Hypervitaminosis A refers to the effects of excessive
vitamin A (specifically
retinoid) intake.
Presentation
Effects include
Birth defects
Liver problems
Reduced bone mineral density that may result in osteoporosis
Coarse bone growths
Skin discoloration
Hair loss
Excessive skin dryness/peeling (desquamation)
Angular cheilitis
Intracranial hypertension (see
Idiopathic intracranial hypertension)
Signs
Signs of acute toxicity include
nausea and
vomiting,
headache,
dizziness, blurred
vision, and loss of
muscular coordination.
Pathophysiology
Hypervitaminosis A occurs when the maximum limit for liver stores of
retinoids is exceeded. The excess vitamin A enters the circulation causing systemic toxicity.
Betacarotene, a
precursor of vitamin A, is selectively converted into retinoids, so it does not cause toxicity.
Although hypervitaminosis A can occur when large amounts of liver are regularly consumed, most cases of vitamin A toxicity result from an excess intake of vitamin A in the form of vitamin supplements. Toxic symptoms can also arise after consuming very large amounts of preformed vitamin A over a short period of time. (See Polar-bear liver below.) The U.S. Institute of Medicine says that the Lowest Observed Adverse Effect Level (LOAEL) for vitamin A, when taken over an extended period of time is 21,600 IU. Most multivitamins contain vitamin A doses below 10,000 IU, therefore multi-vitamins are unlikely to cause vitamin A toxicity when taken at their recommended dosages. But in high doses, its central nervous system toxicity can be enhanced by its lipid solubility because it is readily transported across the blood brain barrier and concentrated in the brain.
Vitamin A causes cells to swell with fluid; too much vitamin A causes them to rupture in hyposmotic environments, hence the toxicity. Toxicity has been shown to be mitigated through vitamin E (tocopherol), cholesterol, zinc, taurine, and calcium. Cholesterol has been shown to prevent retinol induced golgi fragmentation.
Recommended supplement limits
The U.S.
Institute of Medicine has established
Daily Tolerable Upper Levels (UL) of intake for vitamin A from supplements that apply to healthy populations, in order to help prevent the risk of vitamin A toxicity. These levels for preformed vitamin A in
micrograms (µg) and
International Units (IU) are
0–3 years: 600 µg or 2000 IU
4–8 years: 900 µg or 3000 IU
9–13 years: 1700 µg or 5665 IU
14–18 years: 2800 µg or 9335 IU
19+ years: 3000 µg or 10,000 IU
The dose over and above the RDA is among the narrowest of the vitamins and minerals. Possible pregnancy, liver disease, high alcohol consumption, and smoking are indications for close monitoring and limitation of vitamin A administration. However, vitamin A has also been repeatedly tested and used therapeutically over several decades in larger amounts. For example, a total dosage of 100,000 - 400,000 IU has been given for treatment of severe pediatric measles, in areas where vitamin A deficiency may be present, in order to reduce childhood mortality.
Toxicity from eating liver
The
liver of certain animals — including the
polar bear,
seal,
walrus, and
husky — is unsafe to eat because it is extraordinarily high in
vitamin A. This danger has long been known to the
Inuit and has been recognized by 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. In 1913,
Antarctic explorers
Douglas Mawson and
Xavier Mertz were both poisoned (and Mertz died) from eating the liver of their
sled dogs.
Vitamin A itself was not discovered until 1917.
Pathological changes consistent with hypervitaminosis A have been seen in bones of Homo erectus, and have also been attributed to consumption of carnivore liver.
The livers of many other animals have lower levels of vitamin A and are commonly eaten.
See also
Vitamin poisoning
Xavier Mertz
Douglas Mawson
Retinoic acid syndrome
Piblokto
References
External links
Facts about Vitamin A and Carotenoids, from the National Institutes of Health's Office of Dietary Supplements.
Category:Effects of external causes
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