"So what do they believe causes IBS? There are a few theories, but I find treatment is really only effective if you look at all aspects of the condition. Some of the common trends in IBS are as follows:
Low diversity of gut bacteria: Typically the human digestive tract houses several species of lactobacillus, bifidobacterium, etc. Gut bacteria is there to help fight invading strains of bacteria, help you digest your food, and also work in concert with your immune system to maintain balance in your body. Many studies indicate that people with IBS don’t have as many different species, and there is increasing evidence that restoring gut bacteria can eliminate gas and distension in many sufferers."
That Gut Feeling: Understanding IBS | Daily Bulldog