Beef liver is an excellent source of high-quality protein. Not only do pets love the flavor, liver is known to be one of the most concentrated sources of natural vitamin A of any food. Liver is also a great source of folic acid and all of the B vitamins (particularly vitamin B12), which help with fatigue, mental ability and nerve health, as well as preventing anemia. Liver also contains one of the best sources of iron. The primary function of iron is oxygen transport and cell respiration. Iron is also necessary for many functions in the body including formation of hemoglobin, brain development and function, regulation of body temperature, muscle activity and metabolism. The iron in liver is one of most easily digestible and usable sources of iron. Beef liver is also particularly abundant in copper, phosphorus and zinc.