The healthy gastrointestinal tract absorbs only the small molecules like those that are product of complete digestion. These molecules are the amino acids, simple sugars, fatty acids, vitamins, and minerals that the body requires for all the processes of life to function properly. The intestines, small intestine in particular, only allow these substances to enter the body due to the fact that the cells that make up the intestinal wall are tightly packed together. The intestines also contain special proteins called 'carrier proteins' that are responsible for binding to certain nutrients and transporting them through the intestinal wall and into the bloodstream.
Leaky Gut Syndrome (LGS) is the name given to a condition in which the ability of the intestinal wall to keep out large and undesirable molecules is reduced. Hence the name, as substances that are normally kept outside the body and within the intestines, are "leaking" across the intestinal wall and into the body as a whole. This happens when the spaces between the cells of the intestinal wall become enlarged for various reasons.