The bacteria in sourdough pre-digests the flour, releasing micronutrients. This process takes place during a long, slow fermentation, which also gives the loaf a superior taste and texture. Plus, sourdough bread takes longer to digest, helping you feel full after you eat it.
The best sourdough is the one that you bake at home. All you need are flour, water and salt. All of the unnatural ingredients included in commercially produced bread are eliminated. To get started, try this great homemade recipe that I've been using.
If you choose to buy your bread, find a local bakery and buy a fresh sourdough loaf made from whole wheat, rye or spelt.