Which of the following statements about colon physiology is/are correct?

A. Colonic recycling of urea is accomplished by the splitting of urea by bacterial ureases.

B. Fermentation by colonic bacteria may rescue malabsorbed carbohydrates. 

E. Insoluble fibers create bulk in the stool.

DISCUSSION: One of the functions of the colon is to recycle nutrients used in the digestive process, such as bile salts, water, and electrolytes. Urea reaching the colon via either the ileal effluent or the mucosal circulation is split by bacterial ureases. The reabsorbed ammonia is returned to the liver, where it is used for amino acid and protein synthesis. Fermentation is the anaerobic process by which bacteria can degrade carbohydrates and proteins. The normal substrate for colonic fermentation is dietary fiber, which resists degradation by alpha-amylases in the small intestine. Starch polysaccharides are normally degraded by amylases and absorbed in the small intestine; however, when starch polysaccharides are not adequately degraded and absorbed, they can also be fermented and the caloric value recovered as short-chain fatty acids. n-Butyrate, one of the short-chain fatty acids produced by bacterial fermentation, is the preferred fuel of the colonic epithelium. The colonic epithelium utilizes n-butyrate as a fuel for the absorption of sodium and water. Insoluble fibers retain water and are poorly fermented by bacteria, thus producing fecal bulk.