wash, if one ate one kezayit one should wash without a Bracha of Al Netilat Yadayim and if one ate a Kebaytzah (2 Kezaytim) one should wash with a Bracha. (S"A 158:2, Yalkut Yosef 158:4). According to Ashkenazim preferably one should have a Kebaytzah to wash with a Bracha but if one needs to have a kezayit one should wash with a Bracha. If one eats less than a kezayit one should wash without a Bracha. (Vezot HaBracha chap 2, pg 13 based on Igrot Moshe 4:44 and Rav Elyashiv)
IV. Shabbat or Yom Tov meal
Preferably, one should eat slightly more than a Kebaytzah of bread in order to fulfill the obligation of having a Shabbat or Yom Tov meal (S"A 291:1, Or Letzion vol 2, 21:5) however, if one is unable to eat a kebaytzah a kezayit is sufficient (Yalkut Yosef 274:23, Aruch HaShulchan 274:7, Mishna Brurah 291:2). For health reasons (someone on a diet) one may certainly have just a kezayit to fulfilling the meals of Shabbat (Avnei Yishfeh 4:26(1))
V. Covering other foods in the meal
If one eats a regular bread meal, the bread covers the Bracha of all over foods in the meal. (S"A 177:1) However, if one doesn't want to eat bread it's unclear whether it exempts the other foods and so it's preferable to avoid this situation, however, if one is in such a situation one doesn't need to make a Bracha Rishona on each food because there is at least a possibility that the Bracha on bread exempted it. (Magan Avraham 177:1, Avnei Isfeh 4:26(3)). Nonetheless, if one is eating bread on Shabbat or Yom Tov in order to fulfill the obligation of eating the meal all other foods are covered by the bread. (Magan Avraham 177:1, Halachos of Brachos chap 5 pg 98)
VI. How big is the kezayit?
According to Ashkenazim the kezayit is slightly less than one ounze of volume. A good approximation is half the center piece of a 1 lb rye bread (Halachos of Brachos Addendum 1, pg 246). According to Sephardim the kezayit is 27 grams in weight. (Yalkut Yosef 158:4)
On Shabbat or Yom Tov, someone on a low carb diet should wash (without a Bracha of Al Netilat Yadayim according to Sephardim and with a Bracha according to Ashkenazim) and eat a kezayit of bread (half the center piece of a 1 lb rye bread). For having a meal during the weekday see above.