Dietary Reference Intakes for Thiamin, Riboflavin, Niacin,
Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
Institute of Medicine
Food and Nutrition Board
April 7, 1998
Roy M. Pitkin
Professor Emeritus of Obstetrics and Gynecology
University of California, Los Angeles
Chair, Panel on Folate, Other B Vitamins, and Choline
Good morning and welcome. We are pleased to be here today to release the second in a series of reports that we hope will have a lasting impact on nutrition policy in the United States and Canada.
Since the early 1940s, the Food and Nutrition Board of the National Research Council, and later, the Institute of Medicine has set Recommended Dietary Allowances to help policy makers, health professionals, and consumers know which nutrients they need in their daily diets and how much. Recently, the Board undertook an even more ambitious goal when it increased the number of nutrients considered and expanded its review to address how much of a nutrient is too much. Updating the Recommended Dietary Allowances and replacing them with this expanded set of values -- known collectively as Dietary Reference Intakes -- will be completed in stages over the next few years. Today we present the second stage in this review -- our findings on B vitamins and choline.
This category of vitamins -- and folate in particular -- has been the subject of great concern in recent years. Low intake of folate has been linked with a variety of health problems, including spina bifida in infants and heart disease in adults. On the other hand, high intake of folate has been said to delay the diagnosis of pernicious anemia. Because of uncertainties about desirable folate intake and the importance of all B vitamins in promoting and maintaining health, the Department of Health and Human Services and the National Institutes of Health requested a study to give special attention to the needs for folate and vitamin B12; and also to determine recommended intakes and tolerable upper intake limits for all the B vitamins and choline.
With the support of these and other sponsors we conducted a thorough review of all available evidence on these vitamins, relying primarily on peer-reviewed scientific literature. I will highlight a few of our major recommendations today.Neural tube defects such as spina bifida occur in about one birth per 1,000 in the United States and Canada. They result from a disruption during the development of the embryo's central nervous system in the first month of pregnancy, and they are reflected in incomplete closure of the spinal column or even the absence of part of the brain.
Until recently, the link between intake of folic acid, the synthetic form of folate, and neural tube defects was not clearly understood. But in the past decade a strong body of evidence from around the world has shown that women can reduce their chance of having a child with neural tube defects by consuming extra folic acid. We have examined this evidence carefully to formulate very specific guidelines for American and Canadian women who are capable of becoming pregnant. Our report says that to reduce the likelihood of neural tube defects in their children these women should eat a varied diet, and also take an extra 400 micrograms of synthetic folic acid, a synthetic form of the folate found naturally in foods to be absolutely sure they get enough of the nutrient.
Because development of the central nervous system occurs in the first month of pregnancy -- before many women even know they are pregnant -- it is especially critical that women take this extra folic acid in the month just prior to conception and the first month of pregnancy. This extra folic acid should come from fortified cereal grains or from a supplement, since research has not proven that women can get this added protection from the folate that they would get in their foods naturally.
In the United States, meeting recommended intake levels for folate and folic acid is now easier in the United States than it used to be. Since January enriched cereal grains have been fortified with folic acid, which means that enriched bread, pasta, flour, crackers, breakfast cereal, rice, and many other foods all contain an added amount. This fortification should help childbearing women and all adults meet their needs for folate and folic acid.
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, and various psychiatric or mental disorders. Particularly intriguing for researchers has been the finding that consumption of folate and vitamin B6 can reduce elevated levels of homocysteine in the blood. This finding is potentially important, because lower homocysteine concentrations seem to be associated with a decreased risk of cardiovascular disease.
But our examination of the data from these studies uncovered conflicting evidence about whether increasing folate or B6 intake of folate or B6, or both, leads directly to a lower incidence of vascular and heart disease. Likewise, data showing that increased folate intake may protect against colorectal cancer do not provide conclusive evidence of a benefit. We recommend therefore, that these potential health benefits be explored further, but we conclude that evidence is not sufficiently clear to use as a basis for nutritional decisions.
A third important finding concerns the special dietary needs of older adults. Although most Americans and Canadians get sufficient vitamin B12 in their food, between 10 percent and 30 percent of older adults lose their ability to adequately absorb the naturally occurring form of B12 found in food. Why this happens is unclear, but it is important that people of this age do not become B12 deficient. Therefore, we recommend that adults over age 50 meet most of their recommended intake with synthetic B12 from fortified foods or vitamin supplements.
Especially at a time when people are using more supplements, an important new feature of dietary reference values has been to advise people how much of a nutrient is too much. Where there was enough evidence to do so, our committee set tolerable upper intake levels above which people could risk health problems. We recommend that adults do not take more than 100 milligrams of vitamin B6 per day, for example, because intakes above this amount could cause sensory neuropathy, a nerve disorder that can lead to pain, numbness, and weakness in the limbs.
Adults with vitamin B12 deficiency who take excess folic acid may place themselves at greater risk of progressive crippling neurologic damage; we set the tolerable upper intake level for folic acid for adults at 1,000 micrograms, or one milligram, per day.
Individuals who consume too much niacin have been shown to feel a flush, warm sensation, itching, and other symptoms. We set the tolerable upper intake level for niacin at 35 milligrams per day, an amount that some individuals who take high-dose over-the-counter niacin supplements may exceed regularly. For choline, the level was set at 3.5 grams per day for adults; higher intakes could cause low blood pressure and a fishy body odor.
More than 30 scientists from the United States and Canada have contributed to our findings, which we hope will allow policy-makers and individuals to improve their health by making responsible decisions about nutrition. We welcome your questions, first from members of the news media. Prior to asking a question please step to one of the aisle microphones and state your name and affiliation.