Hypervitaminosis A
Toxic effects from excessive preformed vitamin A intake.
Hypervitaminosis A is the condition caused by consuming excessive amounts of preformed vitamin A, which includes retinyl esters, retinol, and retinal. The symptoms stem from disruptions in bone metabolism and the way the body handles other fat-soluble vitamins. Evidence suggests this toxicity has affected humans since prehistoric times. It arises from overconsumption of preformed vitamin A—found in foods like liver, dietary supplements, or prescription drugs—and can be avoided by sticking to the recommended daily intake.
Diagnosing the condition is tricky because blood levels of retinol don’t reliably reflect toxic amounts, though effective tests do exist. Treatment typically involves stopping the source of excess vitamin A, whether it’s a food, supplement, or medication, and most people recover fully. Notably, eating large amounts of provitamin carotenoids, such as beta-carotene from fruits and vegetables, does not lead to hypervitaminosis A.
Symptoms can include a range of effects. Causes center on excessive intake of preformed vitamin A, but individual tolerance varies due to genetic differences. Children are especially vulnerable; daily intakes around 1500 IU per kilogram of body weight have been reported to cause toxicity.
Regarding types of vitamin A, it is “largely impossible” for provitamin carotenoids like beta-carotene to cause toxicity because the body tightly regulates their conversion to retinol. No case of vitamin A toxicity from overconsumption of these compounds has ever been reported. The only consequence of too much beta-carotene is carotenosis, a harmless and reversible condition where the skin turns orange. In contrast, preformed vitamin A is absorbed and stored in the liver very efficiently until a pathological state develops—70 to 90 percent of ingested preformed vitamin A is absorbed.
Sources of toxicity include diet, supplements, and cod liver oil. Liver, particularly from animals like polar bears, bearded seals, fish, and walruses, is extremely high in vitamin A; consuming about 500 grams of polar bear liver would deliver an acute toxic dose for a human. Dietary supplements become toxic when taken above recommended levels.
Quick Facts
- Field
- Toxicology
Facts from the source article.
Lore & Background
Hypervitaminosis A is an ancient phenomenon; fossilized skeletal remains of early humans suggest bone abnormalities may have been caused by it, as observed in a fossilised leg bone of an individual of Homo erectus. Vitamin A toxicity has long been known to the Inuit, as they will not eat the liver of polar bears or bearded seals due to dangerous amounts of vitamin A. It has been known to Europeans since at least 1597 when Gerrit de Veer wrote in his diary that, while taking refuge in the winter in Nova Zemlya, he and his men became severely ill after eating polar bear liver.
Reader's Guide
Hypervitaminosis A is notable for its persistence throughout human history and its potential to cause significant bone and metabolic effects. The condition arises from excessive intake of preformed vitamin A, with children being particularly sensitive. Diagnosis can be difficult because serum retinol is not sensitive to toxic levels, but elevated retinyl esters in the fasting state serve as markers. Treatment involves stopping intake of the offending source, and most people make a full recovery. The condition highlights the importance of adhering to recommended daily upper intake levels, especially given that liver from certain animals (polar bear, bearded seal, fish, walrus) can be acutely toxic. Genetic variations in tolerance mean the toxic dose varies among individuals. Notably, provitamin carotenoids like beta-carotene do not cause toxicity, as their conversion to retinol is highly regulated. The effects include increased bone turnover, altered metabolism of vitamins D, E, and K, and potential mitochondrial toxicity. Recovery depends on removing the causative agent, and liver transplantation may be a valid option if no improvement occurs.
Did You Know?
- Consumption of 500 grams of polar bear liver would result in an acute toxic dose for humans.
- A tablespoon of cod liver oil contains 4,080 μg of vitamin A, exceeding the adult tolerable upper intake level of 3000 μg/day.
- No vitamin A toxicity has ever been reported from ingestion of excessive amounts of provitamin carotenoids like beta-carotene.
- Elevated amounts of retinyl ester greater than 10% of total circulating vitamin A in the fasting state have been used as markers for chronic hypervitaminosis A.
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