The gut represents the largest surface area for contact and exchange in the human body. It allows nutrients to pass through it, but also stops unwanted substances, from food, bacteria, viruses, or other foreign bodies (medicines, pesticides, additives, etc.), via various defence mechanisms (physical barrier, gut microbiota and intestinal immune system).
However, the gut can be adversely affected by a number of factors, including chronic stress or repeated infections, causing loosening of its tight junctions. The gut’s permeability is then increased, allowing it to let unwanted substances through into the blood circulation