Is it permissible to eat homemade fermented foods, such as sauerkraut, soy sauce, and sourdough bread, to improve digestion?

The basic principle regarding foodstuffs is permissibility, and nothing is prohibited unless there is evidence [to the contrary]. Concerning yeast, if its quantity is small and it is consumed in pickles such that it leaves no intoxicating or debilitating effect, then there is no harm. However, if it leaves an intoxicating or debilitating effect, then it is forbidden. As for leavened bread, there is no harm in it, for the yeast used is not an unlawful intoxicant, nor does it leave an intoxicating effect, and cooking by fire removes any potential effect. The fundamental principle regarding edibles and garments is permissibility until there is evidence of prohibition.