Adequate levels are important forgood eyesight, and poor night vision may be one of the first symptoms of a deficiency. It is also necessary for proper function of the immune, skeletal, respiratory, reproductive, and integumentary (skin) systems.
General use
An adequate level of vitamin A unquestionably contributes to good health. It is essential for the proper function of the retina, where it can act to prevent night blindness, as well as lower the odds of getting age-related macular degeneration (AMD), which is the most common cause of blindness in the elderly.
There is also evidence that good levels of vitamin A in the form of carotenoids may decrease the risk of certain cancers, heart attacks, and strokes. The immune system is also strengthened.
It is unclear, however, if supplemental forms of vitamin A have the same benefit as consuming the nutrient in natural foods in the case of a person without deficiency. Taking high levels of vitamin A in any supplemental form is not advisable without the counsel of a healthcare professional.
Preparations
Natural sources
There are two basic forms of vitamin A. Retinoids, the active types, are contained in animal sources, including meat, whole milk, and eggs. Liver is particularly rich in vitamin A, since it is one of the storage sites for excess.
Precursor forms of the vitamin (carotenoids) are found in orange and leafy green produce such as sweet potatoes, carrots, collard greens, spinach, winter squash, kale, and turnip greens. Very fresh foods have the highest levels, followed by frozen foods. Typically, canned produce has little vitamin A.
Preparing vegetables by steaming, baking, or grilling helps them to release the carotenes they contain. Alpha and beta carotene, as well as some of the other lesser-known carotenoids, can be converted to vitamin A in the small intestine. This is done by the body on an as-needed basis, so there is no risk of overdose as there is with the active form.
Supplemental sources
Supplements may contain either the active or precursor forms of vitamin A. The active form may be more desirable for those who may have some difficulty in converting the carotenoids into the active vitamin. This is more often true in those over age 55 or who have a condition that impairs the absorption of fat.
There is a water-soluble form of the vitamin, retinyl palmitate, which may be better utilized in the latter case. Carotenes are also available either as oil-based or natural water-based formulas. Be sure to store both away from light and heat, which will destroy them.
Units
There are several units that can express the amount of vitamin A activity in a product. Many supplements are still labeled with the old International Unit (IU), although the more current and most accurate unit is the Retinol Equivalent (RE). The new measurement distinguishes between the differences in absorption of retinol and beta carotene. One RE is equal to one microgram (MICROg) of retinol, or six MICROg of beta carotene.
Dose limits
Adults should take no more than 25,000 IU (5,000 RE) per day of vitamin A in its active form, except in the case of women who are pregnant or may become pregnant. The latter group should not exceed 10,000 IU (2,000 RE) per day in order to avoid potential toxic effects to the fetus.
The best way to get vitamins is in the natural food form, as the complexities are not always either known or reproducible in a supplement. A diet rich in foods containing carotenoids is optimal, but in the event of nutritional deficiencies, supplements may be needed.
Mixed carotenoids are preferable to either large doses of vitamin A or pure beta carotene supplements to avoid toxicity and maximize healthful benefits. Some of the minor carotenoids appear to have beneficial effects that are still being explored.
A good mixture will contain alpha and beta carotene, as well as lycopene and xanthophylls. Eating foods high in many carotenoids may confer some benefits—such as a lower risk of cancer, heart attacks, and strokes—which a supplement may not.
Deficiency
Levels of vitamin A low enough to cause symptomatic deficiency are uncommon in people of normal health in industrialized nations.
Symptoms of deficiency may include, but are not limited to, loss of appetite, poor immune function causing frequent infections (especially respiratory), hair loss, rashes, dry skin and eyes, visual difficulties including night blindness, poor growth, and fatigue. Generally symptoms are not manifested unless the deficiency has existed for a period of months.
Deficiencies are more likely in people who are malnourished, including alcoholics, the chronically ill, and those with impaired fat absorption. Another group at increased risk of vitamin A deficiency are persons with type 1 diabetes whose disorder is poorly controlled. People with normal health and nutritional status have a considerable vitamin A reserve.
In countries where nutritional status tends to be poor and deficiency is more common, vitamin A has been found to reduce the mortality rate of children suffering from a number of different viral infections.
Experts in plant genetics have been working on a strain of rice that contains beta carotene, hoping to help people in developing countries avoid the risk of vitamin A deficiency. Known as Golden Rice, the new strain is being sent to research institutes in developing countries for further study.
Risk factors for deficiency
Taking the RDA level of a nutrient will prevent a deficiency in most people, but under certain circumstances, an individual may require higher doses of vitamin A.
Those who consume alcoholic beverages may be more prone to vitamin A deficiency. People taking some medications, including birth control pills, methotrexate, cholestyramine, colestipol, and drugs that act to sequester bile will also need larger amounts.
Those who are malnourished, chronically ill, or recovering from surgery or other injuries may also benefit from a higher than average dose. Patients undergoing treatments for cancer, including radiation and chemotherapy, typically have compromised immune systems that may be boosted by judicious supplementation with vitamin A.
Other conditions that may impair vitamin A balance include chronic diarrhea, cystic fibrosis, and kidney or liver disease. Diabetics are often deficient in vitamin A, but may also be more susceptible to toxicity. Any supplementation for these conditions should be discussed with a healthcare provider.
Supplements are best taken in the form of carotenoids to avoid any potential for toxicity. There is not an established RDA for beta carotene. Recommendations for how much to take vary between 6 and 30 mg a day, but the middle range—around 15 mg—is a reasonable average.
Precautions
Overdose can occur when taking megadoses of the active form of this vitamin. Amounts above what is being utilized by the body accumulate in the liver and fatty tissues.
Symptoms may include dry lips and skin, bone and joint pain, liver and spleen enlargement, diarrhea, vomiting, headaches, blurry or double vision, confusion, irritability, fatigue, and bulging fontanel (soft spot on the head) in infants; these are most often reversible, but a doctor should be contacted if a known overdose occurs. Very high levels of vitamin A may also create deficiencies of vitamins C, E, and K.
Symptoms will generally appear within six hours following an acute overdose, and take a few weeks to resolve after ceasing the supplement. Children are more sensitive to high levels of vitamin A than adults are, so instructions on products designed for children should be followed with particular care. Vitamin supplements should always be kept out of reach of children.
It is especially important to avoid overdoses in pregnancy, as it may cause miscarriage or fetal malformations. Using supplements that provide carotenoids will avoid the potential of overdose.
Those with kidney disease are also at higher risk for toxicity due to either vitamin A or beta carotene, and should not take these supplements without professional healthcare advice.
There is some evidence that taking beta carotene supplements puts smokers at a higher risk of lung cancers. The CARET (Beta Carotene and Retinol Efficacy Trial) study is one that demonstrated this effect. Clarification through more study is needed, as evidence also exists showing that beta carotene, along with other antioxidants, can be a factor in cancer prevention.
For example, a team of American researchers has recently reviewed evidence that vitamin A protects against bladder cancer, and a group in Germany is testing an aerosol form of vitamin A to prevent lung cancer.
Some of the lesser known carotenoids may be key factors in the relationship between vitamin intake and cancer. Whole sources of vitamin A are better obtained from foods than from supplements. Smokers should consult with a healthcare provider before taking supplemental beta carotene.
Side effects
Very high levels of carotenoids (carotenemia) may cause an orange discoloration of the skin, which is harmless and transient.
Interactions
Vitamin A supplements should not be taken in conjunction with any retinoid medications, including isotretinoin (Accutane), a drug used to treat acne. There is a higher risk of toxicity.
A very low fat diet or use of fat substitutes impairs absorption of all the fat-soluble vitamins, including A. Mineral oil and aluminum-containing antacids may also inhibit absorption, as do the cholesterol-lowering drugs cholestyramine and colestipol.
Vitamin A reserves of the body are depleted by a number of substances, including alcohol, barbiturates, caffeine, cortisone, tobacco, and very high levels of vitamin E. Overuse of alcohol and vitamin A together may increase the possibility of liver damage.
Taking appropriate doses of vitamin C, vitamin E, zinc, and selenium optimizes absorption and use of vitamin A and carotenoids. As vitamin A is fat-soluble, a small amount of dietary fat is also helpful.
Studies of both children and pregnant women with iron deficiency anemia show that this condition is better treated with a combination of iron supplements and vitamin A than with iron alone.
Posted in: nutrition
Related Post:
Widget by [ Iptek-4u ]
