Surprisingly low doses of vitamin A - as little as the amount contained in two or three multivitamin pills - may increase the risk of birth defects when taken early in pregnancy, a major study found.
The research concludes that anything more than 10,000 international units of vitamin A each day may be dangerous to the fetus.Ordinary multivitamins sold in supermarkets or drugstores typically contain 5,000 units or less. Generic multivitamins prescribed for pregnant women typically contain 4,000 units.
However, some multivitamin brands, especially those sold in health-food stores, can have much more, and straight vitamin A capsules may contain as much as 25,000 units.
The research suggests that one of every 57 babies born to women who take more than 10,000 units of vitamin A will have a birth defect as a result. The problems involved malformations of the face, head, heart and nervous system.
"Any woman who may become pregnant should be aware of the potential risks of excess vitamin A intake," Dr. Kenneth J. Rothman, the principal author, said Friday.
The work, conducted at Boston University, will be published Nov. 23 in the New England Journal of Medicine. Because of its public health importance, the editors took the unusual step of allowing the early release of the study.
Dr. Richard Johnson, medical director of the March of Dimes Birth Defects Foundation, cautioned that the news should not prompt women to shun vitamins. Women are routinely urged to take an ordinary multivitamin every day if they are thinking about getting pregnant, and this advice has not changed.
Indeed, one vitamin, folic acid, has clearly been shown to prevent neural tube defects, which are a serious form of birth defect.
Furthermore, women rarely take excessive amounts of vitamin A. In the Boston researchers' study of nearly 23,000 pregnancies, they estimated that only five or six babies were harmed by vitamin A.
To be safe, experts suggest that women check their multivitamin bottles to make sure vitamin A levels do not exceed 5,000 international units, the current U.S. recommended dietary allowance, or RDA. They should also take no more than one a day and not combine them with straight vitamin A pills.
In addition, the Boston University researchers suggested that women be careful combining vitamin supplements with large servings of liver, which is high in vitamin A, or vitamin-enriched cereals. A bowl of some cereals contains 5,000 units.
They also noted that beta carotene - the vegetable form of vitamin A that is sometimes contained in multivitamins and is also present in carrots and other vegetables - appears to be safe and can be substituted for straight vitamin A, sometimes called the retinol form of the vitamin.
Their findings were based on 22,748 women who got pregnant in the mid-1980s. Since then, vitamin manufacturers have reduced the level of retinol vitamin A in multivitamins, often partially substituting beta carotene.
Vitamin A plays a critical role in regulating the formation of organs during early growth of an embryo in the womb.
Synthetic forms of vitamin A, such as the acne medicine Accutane, are known to cause human birth defects. While high levels of ordinary vitamin A were suspected of having similar effects, the new study is the first to attempt to find the maximum safe dose.
Researchers interviewed women during pregnancy about the food they ate and the vitamins they took. Fewer than 2 percent of the women took more than 10,000 international units of vitamin A daily.