In animals with leaky gut syndrome, the intestinal wall is more permeable than it should be. This means that undigested food, toxins and "bad" bacteria may be able to cross the intestinal wall and enter the blood stream. Some people believe this may trigger an immune reaction in the body that could be responsible for a whole range of symptoms and conditions from gastrointestinal signs such as diarrhea and bloating, to conditions outside of the gastrointestinal system.
However, there is little evidence to support this theory, and currently leaky gut syndrome is not recognized as a distinct medical condition in people or animals.