The ileum is the final and longest section of the small intestine, responsible for absorbing vitamin B12, recycling bile acids, and serving as a major hub of immune surveillance. At roughly 300 cm (about 10 feet) long, it handles tasks that no other part of the digestive tract can perform, making it uniquely important to your overall health.
Where the Ileum Sits
The small intestine has three parts in sequence: the duodenum, the jejunum, and the ileum. The jejunum handles much of the initial nutrient absorption, while the ileum picks up what remains and takes on several specialized jobs. It connects to the large intestine at the ileocecal valve, a muscular junction that controls how quickly digested material moves into the colon. That valve also acts as a one-way barrier, preventing bacteria from the colon from migrating backward into the small intestine.
The entire small intestine is about 2.5 cm in diameter, far narrower than the large intestine. The ileum’s inner lining is slightly different from the jejunum’s: it has fewer and shorter finger-like projections (villi) on its surface, but it compensates with dense clusters of immune tissue and specialized receptor cells that handle absorption tasks the jejunum cannot.
Vitamin B12 Absorption
The ileum is the only place in the body where vitamin B12 can be actively absorbed. This process starts much earlier in the digestive tract. When you eat foods containing B12 (meat, fish, eggs, dairy), your stomach produces a protein called intrinsic factor that binds to the vitamin and escorts it through the intestine. Once this paired complex reaches the ileum, receptor cells on the intestinal wall recognize it and pull it across the lining into the bloodstream.
Without a functioning ileum, this handoff fails. B12 deficiency leads to a specific type of anemia and can cause nerve damage over time. The last 100 cm of the ileum is where B12 absorption is concentrated, so even partial loss of this segment can have serious consequences.
Recycling Bile Acids
Your liver produces bile acids to help digest and absorb fats. Rather than manufacturing a fresh supply for every meal, the body recycles about 95% of its bile acids through a loop between the ileum and the liver. The total bile acid pool in your body is only about 3 grams, but this small supply cycles between 4 and 12 times per day, meaning only 0.2 to 0.6 grams is lost in stool daily and needs to be replaced.
The distal ileum (the portion closest to the large intestine) is where this recapture happens. Specialized transport proteins on the ileal lining actively pull bile acids out of the intestinal contents and send them back to the liver through the bloodstream. When this system breaks down, whether from disease or surgical removal of the ileum, unabsorbed bile acids flood into the colon. The result is chronic watery diarrhea, a condition called bile acid malabsorption. It also leads to poor fat absorption, since there aren’t enough bile acids returning to the liver to keep up with demand.
The Gut’s Immune Surveillance System
The ileum contains the highest concentration of Peyer’s patches in the entire intestine. These are organized clusters of immune tissue embedded in the intestinal wall, and roughly half of all Peyer’s patches in the body sit in the distal ileum. They function as the gut’s dedicated immune monitoring stations.
The surface of each Peyer’s patch features specialized cells called M cells. These cells sample bacteria, viruses, and other particles directly from the intestinal contents and shuttle them to immune cells (dendritic cells, macrophages, and lymphocytes) waiting just beneath the surface. Those immune cells evaluate the threat, and if they identify a pathogen, they activate a broader immune response that spreads to nearby lymph nodes. This system is what keeps the balance between tolerating harmless gut bacteria and mounting a defense against dangerous ones.
The ileum’s immune role is especially important because of its position. It sits right before the colon, where bacterial concentrations explode. The ileum itself hosts around 10 million to 100 million microbes per milliliter of fluid, a significant jump from the upper small intestine (which has only around 10,000 to 100,000 per milliliter) but still far less than the colon. The bacterial communities here tend to be fast-growing species like lactic acid bacteria and enterobacteria, distinct from the colon’s denser, more diverse populations.
Controlling Flow Into the Colon
The ileocecal valve at the end of the ileum does more than just connect two segments of intestine. It regulates transit time, controlling how quickly partially digested material passes from the small intestine into the colon. This matters because the small intestine needs enough contact time with food to extract nutrients. If material rushes through too fast, absorption suffers.
The valve also prevents backwash. Colonic bacteria are present in concentrations vastly higher than what the small intestine is designed to handle. If those bacteria migrate upward past a compromised ileocecal valve, they can colonize the small intestine and cause a condition called small intestinal bacterial overgrowth, which leads to bloating, malabsorption, and nutrient deficiencies.
Magnesium and Remaining Nutrient Pickup
Beyond B12 and bile acids, the terminal ileum also absorbs magnesium. While earlier sections of the small intestine handle most macronutrients (carbohydrates, proteins, and fats), the ileum catches what the jejunum missed. The slightly alkaline environment of the distal ileum, with a pH between 6.5 and 7.5, supports these final absorption steps.
The ileum also absorbs water and electrolytes, further concentrating the intestinal contents before they enter the colon. By the time material leaves the ileum, most of its usable nutrients have been extracted.
What Happens When the Ileum Is Compromised
Diseases like Crohn’s disease frequently target the terminal ileum, and surgical removal of damaged ileal segments is sometimes necessary. Losing more than 100 cm of ileum reliably causes B12 deficiency, bile acid malabsorption, and problems absorbing magnesium. The broader consequences include chronic diarrhea, fatty stools, weight loss, vitamin deficiencies, and electrolyte imbalances. These effects stem directly from the loss of absorptive surface area and the faster transit that follows when there’s less intestine to slow things down.
Fat malabsorption deserves special mention. When bile acids aren’t recycled properly, fat passes through undigested, which also drags fat-soluble vitamins (A, D, E, and K) along with it. Magnesium absorption worsens further because unabsorbed fatty acids in the gut bind to magnesium and prevent it from crossing the intestinal wall. So the effects of ileal loss tend to cascade, with one deficiency compounding another.