Final answer:
Gluten is the protein that people with celiac disease must avoid as it damages their small intestine's microvilli, leading to nutrient absorption problems and other symptoms, requiring a strict gluten-free diet.
Step-by-step explanation:
The protein of concern for those with celiac disease is gluten, which is found in wheat, barley, and rye.
When individuals with celiac disease consume gluten, it triggers an autoimmune reaction that damages the lining of the small intestine, particularly the microvilli. This damage impairs nutrient absorption and can lead to various symptoms such as diarrhea, anemia, and malnutrition.
To prevent these symptoms and associated complications, patients with celiac disease must adhere to a strict gluten-free diet.