Vitamin A and carotene in animal nutrition

Vitamin A, an unsaturated 20 carbon cyclic alcohol, subserves a number of important physiological functions. However, the biochemical basis of these roles is not well understood. The main sources of retinol are liver, egg yolk and milk fat. Several carotenoids show vitamin A activity. The conversion of beta-carotene to retinol is affected by copper-containing dioxygenase and zinc-containing retinene reductase. The efficiency of conversion varies in different species. The latter is defined in units as the daily dose required to produce a weight gain of 3 g/week in young rats between the 4th and 8th week. Retinol is unstable on exposure to light or heat, particularly in the presence of heavy metal ions and water. Much recent work has focused on the absorption, metabolism and excretion of vitamin A. It is now recognized that plasma vitamin A levels do not reflect the nutritional status except in severe hypo- and hypervitaminosis A. Also, many dietary factors may influence vitamin A metabolism. These basic and applied aspects of vitamin A are reviewed.