Vitamin A Deficiency

Updated: Feb 03, 2025
  • Author: George Ansstas, MD; Chief Editor: George T Griffing, MD  more...
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Overview

Background

Vitamins are essential organic compounds that the human body cannot synthesize. Vitamins A, D, K, and E are classified as fat-soluble vitamins, whereas others are classified as water-soluble vitamins. [1]

Vitamin A, the first fat-soluble vitamin to be discovered, consists of a family of compounds called the retinoids. The retinoid designation resulted from finding that vitamin A had the biologic activity of retinol, which was originally isolated from the retina.

There are essentially three forms of vitamin A: retinols, beta carotenes, and carotenoids. Retinol, also known as preformed vitamin A, is the most active form and is mostly found in animal sources of food. Beta carotene, also known as provitamin A, is the plant source of retinol from which mammals make two-thirds of their vitamin A. Carotenoids, the largest group of the three, contain multiple conjugated double bonds and exist in a free alcohol or in a fatty acyl-ester form.

In the human body, retinol is the predominant form, and 11-cis-retinol is the active form. Retinol-binding protein (RBP) binds vitamin A and regulates its absorption and metabolism. Vitamin A is essential for vision (especially dark adaptation), immune response, bone growth, reproduction, the maintenance of the surface linings of the eyes, epithelial cell growth and repair, and the epithelial integrity of the respiratory, urinary, and intestinal tracts.

 
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