Iron absorption in the small intestine

Iron plays an important role in a wide variety of metabolic processes and is an essential nutrient for almost all living organisms.

Iron absorption occurs mainly in the duodenum and upper jejunum, though small amounts may also be absorbed from the stomach, ileum and colon.

Iron enters the stomach where it is exposed to stomach acid and changed into a form that allows it to be absorbed. The portion of the small intestine called the duodenum is the chief area where iron absorption takes place. There may be a second minor absorption site near the end of the small intestinal tract.

Iron is provided to the body in various forms through the diet, but is primarily absorbed as either inorganic iron or as heme iron.

Iron released from food in the stomach appears to be absorbed by the apical villi of duodenal epithelial cells. Intestinal epithelial cells are unique cells with functional and morphological polarity. Ferrous iron passes through the apical membrane (at the luminal surface) and basolateral membrane (at the serosal surface) into the portal vein.

Iron transported into the mucosal cells through the apical membrane, but which is not transported to the portal blood, is stored by intracellular ferritin. Ferritin expression in the small intestinal mucosa is controlled by iron nutritional status, and is lost after 1 – 2 days when the epithelial cells are sloughed at the villus tip.

The second stage of iron absorption is termed basolateral or serosal transfer, where iron is transported from the enterocytes into the intestinal capillaries across the basolateral membrane via the iron transporter ferroportin 1.
Iron absorption in the small intestine