First of all, sourdough is easier to digest than brewer's yeast-based bread, because the wild yeast and lactic acid bacteria produced during fermentation help neutralize antinutrients that grains produce. Gluten, a type of protein in grains can cause gastric issues in those who are allergic to it or sensitive to it.  Sourdough bread does have some gluten content but it's not completely gluten-free because sourdough fermentation doesn't degrade it all. For this reason, those with celiac disease or gluten allergy should avoid it and choose a gluten-free loaf instead.