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Vitamin A (Retinol)

Vitamin A and its metabolites play diverse roles in physiology, ranging from incorporation into vision pigments to controlling transcription of a host of important genes. Health depends on maintaining vitamin A levels within a normal range, as either too little or too much of this vitamin lead to serious disease.

Structure

Vitamin A or retinol has a structure depicted to the right. Retinol is the immediate precursor to two important active metabolites: retinal, which plays a critical role in vision, and retinoic acid, which serves as an intracellular messenger that affects transcription of a number of genes. Vitamin A does not occur in plants, but many plants contain carotenoids such as beta-carotene that can be converted to vitamin A within the intestine and other tissues.

Physiologic Effects of Vitamin A

Vitamin A and its metabolites retinal and retinoic acid appear to serve a number of critical roles in physiology, as evidenced by the myriad of disorders that accompany deficiency or excess states. In many cases, precise mechanisms are poorly understood. Some of the well-characterized effects of vitamin A include:

Sources of Vitamin A

Vitamin A is present in many animal tissues, and is readily absorbed from such dietary sources in the terminal small intestine. Liver is clearly the richest dietary source of vitamin A.

Plants do not contain vitamin A, but many dark-green or dark-yellow plants (including the famous carrot) contain carotenoids such as beta-carotene that serve as provitamins because they are converted within the intestinal mucosa to retinol during absorption.

Vitamin A is stored in the liver as retinyl esters and, when needed, exported into blood, where it is carried by retinol binding protein for delivery to other tissues.

Vitamin A Deficiency and Excess States

Both too much and too little vitamin A are well known causes of disease in man and animals.

Vitamin A deficiency usually results from malnutrition, but can also be due to abnormalities in intestinal absorption of retinol or carotenoids. Deficiency is prevalent in humans, especially children, in certain underdeveloped countries. In herbivores such as cattle, vitamin A deficiency is usually due to lack of green feed, such as in animals coming off of dry summer pastures or those fed poor quality hay. Because the liver stores rather large amounts of retinol, deficiency states typically take several months to develop. Some of the more serious manifestations of vitamin A deficiency include:

Vitamin A excess states, while not as common as deficiency, also lead to disease. Vitamin A and most retinoids are highly toxic when taken in large amounts, and the most common cause of this disorder in both man and animals is excessive supplementation. In contrast, excessive intake of carotinoids are not reported to cause disease - you cannot use the excuse of potential vitamin A toxicity to avoid eating carrots or green vegetables!

Both hypovitaminosis A and hypervitaminosis A are known to cause congenital defects in animals and likely to have deleterious effects in humans. Pregnant women are advised not to take excessive vitamin A supplements, and some medical authorities also recommend that they consume liver only in moderation, which is usually not a hard sell to make.

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