Hypervitaminosis A


Hypervitaminosis A refers to the toxic effects of ingesting too much preformed vitamin A. Symptoms arise as a result of altered bone metabolism and altered metabolism of other fat-soluble vitamins. Hypervitaminosis A is believed to have occurred in early humans, and the problem has persisted throughout human history. Toxicity results from ingesting too much preformed vitamin A from foods, supplements, or prescription medications and can be prevented by ingesting no more than the recommended daily amount.
Diagnosis can be difficult as serum retinol is not sensitive to toxic levels of vitamin A, but there are effective tests available. Hypervitaminosis A is usually treated by stopping intake of the offending food, supplement, or medication. Most people make a full recovery. High intake of provitamin carotenoids from vegetables and fruits does not cause hypervitaminosis A.

Signs and symptoms

Symptoms may include:
Signs

Causes

Hypervitaminosis A results from excessive intake of preformed vitamin A. Genetic variations in tolerance to vitamin A intake may occur, so the toxic dose will not be the same for everyone. Children are particularly sensitive to vitamin A, with daily intakes of 1500 IU/kg body weight reportedly leading to toxicity.

Types of vitamin A

  • It is "largely impossible" for provitamin carotenoids, such as beta-carotene, to cause toxicity, as their conversion to retinol is highly regulated. No vitamin A toxicity has ever been reported from ingestion of excessive amounts. Overconsumption of beta-carotene can only cause carotenosis, a harmless and reversible cosmetic condition in which the skin turns orange.
  • Preformed vitamin A absorption and storage in the liver occur very efficiently until a pathologic condition develops. When ingested, 70–90% of preformed vitamin A is absorbed.

Types of toxicity

  • Acute toxicity occurs over hours or a few days.
  • Chronic toxicity results from adult daily intakes greater than 25,000 IU for 6 years or longer and more than 100,000 IU for 6 months or longer.

Mechanism

Retinol is absorbed and stored in the liver very efficiently until a pathologic condition develops.

Delivery to tissues

Absorption

When ingested, 70–90% of preformed vitamin A is absorbed. Water-miscible, emulsified and solid forms of vitamin A supplements are more toxic than oil-based supplements.

Storage

Eighty to ninety percent of the total body reserves of preformed vitamin A are in the liver. Fat is another significant storage site, while the lungs and kidneys may also be capable of storage.

Transport

Until recently, it was thought that the sole important retinoid delivery pathway to tissues involved retinol bound to retinol-binding protein. More recent findings, however, indicate that retinoids can be delivered to tissues through multiple overlapping delivery pathways, involving chylomicrons, very low-density lipoprotein and low-density lipoprotein, retinoic acid bound to albumin, water-soluble β-glucuronides of retinol and retinoic acid, and provitamin A carotenoids.
The range of serum retinol concentrations under normal conditions is 1–3 μmol/L. Elevated amounts of retinyl ester in the fasting state have been used as markers for chronic hypervitaminosis A in humans. Candidate mechanisms for this increase include decreased hepatic uptake of vitamin A and the leaking of esters into the bloodstream from saturated hepatic stellate cells.

Effects

Effects include increased bone turnover and altered metabolism of fat-soluble vitamins. More research is needed to fully elucidate the effects.

Increased bone turnover

Retinoic acid suppresses osteoblast activity and stimulates osteoclast formation in vitro, resulting in increased bone resorption and decreased bone formation. It is likely to exert this effect by binding to specific nuclear receptors which are found in every cell.
This change in bone turnover is likely to be the reason for numerous effects seen in hypervitaminosis A, such as hypercalcemia and numerous bone changes such as bone loss that potentially leads to osteoporosis, spontaneous bone fractures, altered skeletal development in children, skeletal pain, radiographic changes, and bone lesions.

Altered fat-soluble vitamin metabolism

Preformed vitamin A is fat-soluble and high levels have been reported to affect the metabolism of the other fat-soluble vitamins D, E, and K.
The toxic effects of preformed vitamin A might be related to altered vitamin D metabolism, concurrent ingestion of substantial amounts of vitamin D, or binding of vitamin A to receptor heterodimers. Antagonistic and synergistic interactions between these two vitamins have been reported, as they relate to skeletal health.
Stimulation of bone resorption by vitamin A has been reported to be independent of its effects on vitamin D.
Mitochondrial toxicity
Preformed vitamin A and retinoids exert several toxic effects regarding the redox environment and mitochondrial function.

Diagnosis

Retinol concentrations are nonsensitive indicators

Assessing vitamin A status in persons with sub-toxicity or toxicity is complicated because serum retinol concentrations are not sensitive indicators in this range of liver vitamin A reserves. The range of serum retinol concentrations under normal conditions is 1–3 μmol/L and, because of homeostatic regulation, that range varies little with widely disparate vitamin A intakes.

Retinol esters have been used as markers

Retinyl esters can be distinguished from retinol in serum and other tissues and quantified with the use of methods such as high-performance liquid chromatography.
Elevated amounts of retinyl ester in the fasting state have been used as markers for chronic hypervitaminosis A in humans and monkeys. This increased retinyl ester may be due to decreased hepatic uptake of vitamin A and the leaking of esters into the bloodstream from saturated hepatic stellate cells.

Prevention

Hypervitaminosis A can be prevented by not ingesting more than the US Institute of Medicine Daily Tolerable Upper Level of intake for Vitamin A. This level is for synthetic and natural retinol ester forms of vitamin A. Carotene forms from dietary sources are not toxic. Possible pregnancy, liver disease, high alcohol consumption, and smoking are indications for close monitoring and limitation of vitamin A administration.

Daily tolerable upper level

Life stage group category
Infants
Children and adolescents
Adults
19–70 years		3000

Treatment

If liver damage has progressed into fibrosis, synthesizing capacity is compromised and supplementation can replenish PC. However, recovery is dependent on removing the causative agent: halting high vitamin A intake.

History

Vitamin A toxicity is known to be an ancient phenomenon; fossilized skeletal remains of early humans suggest bone abnormalities may have been caused by hypervitaminosis A, as observed in a fossilised leg bone of an individual of Homo erectus, which bears abnormalities similar to those observed in people suffering from an overdose of Vitamin A in the present day.
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 them containing 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.
It is claimed that, in 1913, Antarctic explorers Douglas Mawson and Xavier Mertz were both poisoned from eating the livers of their sled dogs during the Far Eastern Party. Another study suggests, however, that exhaustion and diet change are more likely to have caused the tragedy.

Other animals

Some Arctic animals demonstrate no signs of hypervitaminosis A despite having 10–20 times the level of vitamin A in their livers as non-Arctic animals. These animals are top predators and include the polar bear, Arctic fox, bearded seal, and glaucous gull. Plasma concentrations are maintained in a non-toxic range despite the high liver content.