V i t a m i n A
Vitamin A is important for growth and development (bone metabolism), for the maintenance of the immune system, healthy skin and good vision.
Vitamin A is a generic term for a number of compounds needed by the body, like retinol, retinal and retinoic acid (retinoids). Beta-carotene and other carotenoids, that can be converted by the body into retinol, are referred to as provitamin A. Hundreds of different carotenoids are synthesized by plants, but only about 10% of them are provitamin A carotenoids.
Vitamin A is commonly known as the anti-infective vitamin. In his retinol form it does maintain the integrity and function of the skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) which form the body's first line of defense against infection. It can be stored and converted to retinal when needed.
Retinal, a metabolite of vitamin A, is required for vision, formating visual purple, a pigment that allows you to see in dim light. Retinal can be converted by the body to retinoic acid.
Vitamin A, in the retinoic acid form, helps to regulate the rate of gene transcription as well in maintaining normal skin health by switching on genes and differentiating keratinocytes (immature skin cells) into mature epidermal cells.
Also stem cells are dependent on retinoids for normal differentiation into red blood cells. Additionally, Vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells.
Vitamin A supplementation has beneficial effects on iron deficiency anemia and improves iron nutritional status. The combination of supplemental vitamin A and iron seems to reduce anemia more effectively than iron or Vitamin A alone.
900 µg RDA - 3000 µg UL
RDA (recommanded dietary allowances) per day 
UL (tolerable upper intake level) per day 
 Dietary Reference Intakes: Vitamins.
The National Academies, 2001