A 63-year-old female complains of general weakness, a feeling of heaviness, compression in the epigastrium, postprandial fullness, nausea, belching after meals. These symptoms have been observed for about 15 years. Objectively: body temperature is 36,4°C, respiratory rate - 20/min, Ps - 88/min, blood pressure - 115/75 mm Hg. Skin and mucous membranes are pale. Blood test results: RBC -2,0-1012/l, Hb -100 g/l. Tests revealed parietalcell antibodies. What is the most likely reason for the development of anemia in this patient?