Ounce for ounce, beef liver offers the greatest amount of protein. Four ounces of raw beef liver provides more than 22 grams of protein. Calf liver contains slightly less, while an equal amount of raw chicken liver comes in at about 20 grams. Because the protein is of animal origin, liver contains all of the amino acids essential to human health.
Liver provides the most concentrated amount of vitamin A of all food sources. It is considered an efficient aid in vitamin A deficiency. Beef liver provides 18,000 mcg of vitamin in 3 1/2 ounces.
Liver has long been a part of the treatment for pernicious anemia because it is a rich source of heme iron (the organic iron in animal foods). This type of iron is five times more easily absorbed than non-heme iron (found in plants). Pork liver contains the highest amount of iron, at 34 mg per 4 ounce serving.
Liver is a good source of the mineral phosphorus. A diet which includes large quantities of liver also should include added calcium since high phosphorus intake can create a deficiency of calcium. Liver is one of the few natural sources of vitamin D, and it is also an excellent source of all the B vitamins (particularly B12), copper, vitamin C, and trace minerals.