The intestine is a part of the digestive tract, which helps in the digestion of food and the passage of nutrients to the blood and the rest of the body in bilaterians. This part of the digestive system extends from the outlet of the stomach to the anus. In humans and most mammals, it is divided into two parts called the small intestine and the large intestine. In animals with a circulatory system, it is the part of the body that assimilates nutrients from food into the blood. The healthy gut is a barrier against some microbes, but permeable to nutrients. It is the site of intense microbial life (intestinal microbiota, human intestinal microbiota). Any abnormal change in its permeability can affect the whole organism.

Small intestine While the stomach mainly “breaks” the molecules that make up food into smaller molecules, digestion continues in the small intestine, where a number of molecules are reduced to nutrients that can be assimilated by the body. The thin membrane of the small intestine is covered with folds and villi in order to increase the exchange surface with the blood network. Most proteins are thus assimilated in the small intestine, as well as carbohydrates and lipids. In humans, it measures between 3 and 6 meters and is made up of the duodenum, jejunum and ileum.

The large intestine in humans measures between 1 and 1.5 meters and has three parts: the cecum (with the vermiform appendix), the colon and the rectum.

The large intestine is home to bacteria living in symbiosis with their host, which can break down molecules that the human body is unable to assimilate. This intestinal flora (or intestinal microbiota) is essential for the proper functioning of the digestion process.

Colon and rectum are sometimes the site of abnormal, uncontrolled proliferation of cells in their mucous membrane. This causes a polyp-shaped lump (adenoma), which can degenerate into invasive colon cancer.

The "second brain" About 200 million neurons are present in the human gut, and the enteric nervous system communicates closely with the central nervous system.

Studies have suggested that the gut microbiota also takes part in communication between the gut and the brain and influences brain function. Some researchers [Which?] Even look into possible links between an imbalance of the intestinal microbiota and certain psychological disorders such as stress, depression, but also neurodegenerative diseases (Parkinson's, Alzheimer's ...).