Could it be that when Bugs Bunny chose his special diet of carrots because he knew that the discovery of vitamin A in 1915 found it was an essential growth factor in animals (1)? Maybe he knew that vitamin A kept his skin healthy under all that fur (1). Or, more likely, that vitamin A kept his eyes healthy for seeing underground (1).
Bugs can also celebrate that his enjoyment of carrots might keep him from later having to say, “What’s up doc?” This is because clinical evidence has led the U.S. Food and Drug Administration to approve a cancer health claim for a low-fat diet rich in fruits and vegetables when it includes vitamin A (1).
Vitamin A’s benefits are all appealing to humans too. But while vitamin A deficiency is not common in developed countries, a few are deficient not being regular eaters of foods high in vitamin A like carrots, sardines or liver (1).
How much should you get? What kind of vitamin A should you be getting? And, how do you know when you should supplement? Clinicians should be familiar with the differences of the various kinds of vitamin A because it would affect recommendations.
Vitamin A references any compound that can produce biological activity of all-trans retinol (1). These include preformed vitamin A retinoids (retinol, retinal, retinoic acid, retinyl esters and others) and provitamin A carotenoids (alpha-carotene, beta-carotene, beta-cryptoxanthin) (1).
Absorption of vitamin A first requires digestion in which enzymes help free up vitamin A from proteins and fats (1). The vitamin A compounds then become solubilized into bile micelles to be transported and absorbed across the brush border membrane of the duodenum and jejunum (1).
The preformed vitamin A retinoids are absorbed easily (about 70-90 percent) as long as a meal includes sufficient fat (1). Provitamin A carotenoids are less absorbed ranging from less than 5 percent in raw foods and juices and up to 60 percent when cooked or taken purely in oil (1).
Retinoids, being lipid-soluble, are not as stable as carotenoids and can oxidize when exposed to light, oxygen, heat or some metals (2). But, again being lipid soluble, about 70-90 percent of the preformed vitamin A retinoids are absorbed (1).
In developing countries, vitamin A deficiency is not common (1). It generally occurs among children leading to increased mortality and infectious morbidity (1). The symptoms of vitamin A deficiency can include xerophthalmia (night blindness, Bitot’s spots, conjunctival abnormalities, corneal scarring and ulcerations), anorexia, retarded growth, karatinization of mucous cells (1).
The best measure of vitamin A from which to make recommendations is retinol activity equivalents (RAE) (1). For example, retinol 1 mcg is equal to RAE 1 mcg, beta-carotene 12 mcg is equal to RAE 1 mcg, and alpha-carotene or beta-cryptoxanthin 24mcg is equal to RAE 1 mcg (1). The requirements of vitamin A intake published by the Institute of Medicine’s Food and Nutrition Board published in 2001 that adult men should consume 625mcg RAE and women 500mcg RAE (1).The Recommended Dietary Allowance (RDA) is 900 and 700 mcg RAE for men and women (1).
Pregnant women have a higher RDA ranging from 770 and 1,300 mcg RAE (1)Smokers, however, should watch any increase because newer research shows vitamin A may increase risk for lung cancer rather than decrease it (1). The biochemical reasons for increasing lung cancer risk are not clear yet, but may have to do with break down products and mitochondriotoxicity (3).
Because main food labels still list vitamin A in the older International Units (IU), it’s important to point out that 1 IU vitamin A is equal to 0.3 mcg regtinal, 3.6 mcg beta-carotene and 7.2 mcg of alpha-carotene and beta-cryptoxanthin (1).
It is possible to get too much vitamin A. Hypervitaminosis A is a disorder that can lead to nausea, vomiting, double vision, headache, dizziness and skin problems (1).
The tolerable upper intake level for preformed vitamin A in adults is 3,000 mcg (1). Beta-carotene and the other provitamin A carotenoids do not have any known tolerable upper intake level (1). A tolerable upper intake level of beta-carotene levels in smokers has yet to be established (1).
Reference List
1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.
2. Carlotti ME, Rossatto V, Gallarate M, Trotta M, Debernardi F. Vitamin A palmitate photostability and stability over time. J Cosmet Sci 2004;55:233-52.
3. Siems W, Salerno C, Crifo C, Sommerburg O, Wiswedel I. Beta-carotene degradation products - formation, toxicity and prevention of toxicity. Forum Nutr 2009;61:75-86.
