I had those symptoms and ended up being diagnosed with Celiac and Crohn's Disease. I had been dealing with the diarrhea for almost two years. People told me I had irritable bowel. I just learned to live with it - until I was diagnosed with renal failure. Then the tests began. There was about a six month period between my diagnosis of renal failure and my Celiac. I switched to McDougall in the beginning and saw no change in my digestive issues. But then I began eating lots of barley soups.
However, when I found out about the Celiac Disease ~ taking gluten out of my diet cleared up my symptoms within just a couple of days.
Now... I have been completely gluten free since Jan or Feb of this year and it is like a switch... if I eat gluten (by accident) ~ I get stomach pain and diarrhea (even on a plant based diet).