diseases that cause malabsorption will be categorized as those that

1) impair intraluminal digestion,

2) impair mucosal absorption, or

3) impair nutrient delivery into the general circulation.

Efficient intraluminal digestion requires regulated release of chyme into the duodenum to allow adequate mixing with pancreatic enzymes and BAs. Pancreatic enzyme activation and micelle formation is most efficient at pH 6.5. Fats are critically dependent on pancreatic enzymes and BAs for transport across the aqueous milieu in the intestinal lumen. Proteins and CHOs are mostly water-soluble and diffuse easily across the unstirred water layer to the brush border membrane of enterocytes. Within the brush border membrane are disaccharidases and proteases which allow adequate digestion of protein and CHO even when more than 90% of the pancreas is destroyed.

Impaired Lipolysis

A number of conditions and drugs impair lipolysis, most common of which is pancreatic insufficiency due to chronic pancreatitis. Fat malabsorption in chronic pancreatitis has two distinguishing features:

1. A high amount of fat in stool, usually greater than 30 g/day. Stools are bulky, greasy, and difficult to flush down the toilet. Watery diarrhea rarely occurs because triglycerides, unlike FAs, do not stimulate water and electrolyte secretion in the colon. Also, large fat droplets exert little osmotic effect.

2. Preservation of fat soluble-vitamin absorption. Gastric lipase (and residual pancreatic lipase) generate adequate FAs for micelle formation in most cases.

Table 1. Clinical Conditions and Drugs Which Impair Lipolysis

Impaired Micelle Formation

In general, fat malabsorption due to impaired micelle formation is less severe than that due to pancreatic lipase deficiency. This relates to the fact that fatty acids (FAs and monaglycerides (MGs), to some extent, can form lamellar structures which diffuse through the aqueous milieu of the lumen to the brush border membrane. Individuals with impaired micelle formation present with watery diarrhea and after prolonged illness, fat-soluble vitamin deficiencies. The diarrhea is watery due to the secretory effects of unabsorbed FAs on the colon.

Table 2. Clinical Conditions and Drugs Which Impair Micelle Formation

Category: Gastroenterology Notes