An article by George Ansstas, MD,taken from medscape reference- http://emedicine.medscape.com/article/126004-overview
The word vitamin was originally derived from Funk’s term “vital amine.” In 1912, he was referring to Christian Eijkman’s discovery of an amine extracted from rice polishings that could prevent beriberi. Funk’s recognition of the antiberiberi factor as vital for life was indeed accurate. Researchers have since found that 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.
Vitamin A was the first fat-soluble vitamin to be discovered. Early observations by ancient Egyptians recognized that night blindness could be treated with consumption of liver. Two independent research teams, Osborne and Mendel at Yale University and McCollum and Davis at the University of Wisconsin, simultaneously discovered vitamin A in 1913. Vitamin A is made up 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 3 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 3, 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. Vitamin A is also important for embryonic development and the regulation of adult genes. It functions as an activator of gene expression by retinoid alpha-receptor transcription factor and ligand-dependent transcription factor.
Deficiency of vitamin A is found among malnourished, elderly, and chronically sick populations in the United States, but it is more prevalent in developing countries. Abnormal visual adaptation to darkness, dry skin, dry hair, broken fingernails, and decreased resistance to infections are among the first signs of vitamin A deficiency (VAD).
In a Venezuelan study, Jimenez et al investigated the effect of a single, oral 200,000 IU dose of vitamin A on iron and vitamin A nutritional status, the phagocytic function of neutrophils, and the rate of anemia, in a population of preschool children with a high prevalence of VAD. The study group consisted of 80 children, including 12 controls; no iron supplementation was provided.
In an assessment of the children 30 days after the administration of vitamin A, the authors found that those who had received the supplement exhibited a significant increase in concentrations of hemoglobin (Hb), mean corpuscular Hb, and serum retinol, while the rates of anemia and VAD among the children fell from 17.6% to 13.2% and from 25% to 13.2%, respectively. In addition, the phagocytic capacity of neutrophils increased in the supplement group. The authors concluded that vitamin A supplementation could help to decrease the frequency of VAD and anemia, as well as to increase the immune response, in preschool children