Check out this post from one of my favorite bloggers, Dr. Sarah Ballantyne a/k/a the Paleo Mom, called The Stomach Acid Connection. Sarah is a PhD in medical biophysics who began writing the blog in response to her own health problems. She explains the science behind a lot of health dysfunction, and why certain approaches (like a Paleo-type diet) are helpful in improving health. I like her because she is science-based and does a great job explaining highly scientific information to the average person … especially those of us who did not excel at biology (ahem!).
Taking Betaine HCL with Pepsin (an acidifier for the stomach) and digestive enzymes have helped improve my IBS-D symptoms over the last couple of months. If you are having digestive problems, looking into the possibility that you have low stomach acid is a good idea.