Colonic digestion and absorption of energy from carbohydrates and medium-chain fat in small bowel failure.

Gut function and the degree of intestinal insufficiency or failure in short bowel patients can be quantified with respect to wet weight and energy absorption by the use of balance studies. This enables the physician to distinguish patients with extreme intestinal failure inconsistent with the restoration of intestinal autonomy by dietary manipulation from short bowel patients with borderline gut failure in whom dietary manipulations may result in the weaning from parenteral support. A high-carbohydrate, low long-chain fat diet and a diet where long-chain fat has been replaced by medium-chain triglycerides increase absorption of energy in patients with small bowel failure, provided that they have a preserved colon in continuity. This is due to the ability of the colonic flora to ferment carbohydrates malabsorbed in the small bowel to the short-chain fatty acids (SCFAs). These SCFAs are easily absorbed across the colonic mucosa resulting in a salvage of carbohydrate energy that otherwise would have been lost in feces. In contrast, long-chain fatty acids are not absorbed by the colon, and long-chain fat malabsorbed in the small bowel of short bowel patients are not retained in the large bowel. Recent work has indicated that the water soluble medium-chain fatty acids are effectively absorbed in the large bowel similar to the SCFAs. This may explain an almost complete absorption of medium-chain triglycerides in short bowel patients, even in patients with virtually no absorption of long-chain fat, and why this only occurs in patients with a colon in continuity. Manipulation of the dietary fat:carbohydrate ratio is much less efficacious in short bowel patients with no colonic function, and the use of medium-chain triglycerides has no proven effect on overall energy absorption from short bowel patients without a large bowel in continuity. Hence, the colon has increasingly important digestive functions as small bowel failure proceeds, not only when it comes to absorption of water and sodium, but also of energy from carbohydrates and medium-chain fat.