WASHINGTON -- Women who might become pregnant need 400 micrograms of folic acid per day to reduce their risk of having a child with neural tube defects, according to the latest report on Dietary Reference Intakes (DRIs) from the Institute of Medicine. The report -- the second in a new series by American and Canadian scientists -- provides Recommended Dietary Allowances (RDAs) and other dietary reference values for B vitamins, of which folate is one, and choline. It says that all adult men and women need 400 micrograms of folate in their diet -- a level that many in the United States have not met, according to surveys completed before January of this year, when many foods began to be fortified with folic acid, a synthetic form of folate. But it emphasized the special needs of childbearing women.
"Research over the past decade strongly indicates that women capable of becoming pregnant should eat a varied diet and also take extra folic acid, especially in the month just prior to conception and the first month of pregnancy," said committee chair Roy M. Pitkin, professor emeritus of obstetrics and gynecology at University of California, Los Angeles. "They can best get folic acid from fortified cereal grains or from a supplement."
Since fortification of enriched cereal grains began, folic acid can be found in enriched bread, pasta, flour, crackers, breakfast cereal, rice, and many other foods in the United States. To reduce the risk of neural tube defects, women capable of becoming pregnant should consume 400 micrograms of folic acid daily from fortified foods, vitamin supplements, or a combination of the two, the report says. This is in addition to the naturally occurring folate they obtain from a varied diet. Whether these women can rely totally on the folate in food is uncertain, since research has involved giving only additional amounts of folic acid.
Neural tube defects -- such as spina bifida -- result from a disruption during the development of the fetus's central nervous system in the first month of pregnancy, when most women do not realize they are pregnant. A common type of congenital malformation of the central nervous system, neural tube defects may appear as incomplete closure of the spinal column or even the absence of part of the brain. In the United States and Canada they occur in about one birth per 1,000.
The report also says that although most Americans and Canadians get sufficient vitamin B12 in their food, between 10 percent and 30 percent of older adults have lost the ability to adequately absorb the naturally occurring form of B12 found in food. People over age 50 should meet most of their recommended intake with synthetic B12 from fortified foods or vitamin supplements.
New Dietary Standards
The committee's report on Dietary Reference Intakes expands on the Recommended Dietary Allowances set periodically by the National Academy of Sciences since 1941. Five additional reports -- on antioxidants, trace elements, electrolytes and water, macronutrients, and other food components -- will follow. Dietary Reference Intakes contain four categories of recommendations, including some values intended to help individuals optimize their health and some to help them avoid consuming too much of a nutrient.
In addition to folate, the report recommends individual intakes for thiamin, riboflavin, niacin, vitamins B6 and B12, pantothenic acid, biotin, and choline. Where possible, it also specifies intake levels above which health problems might occur. Except for folate, recommended intakes for these vitamins have not changed substantially since the last group of recommendations was published in the United States in 1989 and in Canada in 1990. Most Americans and Canadians already meet their requirements for these vitamins through their diet. [The full table of recommended intakes for individuals is available.]
A great deal of research in the past two decades has centered on the roles that B vitamins may play in reducing the risk of cardiovascular disease, cancers, or various psychiatric or mental disorders. Although such research is provocative and promising, it is not yet solid enough to serve as the basis for setting recommendations for nutrient intake, the report says. Therefore, rather than set recommended intake levels to reduce the risk of these diseases, the committee based its recommended intake levels on values shown to guard against anemia or other conditions that can develop when these vitamins are lacking.
Research has shown that consumption of folate and vitamin B6 can reduce elevated levels of homocysteine in the blood, and some studies have linked lower homocysteine concentrations with a decreased risk of cardiovascular disease. But there is conflicting evidence about whether increasing folate or B6 intake leads directly to a lower incidence of vascular and heart disease. Likewise, data showing that increased folate may protect against colorectal cancer do not provide conclusive evidence of a benefit, the report says.
Upper Levels Set
The committee set the tolerable upper intake level for vitamin B6 at 100 milligrams per day for adults. Intakes above this amount could cause sensory neuropathy, a nerve disorder that can lead to pain, numbness, and weakness in the limbs. Likewise, adults with vitamin B12 deficiency who take excess folic acid place themselves at greater risk of progressive, crippling neurologic damage. For folic acid, the committee set the tolerable upper intake level for adults at 1,000 micrograms, or 1 milligram, per day.
Individuals who consume too much niacin have been shown to feel a flush, warm sensation, itching, and other symptoms. The committee set the tolerable upper intake level for niacin at 35 milligrams per day. Some individuals who take high-dose, over-the-counter niacin supplements may exceed this amount regularly. The tolerable upper level for choline was set at 3.5 grams per day for adults. Intakes above that level could cause hypotension, or low blood pressure, and a fishy body odor in some people.
Because the committee was unable to identify studies conducted on the adverse effects of taking large doses of many of the B vitamins, it did not set upper limits for thiamin, riboflavin, vitamin B12, pantothenic acid, and biotin. Due to the lack of data, the committee urged extra caution with regard to excessive consumption of these vitamins.
A list of committee, panel, and subcommittee members follows. The Institute of Medicine is a private, non-profit organization that provides health policy advice under a congressional charter granted to the National Academy of Sciences. This study was funded by the U.S. Department of Health and Human Services, the National Institutes of Health, the Centers for Disease Control and Prevention, Health Canada, the Institute of Medicine, and the Dietary Reference Intakes Corporate Donors Fund. Contributors to the fund include Roche Vitamins Inc., Mead Johnson Nutrition Group, Daiichi Fine Chemicals Inc., Kemin Foods Inc., M&M Mars, Weider Nutrition Group, and Natural Source Vitamin E Association.
Pre-publication copies of Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline are available from the National Academy Press; tel. (202) 334-3313 or 1-800-624-6242. Reporters may obtain a copy from the Office of News and Public Information(contacts listed above).