Vitamin A Toxicity

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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

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.

History

Physical

Causes

Laboratory Studies

Imaging Studies

Other Tests

Procedures

Medical Care

Consultations

Patients with neurologic symptoms may require consultation with a neurologist.

Further Inpatient Care

Further Outpatient Care

Deterrence/Prevention

Education on the proper required daily allowance dose of vitamin A should be provided to modify the individual's current use of vitamin supplements.

Prognosis

Author

Mohsen S Eledrisi, MD, FACP, FACE, Consultant, Department of Internal Medicine, Division of Endocrinology and Metabolism, King Abdulaziz National Guard Medical Center, Saudi Arabia

Disclosure: Nothing to disclose.

Coauthor(s)

Kevin McKinney, MD, Assistant Professor, Department of Medicine, Division of Endocrinology and Metabolism, University of Texas Medical Branch at Galveston

Disclosure: Nothing to disclose.

Mohammad S Shanti, MD, ABEM, Chair, Department of Emergency Medicine, King Faisal Specialist Hospital and Research Center

Disclosure: Nothing to disclose.

Specialty Editors

Harris C Taylor, MD, Clinical Professor of Medicine, Division of Clinical and Molecular Endocrinology, Case Western Reserve University School of Medicine

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Romesh Khardori, MD, PhD, FACP, Professor of Endocrinology, Director of Training Program, Division of Endocrinology, Diabetes and Metabolism, Strelitz Diabetes and Endocrine Disorders Institute, Department of Internal Medicine, Eastern Virginia Medical School

Disclosure: Nothing to disclose.

Mark Cooper, MBBS, PhD, FRACP, Head, Diabetes & Metabolism Division, Baker Heart Research Institute, Professor of Medicine, Monash University

Disclosure: Nothing to disclose.

Chief Editor

George T Griffing, MD, Professor of Medicine, St Louis University School of Medicine

Disclosure: Nothing to disclose.

References

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