Vitamin A Toxicity
- Author: Mohsen S Eledrisi, MD, FACP, FACE; Chief Editor: George T Griffing, MD more...
Background
Vitamin A is an important fat-soluble vitamin. Its basic molecule is a retinol, or vitamin A alcohol. After absorption, retinol is transported via chylomicrons to the liver, where it is either stored as retinol ester or re-exported into the plasma in combination with retinol-binding protein for delivery to tissue sites.
Dietary vitamin A is obtained from preformed vitamin A (or retinyl esters), which is found in animal foods (liver, milk, kidney, and fish oil), fortified foods, and drug supplements. Dietary vitamin A is also obtained from provitamin A carotenoids from plant sources, principally carrots. Dietary vitamin A is available mainly as preformed vitamin A in western countries and as provitamin A carotenoids in developing countries.
The bioavailability of retinol is generally more than 80%, whereas the bioavailability and bioconversion of carotenes are lower. These may be affected by species, molecular linkage, amount of carotene, nutrition status, genetic factors, and other interactions. While in general the body absorbs retinoids and vitamin A very efficiently, it lacks the mechanisms to destroy excessive loads. Thus, the possibility of toxicity exists unless intake is carefully regulated.[1] Revision of earlier estimates of daily human requirements of vitamin A has been suggested; the suggestion is that estimates ought to be revised downwards. Concerns exist about the teratogenicity of vitamin A.[2]
Pathophysiology
The recommended daily allowance for vitamin A is 5000 international units (IU) for adults and 8000 IU for pregnant or lactating women. Being fat-soluble, vitamin A is stored to a variable degree in the body, making it more likely to cause toxicity when taken in excess amounts.[3] In contrast, water-soluble vitamins are generally excreted in the urine and stored only to a limited extent; hence, adverse effects occur only when extremely large amounts are taken.
Epidemiology
Frequency
United States
Nutritional surveys indicate that about 35-50% of adults regularly consume vitamin and mineral supplements. Data are not available for consumption of vitamins in children.
Mortality/Morbidity
Mortality is rare from vitamin A toxicity, although the risk may increase with higher doses.[4]
Morbidity is evident by the wide range of complications observed in this condition.
Race
The use of supplements is generally higher in whites, as well as in individuals with higher levels of education and income.
Sex
The use of vitamin supplements is more common among females.
Age
Single vitamins are consumed more often by adults, while multivitamins are administered more frequently to children.
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