This nutrient is important in preventing night blindness and preserving the health of skin and mucous membranes, as well as in reproductive and immune functions. Until recently, it also was thought to protect against cancer, especially of the lung¾but newer studies suggest that in smokers, it may actually increase risks. See "Vitamin A and Cancer," below.
Among the richest sources of vitamin A are animal-based foods. Prime examples are liver, seafood, egg yolks, and vitamin A-fortified milk.
The body can also manufacture its own vitamin A from precursors that are plentiful in deep green or orange fruits and vegetables, such as carrots, broccoli, cantaloupe, dried apricots, and pumpkin. The color of these foods actually comes from the precursors, which belong to a class of pigments called carotenes. The most abundant and powerful precursor is beta carotene. Two other important precursors are alpha carotene and beta cryptoxanthin. (Many other carotenes, including some famous ones, do not produce vitamin A. Among these are lycopene and lutein.)
To standardize vitamin-A measurement across all foods, scientists calculate the vitamin A-producing power of each precursor in a food, add the powers together, and figure how many micrograms of retinol (the precursor in animal-based foods) the food would contain if it had the same power. The "vitamin A" content is thus expressed in "micrograms of retinol equivalents," or "micrograms RE."*
Food labels still report vitamin A in International Units. To learn how to convert these into micrograms RE, click here.
Double Your Vegetables?
In January 2001, the Food and Nutrition Board (FNB) announced new research indicating that the carotenoid precursors of vitamin A are only half as powerful as formerly believed. The upshot is that if you get most of your vitamin A from deep green or orange vegetables, you're probably getting only half as much as once thought.
Diet Power provides an easy way to correct for this problem: Use the Personal Daily Allowance (PDA) Editor to increase your allowance of vitamin A to an amount reflecting your dependence on deep green or orange vegetables. If you're a vegetarian and get virtually all of your vitamin A from such vegetables, then double the allowance. Or if you get half from your vitamin A from these and half from other foods such as meat and milk, increase the allowance by 50 percent. This will make the "% PDA" bar in your Nutrient History more accurately reflect how well you're meeting your vitamin-A needs.
Be careful not to consume more than the Tolerable Upper Intake Level (UL), however. (The UL is described below.) Getting too much preformed** vitamin A can injure the brain and nervous system and cause blurred vision, headaches, hair loss, diarrhea, menstrual problems, joint pain, insomnia, liver damage, abnormal bone growth, hip fractures in the elderly, and other ailments. It may also increase the risk of birth defects. These risks are especially high in people who consume a lot of alcohol or have liver disease, hyperlipidemia (including high blood cholesterol levels), or severe protein malnutrition.
** According to the FNB, these effects do not apply to vitamin-A precursors.
If you're still taking beta carotene¼
¼you should be aware that expert opinion on this popular supplement has changed in recent years. Because it was thought to promote longevity, many doctors and nutritionists used to recommend daily supplements of 15 to 25 milligrams of beta carotene. In 1996, however, long-term studies showed that beta-carotene supplements were not effective. (See Antioxidants.) Some experts claim that these supplements may actually be harmful, but the evidence is inconclusive. In April 2000 the FNB urged people to use caution before taking carotenes in high doses, and recommended supplements "only to prevent or control a vitamin-A deficiency."
(For notes on vitamin A and specific diseases, see below.)
Vitamin A and Cancer***
The connection between cancer and vitamin-A intake varies with the type of cancer. Here's what the most reliable studies show:
· In the general population, people whose diets are naturally high in carotenes (including alpha and beta carotene, as well as lycopene) have a lower incidence of lung cancer. The evidence is weak, however.
· Among smokers (especially heavy smokers who regularly drink alcohol), beta-carotene supplements are linked to a higher incidence of lung cancer.
· Beta-carotene supplements do not affect the incidence of colorectal cancer, except possibly among people who regularly use alcohol.
· In the general popularion, beta-carotene intake does not appear to affect prostate-cancer risk.
· Among alcohol users, beta-carotene supplements may raise the incidence of prostate cancer¾but among people who don't use alcohol, supplements may reduce the incidence.
· Among people with low levels of beta-carotene in their blood, supplements may help prevent prostate cancer.
These generalizations on prostate cancer are only tentative, however. Experts are calling for more studies.
· A few studies of premenopausal women with a family history of breast cancer show that a diet rich in carotenes and vitamin A reduces breast-cancer risk. Most other studies, however, do not show a link. Experts consider the evidence inconclusive.
· Some studies of vitamin A alone (excluding carotenes) show that the vitamin helps to prevent breast cancer, although the evidence is weak.
· In one large study, researchers tested a vitamin-A analog, called fenretinide, on breast-cancer survivors for an average of five years. The treatment did not reduce recurrences of the disease.
· Diets rich in fruits and vegetables may protect against bladder cancer, but the evidence is not strong. The protective agent is thought to be more likely carotenes than vitamin A itself.
Vitamin A and Heart Disease***
Here, too, the evidence is mixed:
· In the general population, heart-disease risk is not affected by intake of beta carotene.
· Among smokers, beta-carotene supplements may increase heart-disease risk.
Our latest source for the cancer and heart-disease summaries is a review of more than 150 scientific papers, "Vitamins for Chronic Disease Prevention in Adults," published by Drs. Kathleen M. Fairfield and Robert H. Fletcher in the June 19, 2002, Journal of the American Medical Association (JAMA). Reprints can be downloaded (for a fee) from the JAMA Web site at www.jama.ama-assn.org.
Your Daily Allowance
Diet Power sets your Personal Daily Allowance (PDA) of vitamin A at the FNB's Recommended Dietary Allowance (RDA). In January 2001, the FNB slightly lowered the RDA. It is now 900 micrograms RE for males 14**** and older and 700 micrograms RE for females 14**** and older. For pregnant women 18 and younger the figure is 750 micrograms; for those 19 or older it's 770 micrograms. During lactation the allowance is 1200 micrograms for women 18 and younger, 1300 micrograms for all others.
Also in January 2001, the FNB announced a Tolerable Upper Intake Level (UL) for vitamin A. People 14**** to 18 years old should get no more than 2800 micrograms per day; people 19 and older, no more than 3000 micrograms per day. (The same figures apply to women who are pregnant or lactating.)
**** Please remember that Diet Power is not designed for people under 15.
If a label cites vitamin A in International Units (IU)¼
¼you can convert them to micrograms. See International Units of Vitamin A.
Revising Your Allowance
Diet Power automatically sets your Personal Daily Allowance of vitamin A when you enroll in the program, but you can change your PDA to reflect your physician's recommendation. See Personal Daily Allowances, Editing Your.
Color Coding of This Nutrient
The vitamin-A bar in your personal Nutrient History is:
· blue for "good" if you've logged 100 to 150 percent of your PDA
· red for "bad" if you've logged less than 100 percent of your PDA or more than the Tolerable Upper Intake Level (UL).
· yellow for "caution" if you've logged more than 150 percent of your PDA
· missing if you've logged no vitamin A.
In the nutrient profile of a food or recipe, the vitamin-A bar is:
· green for "good" if getting your entire PDA of calories from this item would give you more than 150 percent of your PDA of vitamin A
· magenta for "bad" if getting all your calories from the item would give you less than 50 percent of your PDA of vitamin A
· blue for "neutral" otherwise
· missing if the amount of vitamin A is either zero or (when the term Vitamin A is grayed out) unknown.
How Complete Are Diet Power's Vitamin-A Readings?
For the 8500 generic items in the Food Dictionary: very complete. Only 0.4 percent list their vitamin-A content as "unknown."
For the 2500 chain-restaurant items: not terribly complete. About 41 percent list vitamin A as "unknown."
For all 11,000 items combined: fairly complete. About 9 percent list vitamin A as "unknown."
These figures mean that if you frequently log chain-restaurant foods (or user-added foods with missing vitamin-A readings), your Nutrient History may underreport your intake of vitamin A by a few points.
To see whether a particular food has a vitamin-A reading, open the Food Dictionary and check the food's nutrient profile. If you find a question mark beside "Vitamin A," it means the amount is unknown. (To see all foods with unknown vitamin-A readings, click the dictionary's PowerFoods tab and sort the foods by vitamin-A power; then scroll toward the bottom of the list until you see foods with question marks in the "Power Rating" column.)
Vitamin A on Food Labels
Nearly all food labels are required to report vitamin-A content, as a percentage of Daily Value.*****
The Daily Value for vitamin A is 5000 International Units (IU). This is not necessarily right for you, however¾it's a rough estimate meant to cover most of the U.S. population.
For more on label regulations, see Labels, Food.
Remember, however, that deep green or orange vegetables are not as rich in vitamin A as once believed, or as labels may indicate. See "Double Your Vegetables?" above.