Vitamin B12 is a nutrient that helps keep the body's nerve and blood cells healthy and helps make DNA, the genetic material in all cells. Vitamin B12 also helps prevent a type of anemia called megaloblastic anemia that makes people tired and weak.Two steps are required for the body to absorb vitamin B12 from food.First, hydrochloric acid in the stomach separates vitamin B12 from the protein to which vitamin B12 is attached in food.itamin B12 also helps prevent a type of anemia called megaloblastic anemia that makes people tired and weak. Two steps are required for the body to absorb vitamin B12 from food. First, hydrochloric acid in the stomach separates vitamin B12 from the protein to which vitamin B12 is attached in food.