The liver of mammals, fowl, and fish is commonly eaten as food by humans. Domestic pig, lamb, calf, ox, chicken, goose, and cod livers are widely available from butchers and supermarkets while stingray and burbot livers are common in some European countries. Animal livers are rich in iron, copper, the B vitamins and preformed vitamin A. It is unsure if daily consumption of liver can be harmful, as no conclusive research has been done on the toxicity of preformed vitamin A from food. A single serving of beef liver exceeds the tolerable upper intake level of vitamin A. 100 g cod liver contains 5 mg of vitamin A and 100 µg of vitamin D.