The folic acid (when ionized it is called folate) is a coenzyme that participates in the synthesis and duplication of DNA and for this reason it is fundamental for cell division. If there are not enough folic acid in cells with great turnover, like red blood cells, they have their production reduced. In folic acid deficiency precursor cells (reticulocytes) that would originate erythrocytes (red blood cells) begin cell division but the process is very slow while the cytoplasm growth is normal. So the cells became abnormally large, a typical feature of this kind of anemia called megaloblastic anemia. Megaloblastic anemia can be caused also by vitamin B12 (cyanocobalamin) deficiency since this vitamin is important for cell division too. Both types of anemia are nutrient deficiency anemias. Nutrition Vitamins - Image Diversity: megaloblastic red blood cell