(98) Nutrition
Vitamin A: Benefits, Sources & Side Effects
Vitamin A
What Is Vitamin A?
Vitamin A is actually a family of substances called retinoids that includes retinol, retinal, and retinoic acid. These are called preformed vitamin A because they are in a form that your body readily uses. Retinol is the most usable of the three forms and can be converted to both retinal and retinoic acid in your body.
Preformed vitamin A is found only in foods from animal sources, such as liver and eggs, and is added to all processed milk. Plant food sources do not contain preformed vitamin A, but some do contain provitamin A carotenoids, which can be converted to retinol in your body.
Carotenoids are the yellow-red pigments that give carrots, butternut squash, and cantaloupe their vibrant, deep orange colour.
There are more than 600 different carotenoids, but only 3-beta-carotene (β-carotene), beta-cryptoxanthin (β-cryptoxanthin), and alpha-carotene (α-carotene)-can be converted to vitamin A. These three provide approximately 25 to 35 percent of the dietary vitamin A consumed by adults in the United States, with the majority of it coming from beta-carotene. Other nutritionally significant carotenoids, including lycopene, lutein, and zeaxanthin, may function as antioxidants or provide health benefits, but cannot be converted to vitamin A.
Functions of Vitamin A
Vitamin A Is Essential for Vision
Rays of light are bouncing off this page. For you to read this sentence, your eyes receive this reflection of light and begin the process of translating the light into visible images. After light enters your eye through the cornea, it travels to the back of your eye beta to the macula (which is located in the retina, as shown in figure (a) and allows you to see fine details and things that are straight in front of you).
Vitamin A is a component of two light-sensitive proteins that are essential for vision. The two proteins, rhodopsin and iodopsin, are in the tips of light-absorbing cells in the retina called rods and cones, respectively (b).
As rhodopsin absorbs incoming light, the shape of vitamin A is altered, and it detaches from its protein. This causes a cascade of events that transmits visual messages through your optic nerve to your brain. This change in rhodopsin is called bleaching.
Although the breakdown of iodopsin is similar, rhodopsin is more sensitive to light than iodopsin and is more likely to become bleached. After bleaching, the vitamin A returns to its original shape and becomes part of the protein again, regenerating the eye’s light-absorbing capabilities.
This regeneration process can take a few moments. Have you ever been outside on a sunny day without sunglasses and then entered a dark building? Was it difficult for you initially to see the objects in the room? In this situation, your eyes needed an adjustment period because much of your rhodopsin had been bleached in the bright outdoor sun and your eyes needed to regenerate it once you were in the dark room. Luckily, there is a pool of vitamin A in your retina to immediately help with this regeneration.
Vitamin A Is Involved in Cell Differentiation, Reproduction, Bone Health, and Immunity
Vitamin A plays an important role in cell division and cell differentiation, the processes that determine what a cell becomes in your body.
Vitamin A affects cell division by prompting gene expression, a process that uses genetic information to make the proteins needed to begin the process of cell division. As cells divide and cluster together, changes occur that cause them to become different from their initiating cells.
This differentiation determines what they become in your body. When immature skin cells differentiate into mature skin cells, for example, vitamin A acts as a signal to turn on the genes to create the proteins needed to make healthy skin. This role of vitamin A is one reason dermatologists prescribe retinoid-containing medicines, such as Retin-A or Accutane, to treat acne (see photo). Retin-A is a topical medication that works by enhancing the turnover of skin cells and inhibiting the formation of acne. Accutane is a medication taken orally that affects cell differentiation by manipulating the gene expression of acne producing cells to alter their development in the skin.
During the early stages of pregnancy, vitamin A signals cells to differentiate into tissues that form the baby’s body.
Vitamin A plays a particularly important role in the development of the limbs, heart, eyes, and ears.
Vitamin A may help regulate the cells involved in bone growth through gene expression Too much vitamin A, however, can negatively affect healthy bones. Vitamin A is important for keeping your skin and the mucous membranes of your lungs, intestinal tract, and kidneys healthy and structurally sound. If these linings are weakened or damaged, bacteria and viruses can infiltrate your body and make you sick.
Vitamin A helps keep your skin, which acts as another barrier to infections, healthy to prevent harmful bacteria from entering your body. Vitamin A also works with your immune system to create white blood cells that fight pathogens that enter your bloodstream.
Daily Needs
Vitamin A in foods and supplements can be measured in two ways: in micrograms (μg) of retinol activity equivalents (RAE) and in international units (IU). Because retinol is the most usable form of vitamin A and because provitamin A carotenoids can be converted to retinol, the preferred way to measure vitamin A in foods is its conversion to RAE. However, some vitamin supplements and food labels use the older measure, IU, on their products. (One RAE in micrograms is the equivalent of 3.3 IU.)
Adult females need 700 micrograms RAE of vitamin A daily, whereas adult males need 900 micrograms RAE daily. This is the average amount needed to maintain adequate stores in your body to keep it healthy.
A daily recommendation for betacarotene hasn’t been established, but the Institute of Medicine suggests consuming 3 to 6 milligrammes of beta-carotene every day from foods.7 (Beta-carotene is measured in milligrammes.) You can obtain this easily by eating five or more servings of fruits and vegetables. This amount will provide about 50 percent of the recommended vitamin An intake.
Vegetarians who eat no animal foods, including vitamin A-rich milk and eggs, need to be especially conscientious about eating carotenoids and betacarotene-rich foods to meet their daily vitamin A needs.
Food Sources
Organ meats (liver), milk, and eggs are the most popular sources of preformed vitamin A in the U.S. diet. Carrots, spinach, and sweet potatoes are American favourites for provitamin A carotenoids, including beta-carotene. Similar to vitamin A and other fat-soluble vitamins, carotenoids are absorbed more efficiently when fat is present in your intestinal tract. Adding as little as 1 tablespoon of vegetable oil to your diet can increase the absorption of carotenoids by as much as 25 percent.
Too Much or Too Little
Because vitamin A is stored in your body, excessive amounts of preformed vitamin A can accumulate to toxic levels. The upper level for adults has been set at 3,000 micrograms of preformed vitamin A daily. Overconsumption of preformed vitamin A is usually due to taking supplements and is less likely to occur from overeating vitamin A in foods. Consuming more than 15,000 micrograms of preformed vitamin A at one time or over a short period of time can lead to nausea, vomiting, headaches, dizziness, and blurred vision.
Chronic daily consumption of more than 30,000 micrograms of vitamin A (more than 300 times the amount that adults need daily) can lead to hypervitaminosis A (hyper = over, is = condition), an extremely serious condition in which the liver accumulates toxic levels of vitamin A. Hypervitaminosis A can lead to deterioration and scarring of the liver and even death.
High intake of preformed vitamin A during pregnancy, particularly in the first trimester, can cause birth defects in the face and skull and damage the child’s central nervous system. All women of childbearing age who are using retinoids for acne or other skin conditions should take the proper steps to avoid becoming pregnant.
Although vitamin A is needed for bone health, some research suggests that consuming too much may lead to osteoporosis (osteo = bone, porosis = porous) or thinning of the bone, which in turn increases the risk of fractures.
Osteoporosis-related hip fractures appear to be prevalent in Swedes and Norwegians, who tend to have high consumption of vitamin-A-rich cod-liver oil and specialty dairy products that have been heavily fortified with vitamin A. As little as 1,500 micrograms (3,000 IU) of retinol, which is slightly more than twice the RDA recommended for women, has been shown to be unhealthy for bones. The upper level applies only to preformed vitamin A from foods, fortified foods, and supplements. Provitamin A carotenoids in foods are not toxic and do not pose serious health problems. Your body has a built-in safeguard to prevent provitamin A carotenoids from contributing to vitamin A toxicity, birth defects, or bone damage. If you consume more carotenoids than you need to meet your vitamin A needs, your body will decrease their conversion to retinol. Extra amounts of carotenoids are stored in your liver and in the fat under your skin.
Eating too many carotenoids can cause a nonthreatening condition called carotenodermia (carotene = carotene, dermia = skin) which results in orange tinged skin, particularly on the palms of the hands and soles of the feet. Because these areas are cushioned with fat, they become more concentrated with the pigments and more visibly orange in colour (right hand in photo). Cutting back on carotenoid-rich foods will reverse carotenodermia.
Though a diet abundant in carotenoid rich foods is not dangerous, carotenoid supplements may be. In a study of adult male smokers, those who consumed beta-carotene supplements were shown to have significantly higher rates of lung cancer than those who didn’t take the supplements. However, when these research findings were further analysed, it appeared that only the men in the study who drank one alcoholic drink daily and consumed the beta-carotene supplement experienced the higher incidences of lung cancer. There is no known benefit associated with taking beta-carotene supplements.
Eating a variety of fruits and vegetables is the safest and most healthful way to meet your vitamin A needs.
A chronic vitamin A deficiency can lead to an inability to regenerate rhodopsin, causing night blindness. Individuals with night blindness have difficulty seeing at dusk, because they can’t adjust from daylight to dark, and may not be able to drive a car during this time of the day. If diagnosed early, night blindness can be reversed by taking vitamin A.
A prolonged vitamin A deficiency can also lead to dryness and permanent damage to the cornea, a condition called xerophthalmia (zero = dry, ophthalmic = eye). Up to 10 million children, mostly in developing countries, suffer from xerophthalmia annually, and as many as 500,000 of these children go blind every year because they don’t consume enough vitamin A. Vitamin A deficiency is the number-one cause of preventable blindness in children.
A deficiency of vitamin A is also associated with stunting of bones
Table Tips
Dunk baby carrots in a tablespoon of low-fat ranch dressing for a healthy snack.
Keep dried apricots in your backpack for a sweet treat.
Add baby spinach to a lunchtime salad.
Bake sweet potatoes rather than white potatoes at dinner.
Buy frozen mango chunks for a ready-to thaw beta-carotene–rich addition to cottage cheese or yoghurt.
The carotenoid lycopene, found in tomatoes and tomato products, functions as an antioxidant in the body.
Vitamin A can aid in the treatment of acne.
