ISSA Nutritionist Practice Exam

Where does most protein digestion occur in the digestive system?

1. Mouth

2. Stomach

3. Small intestine

4. Large intestine

The correct answer is: Small intestine

Most protein digestion occurs in the small intestine, making it the correct choice. This process begins in the stomach, where gastric juices, including hydrochloric acid and pepsin, start breaking down proteins into smaller peptides. However, the majority of protein digestion is completed in the small intestine. Once the partially digested proteins enter the small intestine, they encounter pancreatic enzymes such as trypsin and chymotrypsin, which further break down the peptides into even smaller amino acids and peptides. In addition to these enzymes, the intestinal lining also secretes enzymes like peptidases which help in the final stages of protein digestion. The small intestine is specifically adapted for nutrient absorption, including the absorption of amino acids and small peptides formed during protein digestion. The large surface area provided by villi and microvilli in the intestinal lining facilitates this absorption process. While protein digestion starts in the mouth and continues in the stomach, it is in the small intestine where the most significant breakdown and absorption of protein occur, making this section the key player in protein metabolism. The roles of the mouth and large intestine in protein digestion are minimal in comparison, which emphasizes the importance of the small intestine in this process.