Note: this is an article we published in 2002. For an updated article on short bowel syndrome, click here.
To understand about short bowel syndrome, we must first understand the digestive process. The upper part of the GI tract consists of the esophagus, stomach, and duodenum. The lower GI tract contains the intestine, which is approximately 8 metres long and consists of two parts: the small bowel/intestine (about 6 metres in length), and the large bowel/intestine or colon (2 metres). The small bowel is roughly divided into two parts, the jejunum (upper portion), and the ileum (lower portion).
The upper 40% of the small intestine is the jejunum, from Latin, meaning empty. The lower 60% is the ileum, from the Greek, meaning to roll or twist. These portions of the small intestine differ significantly in nature and function. The jejunum wall is thicker and its lumen wider than the ileum. Another significant difference between the upper and lower sections of the small intestine is that peristalsis (intestinal muscle contraction) is more forceful and rapid in the jejunum than in the ileum. Therefore, passage of material is usually faster through the upper section of the small intestine and slower as it goes further along.
The principal function of the small intestine is to digest (break down) and absorb (cross the intestinal wall into systemic body fluids) all the dietary nutrients, including proteins, carbohydrates, fats, vitamins, and minerals. The small and large intestines work together in cooperation with the liver and the pancreas, as well as colonic bacteria, to break down complex foods and to extract the right amount of each nutrient. Water and salt are absorbed into the body via the colon.
What is Short Bowel Syndrome?
Short Bowel Syndrome (SBS) occurs when the small intestine ceases to function properly due to trauma, disease activity, or when too much of it has been removed. A bowel resection (surgical removal of a portion of the intestine) may occur for various reasons, such as reduced blood supply to the bowel and, less commonly, for strangulated hernia, Crohn’s disease, or trauma. The small intestine is amazingly adaptive; in fact, up to 40% of it can be removed without significant loss of digestion. However, adaptation depends upon which portion of the small bowel is removed, and the function of that specific part. Patients do better if the duodenum, proximal jejunum, distal half of the ileum, and the ileocecal valve are spared. Therefore, if the “middle” part of the small bowel is removed, the patient will usually have the least amount of digestion complications. Conversely, if just 25% of the distal ileum is removed, severe diarrhea and significant malabsorption can occur.
Two types of diarrhea can result after a massive ileal resection – one induced by malabsorbed bile salts and the other from malabsorbed fat. In the first, bile acids are not absorbed in the small intestine where they should be, but carry on to the colon where they interfere with the body’s ability to absorb electrolytes and water. If the bile salts are not high enough in the small intestine, the ability to digest fat is also impaired and fat proceeds to the colon, where it too interferes with absorption of electrolytes and water, thereby producing diarrhea.
Most of the crucial absorption of fat takes place in the ileum. Here, removal of intestinal sections results in reduced ability to digest fats. Direct nutritional consequences of decreased fat absorption, aside from weight loss, are the decreased absorption of vitamins A, D, K, and E.
A common pitfall for patients with SBS is to overcompensate for this decreased fat absorption by overeating. Interestingly, patients do much better on a low fat, high protein, and low simple sugar diet, so that the difficulty the ileum has in absorbing fat does not interfere with the rest of the digestive process. Because carbohydrates are primarily broken down by bacteria in the colon, it is difficult to know what affect small bowel resection has on carbohydrate absorption. If the jejunum is resected, the patient usually tolerates the procedure well but may develop lactose or disaccharide intolerance.
Most absorption of water and electrolytes occurs in the colon, so resections of large amounts of the small bowel have very little affect on digestion of these components. However, a diet high in salt may result in diarrhea. Absorption of calcium, magnesium, and zinc is also reduced.
Management of diarrhea in SBS
In cases where diarrhea is severe, intravenous fluids may be required to replace fluid and electrolyte losses. The key electrolytes that require frequent monitoring are sodium, potassium chloride, and magnesium. The amount of fluid infused is measured according to the amount of loss incurred, as well as adequate urine output. As oral intake increases, infusions begin tapering. There is also medicinal management of diarrhea with antidiarrheal agents such as loperamide (Imodium®) and diphenoxylate (Lomotil®), used only on the advice of a doctor.
Therefore, the affect of short bowel syndrome on the body’s ability to absorb nutrients and the amount of diarrhea experienced depends on which portion of the small intestine is removed or affected by disease. Patient experience is varied but the usual occurrence is frequent diarrhea.