Overview of vitamin A

INTRODUCTION

Vitamins are a number of chemically unrelated families of organic substances that cannot be synthesized by humans but need to be ingested in the diet in small quantities to prevent disorders of metabolism. They are divided into water-soluble and fat-soluble vitamins (table 1).

Ancient Egyptians recognized that night blindness could be treated by consumption of liver [1]. In the late 1920s, through the efforts of a Swiss scientist named Karrer and his colleagues, the fat-soluble compound in liver was isolated and termed vitamin A [2].

This topic review will focus on issues related to vitamin A. Overviews of the other fat-soluble vitamins, minerals and water-soluble vitamins are available elsewhere. (See "Overview of vitamin D" and "Overview of vitamin E" and "Overview of vitamin K" and "Overview of dietary trace minerals" and "Overview of water-soluble vitamins" and "Vitamin supplementation in disease prevention".)

CHEMISTRY

Vitamin A is a subclass of a family of lipid-soluble compounds referred to as retinoic acids. These consist of four isoprenoid units joined in a head to tail fashion. There are two main forms of vitamin A: provitamin A carotenoids (beta-carotene and others), and preformed vitamin A.

• Provitamin A carotenoids are found in plants. There are many forms of provitamin A, but beta-carotene is only one that is metabolized by mammals into vitamin A.
• Preformed vitamin A (retinol, retinal, retinoic acid, and retinyl esters) is the most active form of vitamin A; it is mostly found in animal sources of food and is also the form supplied by most supplements.