Which substances are reabsorbed in the colon?

• A. Water, electrolytes, and short-chain fatty acids produced by bacterial fermentation
• B. Glucose and amino acids
• C. Bile acids only
• D. Vitamin B12 and intrinsic factor

Answer: (B)

The main concept here is that the colon handles what the small intestine leaves behind by focusing on fluid and electrolyte balance and harvesting energy from bacterial fermentation. The colon reabsorbs water and electrolytes from the luminal content, and the bacteria living in the colon ferment undigested carbohydrates to short-chain fatty acids. These fatty acids, especially butyrate, are absorbed by colonocytes and provide an important energy source while also helping to regulate the gut environment. Water follows the movement of ions like sodium, so overall most of the water that enters the colon is reabsorbed, which concentrates the stool.

Glucose and amino acids are absorbed primarily in the small intestine through specific transporters, not in the colon. Bile acids are mainly reabsorbed in the ileum, with only a small portion interacting with the colon. Vitamin B12 and intrinsic factor require absorption in the terminal ileum, not the colon. So the substances constituting the colon’s primary reabsorption role are water, electrolytes, and short-chain fatty acids produced by bacterial fermentation.