There are several forms of vitamin A found in the foods we eat, and there are also other metabolically active forms that are found in the body. Pre-formed vitamin A is found in foods of animal origin, or in vitamin supplements. The main form of this is retinol , formula C20H29OH, which got its name from its affect on the retina of the eye. Retinyl esters , found in food and in the body, are a form of vitamin A that is esterified with a fatty acid, most commonly palmitic acid. Two metabolically active forms of retinol in the body are retinaldehyde , and precursor to rhodopsin, and retinoic acid , the principle metabolite of retinol.
Retinoic acid has a hormone-like action.
It carries oligosaccharides in glycoprotein synthesis,it increases epidermal growth factors, it supports growth and cell development, and has many other functions related to its regulation of a large number of genes. Retinal is needed for the synthesis of rhodopsin. In a deficiency, rods and cones in the eye cannot adjust to light changes, which results in night blindness. It also plays a role in bone and teeth development. Provitamin A carotenoids are found in fruits, vegetables and dairy products.