Dietary Reference Intakes
For Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc
Institute of Medicine
January 9, 2001
Professor of Medicine and Nutrition, School of Medicine, and
Associate Director, Jean Mayer USDA Human Nutrition Research Center on Aging
Tufts University, Boston
Chair, Panel on Micronutrients
Good morning. Thank you for joining us for the public release of a report that we hope will guide policy-makers, health professionals, and others as they grapple with complex nutritional issues. Today we offer our findings and recommendations on several micronutrients, which are based on a comprehensive review of the science in this area.
Since 1941, the Food and Nutrition Board of the National Academies has set Recommended Dietary Allowances on the types and quantities of nutrients that are needed for healthy diets. Over the past five years the board has implemented a new system for determining the RDAs and other values now called Dietary Reference Intakes or DRIs. The report we are releasing today is the latest in a series on DRIs, developed jointly by American and Canadian scientists. It focuses on vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc.
How much of these nutrients does one need to take on a daily basis to ensure good health? To answer that question we offer several categories of numbers that constitute the DRIs. The Recommended Dietary Allowance, or RDA, represents a daily nutrient intake goal for healthy individuals. There is no proven benefit of consuming levels greater than the RDA. The Adequate Intake, or AI, also is a goal for nutrient intake for healthy individuals, which is set when the scientific evidence is insufficient to determine an RDA. The AI is usually the amount found in a nutritionally adequate diet and individuals should use it as a goal for intake when no RDA exists. We also offer "tolerable upper intake levels," known as ULs. These represent a ceiling -- the largest amount of a nutrient that healthy individuals can take each day without being placed at increased risk of adverse health effects. I should point out that this is the first time the Food and Nutrition Board has set upper levels for some of the nutrients included in this review.
After extensive review of the scientific literature having to do with these 14 micronutrients, we made the following recommendations.
For vitamin A, the panel set the RDA for men and women at 900 and 700 micrograms per day, respectively. These RDAs assure adequate stores of vitamin A in the body. We also evaluated the vitamin A activity of provitamin A carotenoids that are found in green leafy vegetables and darkly colored fruits. We determined that the provitamin A carotenoids from fruits and vegetables provide half the vitamin A activity than previously reported by the Food and Nutrition Board in 1989. We have thus established a new measurement unit for indicating the vitamin A activity for provitamin A carotenoids, called retinol activity equivalents (or RAEs). Specifically, the new RAEs mean that foods containing 12 micrograms of beta-carotene provide the equivalent of 1 microgram of vitamin A, while 24 micrograms of dietary alpha-carotene or beta-cryptoxanthin is equivalent to 1 microgram of vitamin A. This means that provitamin A carotenoids in fruits and vegetables provide the body with half the amount of vitamin A as previously thought. This doesn't mean fruit and vegetable consumption must double. Individuals, especially vegetarians, whose main source of vitamin A comes from provitamin A carotenoids in fruits and vegetables, will need to make sure they choose enough deeply colored green, yellow, and orange fruits and vegetables to meet their vitamin A requirement. The RDA for this nutrient can be met by choosing foods rich in provitamin A and vitamin A without taking supplements. Among foods containing preformed vitamin A are organ meats, certain fish, and dairy products.
The upper level for vitamin A intake -- from both foods and supplements -- is set at 3,000 micrograms per day for adults. Taking more than this amount on a daily basis may increase the risk of liver damage and birth defects.
In attempting to set the recommended intake for vitamin K using the DRI process, we determined that there were insufficient data to establish an RDA. Thus adequate intakes, or AIs, of 120 and 90 micrograms per day for men and women, respectively, were set based on estimated median intakes from national survey data. Green leafy vegetables such as spinach, broccoli, and cabbage, and some vegetable oils are all good sources of vitamin K. There were insufficient data to establish an upper intake level.
Insufficient intake of iron can result in iron-deficiency anemia -- the most prevalent nutrient deficiency in the world -- as well as impaired cognitive development in infants and young children. The RDA for men and post-menopausal women is 8 milligrams per day. The RDA for pre-menopausal women is 18 milligrams per day. The RDA for pregnant women is 27 milligrams per day. Based on U.S. national survey data, a high percentage of pregnant women are consuming inadequate amounts of dietary iron. The RDA can be met during pregnancy by increasing the consumption of foods highly fortified with iron or by the use of iron supplements. Because the intestinal absorption of iron is less from plant foods than from animal foods, vegetarians need to consume twice as much dietary iron to meet their requirement. The upper intake level for iron is 45 milligrams per day, based on findings of adverse gastrointestinal effects, such as constipation and nausea, that can occur when consuming iron supplements, especially when taken on an empty stomach. This upper level is for the general healthy population. At this time, there is insufficient evidence to establish an upper level for individuals with hereditary hemochromatosis, a genetic disorder that results in unregulated iron absorption.
There is evidence to suggest that chromium enhances insulin activity; however, there were insufficient dose-response data to establish an RDA. Based on estimates of chromium intake from nutritionally adequate diets, we recommend an AI of 35 and 25 micrograms per day for men and women, respectively. The data on the adverse effects of high levels of intake of the chromium contained in supplements were not adequate to establish an upper level for this nutrient.
The report establishes an RDA for copper of 900 micrograms per day for both men and women, based on studies demonstrating the amount of dietary copper needed to maintain adequate copper status. The upper level is set at 10 milligrams (that is 10,000 micrograms) per day. Consuming more than this amount on a daily basis increases the risk of liver damage.
Iodine is an essential component of the body's thyroid hormones. The RDA is 150 micrograms per day for both men and women. The upper level is 1,100 micrograms per day, based on concern of increased risk of hypothyroidism.
Manganese is involved in the formation of bone and in protein, fat, and carbohydrate metabolism. The AI for manganese is 2.3 milligrams per day for men and 1.8 milligrams per day for women, based on median intakes of manganese from national surveys. The upper level is 11 milligrams per day, based on concerns related to neurological side effects observed at higher levels of intake.
Molybdenum is a component of a limited number of enzymes in the human body. The RDA is 45 micrograms per day for both men and women. The upper level is 2 milligrams per day based on concern for reproduction and growth effects seen in animal studies. Plant foods such as legumes, grains, and nuts are primary sources of molybdenum.
Zinc has multiple metabolic roles in the human body. The RDA is 11 milligrams per day for men and 8 milligrams per day for women. Like iron, the intestinal absorption of zinc from plant foods is not as great as from animal foods. Therefore, vegetarians may need up to 50 percent more dietary zinc than nonvegetarians. The upper level for zinc is 40 milligrams per day. Consuming more than this amount may increase the risk of copper deficiency by blocking its absorption.
The role of certain trace elements -- arsenic, boron, nickel, silicon, and vanadium -- in human health was evaluated. There is evidence from animal studies that these five nutrients do play a beneficial role in some physiological processes. For boron, silicon, and vanadium, measurable responses of humans to variations in dietary intake have been demonstrated. However, the available data are not as extensive as for the other nutrients studied and the responses are not as consistently observed. Therefore, neither an RDA nor an AI was established. In terms of establishing upper levels, although inorganic arsenic is known to be toxic, there is insufficient evidence on the adverse effects of dietary arsenic, which primarily occurs as organic arsenic in foods. Therefore an upper level for arsenic was not established. There were insufficient data to establish an upper level for silicon. The upper levels for boron and nickel are based on adverse reproductive and developmental effects observed in laboratory animals at high intakes. The upper level for vanadium is based on renal toxicity observed in laboratory animals. The upper level for boron is 20 milligrams per day; the upper level for nickel is 1 milligram per day; and the upper level for vanadium is 1.8 milligrams per day.
These recommendations represent a snapshot of a very active area of scientific research. Studies are ongoing, and there is still much to be learned. Our report highlights areas where more research is needed. In general, better biomarkers that can be used to assess adverse outcomes of both inadequate and excess intakes of specific vitamins and trace elements are needed. Also, too little is known about the nutrient needs of specific population groups, including children and the elderly.
Finally, while a good deal of interest now focuses on supplements, we should note that most adults in Canada and the United States get enough vitamin A, vitamin K, chromium, copper, iodine, iron, manganese, molybdenum, and zinc from their normal diets to meet our recommendations. And those who do not get enough of these nutrients could do so by making modest changes to their diets.
This concludes my opening statement. My colleagues and I would now be glad to take your questions. Those of you listening to our Webcast can send in questions by e-mail, using a link on the National Academies home page. We ask those of you in the room to step to a microphone and identify yourself and your organization when you ask your question. We'll begin with a question in the room.