What is the underlying cause of this syndrome
Are you asking from a western or a Chinese Medicine perspective? From a TCM perspective there are multiple potential underlying causes - most would involve the liver and/or spleen systems. In “syndromes” particularly is where proper Chinese Medicine diagnostic skills are crucial. Using all of the proper tools to arrive at that particular individuals underlying diagnosis is the only way to get good consistent results with IBS. In other words, the underlying cause, is the set of conditions unique to that individual.
In my clinical experience IBS is often just the result of a poor diet. I’ve seen many patients able to completely eliminate their symptoms of IBS by eliminating diary, soda, processed foods, and refined sugars. That would be the first step. If IBS symptoms persist even after making dietary changes, acupuncture can be extremely effective in eliminating them.