Q. A 68-year-old male with a long history of chronic stomach pain and indigestion consults a gastroenterologist. His symptoms, including upper abdominal pain, nausea, and a burning sensation, have been occurring intermittently for years and have been managed with over-the-counter antacids with partial relief. Recent endoscopy shows chronic inflammation with significant loss of stomach mucosa and the replacement of normal glands by mucus-secreting cells. He has been diagnosed with atrophic gastritis. Given the patient's diagnosis, which of the following nutrients would mostly be affected due to the atrophic changes in the gastric mucosa?