Small Intestinal Bacterial Overgrowth (SIBO) I believe has an underlying cause from the use of oral probiotics in extreme amounts. People are taking millions, and sometimes billions daily without understanding their digestive system. This overcompensation is disrupting the balance in the small intestine.

In a nutshell, digestion starts with chewing our food, which mixes it with saliva rich in enzymes to begin the breakdown of carbohydrates. Hopefully, we chew sufficiently so that our food is like a mush. Once it reaches the stomach, the hydrochloric acid (HCl) and pepsin digests the proteins and fats from our meal. 

Another component of HCl is a mainline of defense against bacterial invasion such as E. coli. However, this defense is non-discriminatory, and will destroy good bacteria (probiotics) as well. The outcome of this function is partially determined by the balance of our stomach acid, some people are over-acidic, some may be taking "acid blockers" (either over the counter or prescription), and others just have low levels of HCl. 

While you want probiotics to get into your intestinal system, the delivery is inadequate in my opinion. The level of bacteria (good and bad) that make it through the stomach can grow rapidly in the pH of the small intestine. This can lead to an overgrowth of the bad bacteria that for many will result in SIBO.

When we understand that the function of the small intestine is to continue the digestion of carbohydrates and sugars, the importance of enzymes in this organ is of greatest importance. The pancreatic enzymes continue to digest the food and the villi (tiny hair-like follicles) of the small intestine absorb the nutrients into the blood stream for use as energy and to promote healing within the body. If one has leaky gut, the digestion is incomplete and macro-molecules are absorbed into the bloodstream creating allergens and reactions.

Once these contents move through the 22 feet of the small intestine, they pass into the large intestine at the ileocecal valve (lower right of the abdomen). The purpose of the large intestine is many, however most people think of it as the decomposition of our food into fecal material. This is one of the function of the probiotics; decomposition.