A 52-year-old man from Delhi presents with a 6-week history of epigastric pain, heartburn, and regurgitation. Upper endoscopy reveals erosive esophagitis (Los Angeles Grade C) and a gastric ulcer. He is started on omeprazole 40 mg once daily. After 4 weeks of therapy, repeat endoscopy shows complete healing of the esophagitis and ulcer. However, the patient develops chronic diarrhea, and serum vitamin B₁₂ level is found to be 180 pg/mL (normal 200–900). Which of the following best explains the vitamin B₁₂ deficiency in this patient?