In some people with irritable bowel syndrome (IBS), ingested FODMAPS carbohydrates are not absorbed as they should be in the small intestine; instead they pass into the large intestine. Humans have large numbers of bacteria living in the large intestine. This is normal. The problem is that FODMAPS act as "fast food" for the bacteria, which give off a lot of gas as they ferment the food. The gas makes your large intestine bloat (swell). If you have IBS, your intestines may be extra sensitive to this, and it hurts. It may cause constipation or diarrhea.
Another problem with FODMAPS is the way they pull water into your large intestine. This is called osmosis. One way to imagine osmosis is to picture what happens when you sprinkle sugar on freshly cut strawberries. The sugar attracts the water from the strawberries to itself. The sugar pulls the juice right out of the strawberries and into the bowl. In your large intestine, FODMAPS pull water out of your cells and into your large intestine in the same manner, causing it to bloat. Pain and watery diarrhea can result.
All FODMAPS are believed to cause IBS symptoms the same way: too much gas and water in your large intestine.
Find out more about this book:IBS: Free at Last! Change Your Carbs, Change Your Life with the FODMAP Elimination Diet, 2nd Edition