Vitamin A Use Today and Its Potential Toxicities
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Introduction
Deficiency of vitamin A is rare in the United States, yet many people take supplemental vitamins that contain vitamin A to prophylactically manage what potentially may ail them. Vitamin A has received much attention, both in the scientific and lay press, that is established in fact, assumption, misinformation, as well as hype. Recent findings on the nutritional and metabolic differences between young and old persons have raised the concern of vitamin A toxicity. Therefore, healthcare professionals should be cognizant of the physiology of retinol, and the benefits and risks of supplementation.
Vitamin A and its natural and synthetic analogs are referred to as retinoids. There are several forms of retinoids: retinal (aldehyde); retinoic acid, which is also known as tretinoin (acid); and retinol (alcohol). Vitamin A is acquired through the diet and is ingested through animal sources as retinyl esters and through plant sources as carotenoids, and converted to retinol (Figure).1
Vitamin A is a group of compounds that plays an important role in vision, bone growth, reproduction, cell division, and cell differentiation. It helps regulate the immune system and may help lymphocytes. Vitamin A is found commonly in many animal and plant food sources and is a common component of fortified foods. In addition to foods, vitamin A is a common component of vitamin and mineral supplements.
Consumption of vitamin and mineral supplements is a common behavior in the United States, especially in older adults. Vitamin and mineral supplement use is the third largest over-the-counter drug category, with approximately $1.5 billion spent annually.2 Reasons for taking these supplements include decreasing susceptibility to health problems such as stress, colds, heart attacks, and cancer, and increasing energy.
Vitamin A in Animal and Plant Food Sources
Vitamin A found in foods that come from animals is called preformed vitamin A (Table I).3 It is absorbed in the body in the form of retinol. Retinol, one of the most usable (active) forms and the most reduced form of the vitamin, satisfies requirements for all known functions of vitamin A. Sources include liver, whole milk, and some fortified food products. The body can convert retinol into the other active forms, retinal and retinoic acid.1
Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid.3 Twenty-six percent of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids.3 Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.1
Fewer than 10% of the 563 identified dietary carotenoids can be made into vitamin A in the body. However, these carotenoids are powerful antioxidants with greater protective effects against free radical damage than beta-carotene. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health-promoting properties.
Measurements of Vitamin A Activity
Vitamin A activity is measured relative to retinol as retinol equivalents (RE). One RE is equal to 1 microgram (mcg) of retinol. International unit (IU) is a measurement of vitamin A commonly used for vitamin A supplement products.
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