New Advice About Vitamin A
For: Wellness Made
Easy, May 2003
Vitamin A is essential for good health—notably for eyes and skin, immune function, reproduction, and bone growth. That's why nonfat and low-fat milk, margarine, and many breakfast cereals are fortified with it. However, high doses of vitamin A taken over time can be dangerous, especially for pregnant women (the vitamin is fat-soluble and is stored in the body, so levels can build up). Now a Harvard study suggests that Levels of vitamin A once considered safe and healthful may weaken women's bones and increase the risk of fractures.
The study of 70,000 postmenopausal nurses found that those who consumed the most vitamin A (equal to at least 6,600 international units, or IUs) from foods and/or supplements over an 18-year period had nearly double the risk of fractures com-pared with those consuming the least. A Swedish study in 1998 had similar findings, though other studies have found no such increased risk. It's theorized that moderate to high levels of vitamin A reduce bone growth and interfere with the ability of vitamin D to help the body utilize calcium.
However, both the Harvard and Swedish studies found that only "preformed" vitamin A, also called retinol, was linked to bone loss and fractures. The body converts beta carotene and many other carotenoids in vegetables and fruits into vitamin A, as needed. The studies found no adverse effect from carotenoids.
Sorting through the numbers
The newly revised daily Recommended Dietary Allowance (RDA) for vitamin A is equal to 2,333 IU daily for women, and 3,000 IU for men. The labels on foods and supplements use an older set of daily requirements called the "Daily Values," which includes 5,000 IU for vitamin A. Multivitamins typically contain 100% of the Daily Value. Recently, the Institute of Medicine, which determines the RDAs, set 10,000 IU as the safe upper limit for A.
So the amount found to be risky in the Harvard study (6,600 IU) is nearly three times the RDA for women, and one-third more than the Daily Value. But it is far below the "upper limit." That's why this finding is surprising.
(Note: To add to the confusion, two different units of measure are used for vitamin A. We've used IU here because that's what the FDA uses for the Daily Values—and thus that's what you'll see on the labels of foods and supplements. However, researchers express vitamin A activity in micrograms [a millionth of a gram]. The Institute of Medicine also uses micrograms for the RDA. If you want to convert IUs to micrograms, click here.)
Where it comes from
• Liver, which stores the vitamin, is very rich in it: a whop-ping 30,000 IU in 3 ounces of beef liver, and 13,000 IU in the same amount of chicken liver.
• Fish oil, from fish livers, is the most concentrated source of A (though the flesh of most fish has only modest amounts of the vitamin).
• Fortified breakfast cereals, including most instant oatmeal, have a fair amount—usually 500 to 1,000 IU (10 to 20% of the Daily Value) per serving.
• Whole milk has 300 IU per cup; nonfat and low-fat milk (which are fortified with A), 500 IU; butter or margarine, 400 to 500 IU per tablespoon; most hard cheese, about 250 IU per ounce.
• Most multivitamins contain 5,000 IU of vitamin A. But in some brands, 20 to 50% is in the form of beta carotene. Half the women in the Harvard study took multivitamins, which were the single largest source of A.
All you need to know
If you're a postmenopausal woman, keep track of your vitamin A intake:
• If you take a multivitamin, check the label. It should contain no more than 5,000 IU of vitamin A, and at least 20% of this should be in the form of beta carotene (the label will say, for instance, "50% as beta carotene" under vitamin A). There is no reason to stop taking your multi, which contains many nutrients important for healthy bones.
• Don't take separate vitamin A supplements. Even more important, women of childbearing age must not take them (doses as low as 10,000 IU a day increase the risk of birth defects). Men don't need them either.
• Don't worry about the vitamin A in milk or most other foods. It is hard to get high levels of vitamin A just from food, unless you eat liver or several servings of highly fortified cereal every day.
• Don't worry about carotenoids. These do not endanger your bones, since your body carefully regulates their conversion to vitamin A.
• Don't take cod liver oil.
• Don't go overboard and try to avoid vitamin A altogether. However, if you eat a variety of fruits and vegetables, which are rich in carotenoids, you don't need to worry about getting enough vitamin A.
Good news: Some makers of multivitamins have announced that they will soon lower the amount of preformed vitamin A in their pills and/or substitute more beta carotene for it. Similarly, in many margarines, some of the added vitamin A has already been replaced by beta carotene. Check the labels.
UC Berkeley Wellness Letter, July 2002