A patient with chronic nephritis for 5 years, on a long-term low-salt and low-protein diet, presents with fatigue, nausea, and vomiting for 20 days. BP is 140/100 mmHg, no edema, Hb 60 g/L, urinary protein (+), granular casts 0-3/HP, serum albumin 30 g/L, globulin 25 g/L, BUN 20 mmol/L, serum Cr 1220 μmol/L, serum Na 125 mmol/L. What is the main cause of anemia in this patient?