Absorption of nutrients across intestines

To start my section on nutrition and gastroenterology, I thought I would begin with the basics of what we absorb where within the GI tract. A throwback to a blog post I wrote years ago about bariatric surgery and microbes.

It is really important to understand this process when seeing patients, as they may have had bowel resection surgery, may have inflammation within the bowel causing difficulties with absorption, and they may have losses from the bowel from a stoma or fistula. Considering which macro and micronutrients may need to be consumed in larger amounts, or may need replacing intravenously is really important for the health of the individual.

So here it is:


I have a similar picture hanging above my desk, as it comes in handy so frequently!

The main patient groups I see where I need to consider absorption are:

  • Crohn’s disease
    • As you can see from the above picture, the majority of our nutrients are absorbed across the small intestine, predominately the duodenum and jejunum. Although Crohn’s disease can affect any part of the GI tract, lots of patients have an affected part of small bowel. The inflamed sections of small bowel are unable to digest and absorb nutrients as well, and therefore diarrhoea can occur as these instead travel to the colon. If the disease affects the ileum, then vitamin B12 absorption can be affected (B12 is only absorbed in the ileum), meaning the patient may require B12 injections as supplements.
  • Bowel resection/short bowel syndrome 
    • Patients with IBD may require surgery in which the affected part of the bowel is removed. Depending on the part resected, absorption of certain nutrients can be affected
    • Short bowel syndrome is where <100cm of small bowel remains, and therefore absorption is severely affected. Most patients with this need long term parenteral nutrition and/or intravenous fluids.
  • High output stomas/fistula
    • After bowel resections, some people require ileostomies, colostomies, and may end up with enterocutaneous fistulas (a connection between the intestine and the skin). As an ileostomy is higher up anatomically than a colostomy, everything past the ileum will not be absorbed, and the output is likely to be watery as the water has not had chance to be reabsorbed into the colon. Consideration therefore needs to take place as to where the stoma or fistula lies, and what may not be absorbed. Due to this high output, not only water is being lost, but also essential salts. These patients may need long term electrolyte replacement in the way of double strength Dioralyte or St Marks Solution to avoid dehydration.


I could talk about this topic for ages, and maybe I shall revisit it in the future. Please let me know if you have any comments, or any topics you would like me to cover.

With thanks to discover therapies for the image above