Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters [1-3].
TWO FORMS OF VITAMIN A ARE AVAILABLE IN THE HUMAN DIET:
1.) Preformed Vitamin A (retinol and its esterified form, retinyl ester) – found in foods from animal sources, including dairy products, fish, and meat especially liver.
2.) Provitamin A Carotenoids [1-5] – found in plant foods such as sweet potato, spinach, carrots and canteloupe, etc.
The most important Provitamin A carotenoid is beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A.
Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions [2,3]. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.
Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing Vitamin A deficiency. However, their value for assessing marginal vitamin A is limited because they do not decline until vitamin A levels in the liver are almost depleted . Liver vitamin A reserves can be measured indirectly through the “relative dose-response test”, in which plasma retinol levels are measured beforeand after the administration of a small amount of vitamin A . A plasma retinol level increase of at least 20% indicates an inadequate vitamin A level [3,5,6].
For clinical practice purposes, plasma retinol levels alone are sufficient for documenting significant deficiency.
Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea [2-4]. It also supports cell growth playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs .
Sources of Retinoids: beef, liver, eggs, shrimp, fish, fortified milk, cheddar cheese and, Swiss cheese
Sources of Beta Carotene: sweet potatoes, carrots, pumpkins, squash, spinach, mangoes and, turnip greens
BEST AS SUPPLEMENT
Retinyl pamitate (fat soluble): it’s important to get adequate levels of vitamin A from retinyl palmitate because it plays a vital role in many functions in the body including vision, reproduction, bone growth, blood cell generation, and brain development.
Beta carotene (water soluble): whatever your body doesn’t need is passed in your urine, which means you can’t build up toxic levels, but you can end up with some very bright yellow urine. Since carotenoids can be converted into a different form of vitamin A (retinol) when necessary, it’s a good idea to give your body enough of them to use when it needs it.
INTERACTIONS WITH MEDICATIONS
Vitamin A can interact with certain medications, and some medications can have an adverse effect on vitamin A levels. Individuals taking these and other medications on a regular basis should discuss their vitamin A status with their health care providers.
Orlistat (Alli, Xenical), a weight-loss treatment, can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene, causing low plasma levels in some patients . The manufacturers of Alli and Xenical recommend encouraging patients on orlistat to take a multivitamin supplement containing vitamin A and beta-carotene, as well as other fat-soluble vitamins [40,41].
Retinoids such as psoriasis treatment acitretin (Soriatane) and bexarotene (Targretin), used to treat the skin effects of T-cell lymphoma. Retinoids can increase the risk of hypervitaminosis A when taken in combination with vitamin A supplements .