"In the small intestine nutrients are absorbed through a semi-permeable lining or hair like structures of the intestinal wall. The lining of the intestine regulates what gets in or out of the body. When you have a leaky gut syndrome this regulation is lost and your intestinal lining becomes permeable which means anything will be able to get in or out , the stomach lining is damaged. Impaired digestion leads to an excessively permeable or “leaky” gut. The bacteria in the intestine, during the process of digesting the food will produce toxic chemicals and gas. These toxins, are called endotoxins which can damage the mucus lining which results in increased intestinal permeability. Over time the lining of the intestine erodes because of these toxins. This is called a leaky gut syndrome."
Leaky Gut :Your Immunity Depends On It ! - nyalife.com