What the researchers know (which may be different then what the doctors say or know).IBS is a disorder of the nervous system controlling the gut, and a part of the immune system that is also involved in controlling the gut as the immune system and nervous system often work together.You have a huge number of nerves in a neural net that control and sense all parts of the gut.IBSers have a disruption in this neural net, often from a GI infection and the damage that happens to the nerves in the gut wall as the infection is fought off.There are a number of biomarkers which show the dysfunction of the nerves and of some of the immune cells in the lining of the gut. There is no over inflammation with redness and ulcers but there are some differences in the parts of the immune system that control the gut, rather than the part that can attack the gut.Most drugs developed for IBS are directed at the nervous/control system of the gut. Not the lining, not the part that absorbs things, but the nerves that tell it when and where to do every single thing. If it isn't told what to do, it won't do the right things.There is a blood test that measures those biomarkers and the biomarkers for other illnesses that you see in the blood that can be used to rule IBS in.It is not an imaginary disease with no physical cause.Basically I think of it like a TV and a remote control. You pour a soda into the remote and short out some of its circuitry then the TV doesn't work right. The TV can do all the things a TV does, but it isn't getting the right signals so may do the wrong thing at the wrong time or not get certain signals for some things.So in IBS-D it isn't that you colon lining can't absorb water or anything like that. It is that the "move it along" signals make the stool that starts out as a liquid ends up moving though the system faster than the water is taken out.The times when IBS-D people tend to have problems are the times the colon normally gets a move it along signal. Like around the time you get up, and after meals.