When is food completely digested?

• A. When it passes from the esophagus and into the stomach
• B. When it leaves the first section of the small intestine
• C. While it's in the large intestine
• D. When it leaves the body as a bowl movement

Answer: (B)

Most chemical digestion and the start of nutrient absorption happen in the small intestine, especially when chyme first meets the digestive juices in the duodenum. This first section mixes chyme with pancreatic enzymes and bile, so carbohydrates, proteins, and fats begin to break down there. The lining also provides brush-border enzymes that finish breaking down small molecules. By the time the contents move into the next portion of the small intestine, most nutrients have been digested and are ready to be absorbed. So the point at which chyme leaves the first section of the small intestine is when digestion is effectively complete.

In contrast, passing through the esophagus into the stomach is mainly transit with some initial digestion, the large intestine does little chemical digestion, and expelling waste is not digestion.