It has long been known among Eskimos and arctic travelers that eating polar bear liver can cause severe illness in men and dogs. Nearly 80 years, a researcher Norway named Kåre Rodahl set out to find out why.

“During a recent expedition made by one of us to north-east Greenland, 1939-40, specimens of polar- bear liver were collected with a view to identifying the toxic substance,” wrote Kåre Rodahl and his collaborator Thomas Moore. “On chemical and biological examination these specimens were found to be very rich in vitamin A.”

Very rich was an understatement. Their laboratory analysis showed that polar bear liver could contain more than 8,000,000 IU per pound. Contrast that with the highest level of vitamin A that’s considered safe by the U.S. Food and Nutrition Board: 10,000 IU per day.

“It seems probable that this high concentration of vitamin A is the cause of toxicity, and that the ingestion of more than small amounts of liver leads to hypervitaminosis A, the two researchers concluded.

Consuming excessive amounts of vitamin A over long periods of time can cause increased intracranial pressure (pseudotumor cerebri), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and even death, according to the U.S. National Institutes of Health.

K Rodahl and T. Moore. The vitamin A content and toxicity of bear and seal liver. Biochem J. 1943 Jul; 37(2): 166–168. (full text)