What is the primary role of the large intestine in digestion, and what processes occur there?

• A. Absorption of most nutrients occurs in colon
• B. Water and electrolyte absorption; formation and storage of feces; fermentation producing short-chain fatty acids and gases
• C. Primary site of protein digestion
• D. Primary site of bile acid synthesis

Answer: (B)

The main idea is that the large intestine’s job is to reclaim water and electrolytes, form and store feces, and host bacteria that ferment remaining material to produce short-chain fatty acids and gases. While the small intestine does most of the digesting and absorbing of nutrients, the colon concentrates the waste by absorbing water and salts and then shaping it into stool. The bacteria lining the colon break down undigested carbohydrates and other materials, yielding short-chain fatty acids that serve as an important energy source for colon cells, along with gases like carbon dioxide and methane. This function of fermentation and feces formation is unique to the large intestine, while other processes—protein digestion and bile acid synthesis—occur elsewhere (protein digestion begins in the stomach and small intestine; bile acids are made by the liver).