WASHINGTON -- Women who might become pregnant need 400 micrograms of folic acidper day to reduce their risk of having a child with neural tube defects,according to the latest report on Dietary Reference Intakes (DRIs) from theInstitute of Medicine. The report -- the second in a new series by American andCanadian scientists -- provides Recommended Dietary Allowances (RDAs) and otherdietary 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 surveyscompleted before January of this year, when many foods began to be fortifiedwith folic acid, a synthetic form of folate. But it emphasized the specialneeds of childbearing women.
"Research over the past decade strongly indicates that women capable of becomingpregnant should eat a varied diet and also take extra folic acid, especially inthe month just prior to conception and the first month of pregnancy," saidcommittee chair Roy M. Pitkin, professor emeritus of obstetrics and gynecologyat University of California, Los Angeles. "They can best get folic acid fromfortified cereal grains or from a supplement."
Since fortification of enriched cereal grains began, folic acid can be found inenriched bread, pasta, flour, crackers, breakfast cereal, rice, and many otherfoods in the United States. To reduce the risk of neural tube defects, womencapable of becoming pregnant should consume 400 micrograms of folic acid dailyfrom fortified foods, vitamin supplements, or a combination of the two, thereport says. This is in addition to the naturally occurring folate they obtainfrom a varied diet. Whether these women can rely totally on the folate in foodis uncertain, since research has involved giving only additional amounts offolic acid.
Neural tube defects -- such as spina bifida -- result from a disruption during thedevelopment of the fetus's central nervous system in the first month ofpregnancy, when most women do not realize they are pregnant. A common type ofcongenital malformation of the central nervous system, neural tube defects mayappear as incomplete closure of the spinal column or even the absence of part ofthe brain. In the United States and Canada they occur in about one birth per1,000.
The report also says that although most Americans and Canadians get sufficientvitamin B12 in their food, between 10 percent and 30 percent of older adultshave lost the ability to adequately absorb the naturally occurring form of B12found in food. People over age 50 should meet most of their recommended intakewith synthetic B12 from fortified foods or vitamin supplements.
New Dietary Standards
The committee's report on Dietary Reference Intakes expands on the RecommendedDietary Allowances set periodically by the National Academy of Sciences since1941. Five additional reports -- on antioxidants, trace elements, electrolytesand water, macronutrients, and other food components -- will follow. DietaryReference Intakes contain four categories of recommendations, including somevalues intended to help individuals optimize their health and some to help themavoid 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 problemsmight occur. Except for folate, recommended intakes for these vitamins have notchanged substantially since the last group of recommendations was published inthe United States in 1989 and in Canada in 1990. Most Americans and Canadiansalready meet their requirements for these vitamins through their diet. [Thefull table of recommended intakes for individuals is available.]
A great deal of research in the past two decades has centered on the roles thatB vitamins may play in reducing the risk of cardiovascular disease, cancers, orvarious psychiatric or mental disorders. Although such research is provocativeand promising, it is not yet solid enough to serve as the basis for settingrecommendations for nutrient intake, the report says. Therefore, rather thanset recommended intake levels to reduce the risk of these diseases, thecommittee based its recommended intake levels on values shown to guard againstanemia or other conditions that can develop when these vitamins are lacking.
Research has shown that consumption of folate and vitamin B6 can reduce elevatedlevels of homocysteine in the blood, and some studies have linked lowerhomocysteine concentrations with a decreased risk of cardiovascular disease. But there is conflicting evidence about whether increasing folate or B6 intakeleads directly to a lower incidence of vascular and heart disease. Likewise,data showing that increased folate may protect against colorectal cancer do notprovide conclusive evidence of a benefit, the report says.
Upper Levels Set
The committee set the tolerable upper intake level for vitamin B6 at 100milligrams per day for adults. Intakes above this amount could cause sensoryneuropathy, a nerve disorder that can lead to pain, numbness, and weakness inthe limbs. Likewise, adults with vitamin B12 deficiency who take excess folicacid place themselves at greater risk of progressive, crippling neurologicdamage. For folic acid, the committee set the tolerable upper intake level foradults at 1,000 micrograms, or 1 milligram, per day.
Individuals who consume too much niacin have been shown to feel a flush, warmsensation, itching, and other symptoms. The committee set the tolerable upperintake level for niacin at 35 milligrams per day. Some individuals who takehigh-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 afishy body odor in some people.
Because the committee was unable to identify studies conducted on the adverseeffects of taking large doses of many of the B vitamins, it did not set upperlimits for thiamin, riboflavin, vitamin B12, pantothenic acid, and biotin. Dueto the lack of data, the committee urged extra caution with regard to excessiveconsumption of these vitamins.
A list of committee, panel, and subcommittee members follows. The Institute ofMedicine is a private, non-profit organization that provides health policyadvice under a congressional charter granted to the National Academy ofSciences. This study was funded by the U.S. Department of Health and HumanServices, the National Institutes of Health, the Centers for Disease Control andPrevention, Health Canada, the Institute of Medicine, and the Dietary ReferenceIntakes Corporate Donors Fund. Contributors to the fund include Roche VitaminsInc., Mead Johnson Nutrition Group, Daiichi Fine Chemicals Inc., Kemin FoodsInc., M&M Mars, Weider Nutrition Group, and Natural Source Vitamin EAssociation.
Pre-publication copies of Dietary Reference Intakes: Thiamin, Riboflavin,Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, andCholine are available from the NationalAcademy Press; tel. (202) 334-3313 or 1-800-624-6242.
Materials provided by National Academy Of Sciences. Note: Content may be edited for style and length.
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