Alas, the fermenting occurs in the starter, wherein the beasties do SOME conversion of complex carbohydrates to lactic acid... but as a former sourdough baker, Rob, surely you have an idea of what a SMALL proportion of the loaf is the sourdough starter.

The sourdough's distinct flavour (from the lactic acid) is a mere sideshow to the gas bubble producing and leavening main action of the starter in sourdough breads (so the carbs remain intact in the remainder of the ingredients).

There's a few very good and detailed items about the composition and making (fermenting) of yogurts and cheeses in the Health::Candida (if I remember correctly) Forum.