The body uses vitamin B12 to create DNA and keep blood and nerve cells healthy, according to the National Institutes of Health. Folic acid, on the other hand, is used in producing DNA, making RNA, converting food into fuel and making the skin healthy, states the University of Maryland Medical Center.
Vitamin B12 is also essential for preventing anemia, the National Institute of Health states. When someone eats food containing it, hydrochloric acid in the stomach frees B12 from its carrier protein. Intrinsic factor, another protein, then binds to it so that the body can absorb it. When someone has pernicious anemia, they do not produce intrinsic factor, which causes an inability to absorb vitamin B12. Good sources of vitamin B12 include fish, beef, eggs, milk and fortified breakfast cereals.
Vitamin B9, also known as folic acid, is essential for turning carbohydrates into glucose for fuel;, keeping the skin, hair and eyes healthy, maintaining the nervous system and liver function and producing DNA and RNA, the University of Maryland Medical Center notes. It works with other vitamins, including B12, to regulate homocysteine, which is an amino acid that can cause heart disease when it is present in high levels. When pregnant women take vitamin B12, they reduce the risk of fetal nervous system defects, including spina bifida and brain damage. Good sources of folic acid include dark leafy greens, lima beans, whole grains and supplements.