The intestines are the entry point for most of the body’s nutrients. Because of the damage caused by Crohns Disease and Ulcerative Colitis, that absorption is negatively impacted, resulting in malabsorption. What specific nutrients are malabsorped (and may require supplementation) depends greatly on what areas the IBD has damaged. What specific areas of the intestines, therefore, are primarily* responsible for absorbing specific nutrients?(1)
The intestines are broken up into two halves – the small intestine and the large intestine (or colon). Each have different patterns of nutrient absorption, and are further delineated into subregions.
The small intestine extends from the pyloric sphincter (marking the end of the stomach) to the ileocecal valve (marking the beginning of the large intestine). It is further broken up into three separate regions – the duodenum, the jejunum, and the ileum (in order from the stomach).
The duodenum isn’t directly involved in absorption, but it is the processing center for breaking down nutrients for absorption later in the intestines. Chyme (the liquefied contents that are the results of the stomach breaking down food) release is regulated by the duodenum, so the speed of gastric emptying can be impacted by damage here. Additionally, the duodenum’s cells signal the pancreas, liver, and gall bladder to release digestive enzymes. Damage to or excitation of the receptors can result in too little (poor breakdown of nutrients) or too much (depends on the enzyme). Duodenal damage can sometimes be detected by color changes in the stool, ranging from whitish color (potential liver disease or gallbladder issues) to yellow stool (potential bile issues), but any sustained change in color could also be the result of secondary infection and warrants a visit to the GI. Finally, the proximal duodenum is primarily associated with iron absorption and constant anemia may be a result of duodenal damage (though it can also result from bleeding anywhere in the GI tract).
The jejunum is the area responsible for the absorption of most nutrients in the intestines. The villi are larger there, and the surface area is greater than the duodenum. Once the broken down nutrients pass through the duodenum, the enterocytes, or absorption cells, take in vitamins, sugars, amino acids, and water. They then enter the bloodstream via the liver. Blood tests showing low vitamin levels may be indicative of active disease or damage to the jejunum. While the jejunum does secrete some lactase, lactose intolerance is more often, but not exclusively, associated with duodenal damage.(2)
The ileum digests everything that is left after the jejunum. Carbohydrate and protein digestion are completed through protease and carbohydrase enzyme activity. Additionally, vitamin B-12 is primarily absorbed in the ileum, and ileum damage may show up as lower B-12 levels on blood tests (and require booster shots). Finally, bile salts are absorbed in the ileum and damage may result in bile acid being passed along into the large intestine and causing rectal discomfort. Rectal pain after eating fatty foods may indicate ileum-based disease. While duodenal and jejunal damage are related to Crohn’s Disease, both Crohn’s and Ulcerative Colitis can impact the ileum.
The large intestine is broken up into the cecum, the rectum, and the anal canal. It has a smaller role in absorption than the small intestine, and is much shorter (though of a larger diameter) than it’s digestive predecessor. Because of this, individuals can survive the removal of their large intestine (as a final treatment of ulcerative colitis) through an ileostomy.
Because ileostomies are fairly well understood, the symptoms of those with ileostomies mimic those with severe large intestinal damage. The two primary absorptions of the colon are water and electrolytes, principally sodium and potassium. Damage to the large intestine may result in watery stools and diarrhea, or in systemic electrolyte imbalances.
While not primarily involved in asbsorption, rectal damage can cause increased urgency. The flow of stool to the rectum triggers the need to defecate. If the need is suppressed in normal guts, the stool is generally moved back into the cecum. In those with damaged rectal tissue, the urgency may not subside and may actually force the anal canal to open, resulting in unpleasant feelings and potential accidents.
· Damage to different areas of the intestines due to inflammatory bowel disease causes different malabsorption issues.
· Blood tests, stool color and consistency, and related symptoms can help pinpoint areas of active disease and assist in targeted treatments.
*Other areas can absorb smaller amounts of nutrients – only the primary absorption areas are noted.
1. Heizer, William D. "Normal and abnormal intestinal absorption by humans."Environmental health perspectives 33 (1979): 101.
2. Langman, J. M., and R. Rowland. "Activity of duodenal disaccharidases in relation to normal and abnormal mucosal morphology." Journal of clinical pathology 43, no. 7 (1990): 537-540.