A 60-year-old woman complained of weakness, lethargy and easy fatigability. Investigation showed that she had iron deficiency anaemia (Hb. 8 g/dl). She was prescribed cap. ferrous fumarate 300 mg twice daily. She returned after one month with no improvement in symptoms. Her Hb. level was unchanged. On enquiry she revealed that she felt epigastric distress after taking the iron capsules, and had started taking antacid tablets along with the capsules. What could be the possible reason for her failure to respond to the oral iron medication?