Vitamin A
Description
Vitamin A is a stable fat-soluble vitamin that is involved in a wide variety of bodily functions.
As a constituent of the visual pigment rhodopsin, it plays an important role in the chemical initiation of visual excitation.
Vitamin A also has an effect on the development and maintenance of epithelial cells and the development of bone. Vitamin A is necessary for the differentiation of underlying cutaneous cells into mucous epithelial cells, and hence is an important factor in determining the integrity of mucous membranes, such as that in the respiratory tract, urinary tract, and digestive tract.
Vitamin A is essential for the proper synthesis of mucoproteins and glycoproteins found in mucous membrane secretions which act to inhibit infection by maintaining epithelial integrity.
By virtue of it roles in epithelial cell differentiation and the effect of retinoids on cancer malignancies, vitamin A has been hypothesized to serve an anticancer function as well.
Method of Action
Vitamin A occurs in several forms; retinol, retinal (or retinene), and retinoic acid. It can be metabolically converted to retinal ester. Sources of vitamin A are the carotenes, yellow-red crystalline compounds synthesized by plants.
Vitamin A is split by various enzymes in the pancreatic juice to free retinol. The retinol form is then absorbed by the mucosal cells of the intestine, where it is incorporated in small transport particles of fat called chylomicrons. These particles are released into the lymphatic system and eventually are carried to and stored in the liver. Factors affecting fat absorption affect vitamin A absorption since vitamin A is a fat-soluble vitamin.
Vitamin A is mobilized from the liver bound to the retinol-binding protein (RBP) which serves to transport vitamin A into the circulatory system for subsequent use.
Vitamin A is an essential component of photoreception. As a visual pigment, it is combined with a protein opsin and is itself what is called the II-cis form. This form changes to the II-trans retinaldehyde upon light exposure, releasing the protein opsin and invoking visual excitation response.
Vitamin A also plays an important role in the development of bone and maintenance of epithelial cells.
Beta-carotene, a vitamin A precursor yellow in color and a constituent pigment of green, orange, and yellow vegetables, is one of the most important sources of vitamin A to humans. Beta-carotene yields two molecules of vitamin A per molecule of beta-carotene metabolized, but this conversion is only about 50% efficient in the body.
Properties & Uses
One form of vitamin A, retinoic acid, has proven to be effective in some cases for the treatment of acne and various skin disorders when applied topically.
Vitamin A has been shown to be of value in protection against cancer. This has been related to the function of retinoids in promoting normal differentiation of epithelial cells during premalignancy periods.
Vitamin A supplementation has proved effective in reversing symptoms of night blindness and corneal damage.
Vitamin A has been considered an anti-infective vitamin because deficiency is directly linked to high incidence of respiratory ailments. Adequate vitamin A intake will decrease the added risk of respiratory infection by maintaining the body's stores at a protective level.
Consequence of Deficiency
Vitamin A deficiencies occur from lack of adequate dietary intake or upon interference with its absorption, storage, or transport. Vitamin A metabolism is determined by available supply. With a decreased supply, the body slows the rate of vitamin A metabolism in an effort to conserve the current supply.
Severe deficiency symptoms include skin changes, night blindness and corneal ulcerations. In the most advanced states of deficiency, the mucous membranes of the respiratory system, gastrointestinal system, and urinary tract may be affected.
Vitamin A deficiency can also cause xerophthalmia or xerosis conjunctivae. These rare conditions may also be caused by weight loss from a disease such as cancer or malabsorption. Xerophthalmia can eventually lead to blindness after keratomalacia. Eye disorders caused by vitamin A deficiency are more rapid and severe in children. Skin changes also occur in vitamin A deficiency. Goose flesh or xeroderma (alligator skin) are characteristic changes. Hair follicles become plugged with keratin, making skin rough, dry, and scaly. Other symptoms of vitamin A deficiency include anorexia, inhibited growth, decreased immunity to infection, and skeletal abnormalities.
Toxicity Factors
Hypervitaminosis has been observed in individuals ingesting in excess of 50,000 IU of vitamin A daily for several years. Symptoms of vitamin A toxicity include: bone pain and fragility, coarsening or loss of hair, dry and fissured skin, irritability, fatigue, severe headaches, anorexia, and abdominal discomfort.
Toxicity in children can result from prolonged intake of vitamin A in excess of 25,000 IU daily. Some symptoms include: drowsiness, vomiting, loss of hair, double vision, skin rashes and skin cracking.
Another special interest group is pregnant women. They must also be careful about selecting vitamin A supplements. Retinol forms are contra-indicated during pregnancy.
Toxicity symptoms, in most instances, will subside within a few days after curtailed ingestion of vitamin A.
Recommended Dietary Allowance
| age | RDA (RE) | RNI (mcg) | IU |
| infant/children | |||
| 0-12 months | 375 | 350 | 1167 |
| 1-3 years | 400 | 400 | 1333 |
| 4-6 years | 500 | 400 | 1333 |
| 7-10 years | 700 | 500 | 1667 |
| males | |||
| 11-14 years | 1000 | 600 | 2000 |
| 15+ years | 1000 | 700 | 2333 |
| females | |||
| 11+ years | 800 | 600 | 2000 |
| pregnant | 800 | 700 | 2333 |
| lactation(1st 6 mth) | 1300 | 950 | 3167 |
| (2nd 6 mth) | 1200 | 950 | 3167 |
Vitamin A needs are modified by a number of variables. Although vitamin A is generally ample in most diets, many factors may alter its need, such as the amount stored in the liver, the form in which it is taken (as carotene or vitamin A), illness, and gastrointestinal or hepatic defect. Deficiency must be considered in any chronic disease of the liver, pancreas, or intestinal tract.
The Recommended Dietary Allowance (RDA) standard recommends a margin of safety above minimal needs. Traditionally, vitamin A has been measured in international units (IU). One IU is equivalent to the biologic activity of 0.6 micrograms of beta-carotene or 0.3 micrograms of retinol. Biologic activity of a vitamin is measured in rats according to its ability to forestall the development of a disease associated with a deficiency of that specific vitamin.
For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.
The RDA standard uses the term retinol equivalents (RE), a more accurate term because individual absorption and conversion of provitamin A carotenoids are variable factors. With this variance included in the newer term, a means is provided for clearer information exchange. Complete transition to retinol equivalents from IU as a means of measuring vitamin A activity will await the listing of various food composition tables in terms of contents of retinol, beta-carotene, and other provitamin A carotenoids separately on the basis of weight. In the interim, the RDA standard will be given in both RE and IU.
Research indicates no more than 50,000 IU of vitamin A on a daily basis can be utilized by the body, except in therapeutic situations, in which case 100,000 IU is recommended. Samples of meals served in 50 colleges, analyzed for nutrient content, were found to have a vitamin A value more than double the 5000 IU RDA for men and triple the amount for women. The range averaged between 6,819 to 19,424 IU per person daily.
Food Sources
High: (10,000 - 76,000 IU/100 g)
| Beef liver | Mint |
| Carrots | Palm oil |
| Chicken liver | Parsley |
| Dandelion greens | Pork liver |
| Kohlrabi | Spinach |
| Lamb liver | Turnip greens |
| Liver oil: | Veal liver |
| Cod | |
| Halibut | |
| Salmon | |
| Shark | |
| Sperm whale |
Medium: (1,000 - 10,000 IU/100 g)
| Acorn squash | Fennel |
| Apricots | Hubbard squash |
| Beef kidney | Kale |
| Beet greens | Lamb kidney |
| Broccoli | Leek greens |
| Butter | Whole milk (dry) |
| Butternut squash | Mango |
| Cantaloupe | Mustard |
| Chard | Nectarines |
| Cheese (except cottage cheese) | Peaches |
| Cherries (sour) | Pork kidney |
| Chicory greens | Pork liver |
| Chives | Pumpkin |
| Collard greens | Romaine |
| Cream | Sweet potatoes |
| Eel | Tomatoes |
| Egg yolk | Watercress |
| Endive | Whitefish |
Low: (100 - 1,000 IU/100 g)
| Artichokes | Lentils (dry) | |
| Asparagus | Leeks | |
| Avocados | Lettuce | |
| Beans (except kidney beans) | Loganberry | |
| Black walnuts | Milk | |
| Blackberry | Okra | |
| Blueberry | Olive | |
| Boysenberry | Orange | |
| Brussels sprout | Oyster | |
| Cabbage | Peanut | |
| Carp | Pecam | |
| Cashew | Pineapple | |
| Celery | Pistachio | |
| Cherries (sweet) | Plums | |
| Clams | Prunes | |
| Cowpea | Raspberry | |
| Cucumber | Red currants | |
| Gooseberry | Rhubarb | |
| Grapes | Rutabaga | |
| Green pea | Salmon | |
| Hazelnut | Sweet Corn | |
| Herring | Summer squash | |
| Hot chili pepper | Tangerine | |
| Kumquat | Zucchini squash |
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