Shalom,   According to the Hida (Rabbi Haim Yosef David azulai). the reasons as to why Birkat hamazon has such a powerful effect on parnassa is that  pronouncing the blessings within it while singing aloud is a powerful segula for elevating the spiritual light within food. This in turn furthers the spiritual process of rectification which is the purpose of creation. The minimum amount of bread one needs to have is a kzayit or about 29 grams  The way to maximize the positive effect on your body is to have the type of bread. Be it oat,spelt,whole wheat that agrees with your body while also eating a full meal/ snack during the time of the meal and minimizing the amount of bread that you consume.   Also you can join others that are eating a meal with bread and have at least a reviit of wine about 4 fluid ounces instead of bread so as to make the birkat hamazon with the group. Best wishes——   R.YosefKarpman