||Fruits And Vegetables Yield Less Vitamin A Than Previously Thought;
Upper Limit Set For Daily Intake Of Vitamin A And Nine Other Nutrients
Darkly colored, carotene-rich fruits and vegetables -- such as carrots, sweet potatoes, and broccoli -- provide the body with half as much vitamin A as previously thought, says the latest report on Dietary Reference Intakes (DRIs) from the National Academies' Institute of Medicine. This means people need to make sure they eat enough of these fruits and vegetables to meet their daily requirement for vitamin A, especially if they do not eat animal-derived foods, which serve as abundant sources of the nutrient for most people.
Darkly colored fruits and vegetables are still good sources of vitamin A," said Robert Russell, professor of medicine and nutrition, Tufts University School of Medicine, Boston, and chair of the panel that wrote the report. "But new evidence shows that it takes twice as much of them to yield the same amount of vitamin A in the body as we previously understood. People need to take this into consideration and make sure they select enough carotene-rich fruits and vegetables to meet their daily vitamin A requirement. This is especially true for those who don't eat meats, fish, eggs, or vitamin A-fortified milk or cereal. They may need to significantly increase their consumption of such fruits and vegetables."
Three carotenoids -- alpha-carotene, beta-carotene, and beta-cryptoxanthin -- are present in certain orange, red, green, and dark-yellow fruits and vegetables. They are referred to as provitamin A carotenoids because they can be converted in the body to retinol, an active form of vitamin A. Based on a comprehensive review of recent research, the panel found that the amount of provitamin A carotenoids required to create a unit of retinol is twofold higher than the amount believed needed in 1989, when the National Academy of Sciences last issued recommendations for vitamin A.
In addition to vitamin A, the report examines the nutritional value of the micronutrients vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. It sets a daily maximum level of intake for vitamin A as well as for boron, copper, iodine, iron, manganese, molybdenum, nickel, vanadium, and zinc. Specific recommended intakes are given for vitamins A and K, chromium, copper, iodine, iron, manganese, molybdenum, and zinc.
New Dietary Recommendations
The new DRI report is the fifth in a series that updates and expands on the Recommended Dietary Allowances (RDAs) in the United States and Recommended Nutrient Intakes in Canada. Although DRIs are designed for use in the United States and Canada, they can provide guidance to researchers and policy-makers coping with malnutrition elsewhere in the world. For example, while iron deficiency, especially among pregnant women, is of concern in this country and Canada, it also is known to be prevalent -- along with vitamin A, zinc, and iodine deficiencies -- in developing countries.
DRIs are established using a new paradigm based on indicators of good health and the prevention of chronic disease developed by U.S. and Canadian scientists. They encompass not only recommended daily intakes that are intended to help people maintain their health, but also tolerable upper intake levels (ULs) that help them avoid harm from taking too much of a nutrient. An adequate intake (AI), based on diets known to be nutritionally adequate for the U.S. and Canadian populations, is recommended when not enough evidence exists to set an RDA. The reference intake values are designed to meet the needs of individuals in the United States and in Canada who are healthy and free from specific diseases or conditions that may alter their daily nutritional requirements.
Based on national nutrition surveys, the report says that daily requirements for the nutrients it examined can be met, in almost all instances, without taking supplements. One exception, however, is that pregnant women usually need iron supplements to meet their increased daily requirements. In fact, surveys in the United States show that only half of all pregnant women who live here consume adequate amounts of iron in their diets.
Below are highlights of the report's recommendations. The full report contains dietary recommendations -- when the data allows -- for all age groups, as well as for pregnant and lactating women.
Vitamin A. Besides being important for normal vision, vitamin A plays a vital role in gene expression, reproduction, embryonic development, growth, and immune function. To ensure adequate stores of vitamin A in the body, men should consume 900 micrograms daily and women should consume 700 micrograms daily. The UL was set at 3 milligrams, or 3,000 micrograms, per day. Recent research shows that excess vitamin A intake may increase the risk of physical birth defects, liver abnormalities in adults, and bulging of the skull where bone has not yet formed in infants and young children.
The most obvious symptom of inadequate vitamin A consumption is vision impairment, especially night blindness, which occurs after the body's vitamin A stores have been depleted. Up to 500,000 children worldwide go blind each year because of vitamin A deficiency. Night blindness induced by vitamin A deficiency is rarely observed in the United States and Canada, and determining from survey data whether people in these two countries get enough vitamin A is more difficult. For example, estimates of intake from national surveys of food and nutrient consumption indicate that between 25 percent and 50 percent of young adults in the United States may not get enough of the nutrient in their diets to assure adequate vitamin A stores. However, these figures should be interpreted with caution since the surveys only measure consumption over one or two days, when in fact it takes several weeks of data collection to accurately assess intake of this vitamin.
Vitamin K. This nutrient plays an essential role in the coagulation of blood and is found in green leafy vegetables. An AI of 120 micrograms for men and 90 micrograms for women was determined based on consumption levels of healthy individuals. No adverse effects have been reported for vitamin K, so a UL was not established. There have been reports that a lack of the vitamin may be related to bone disease, including the development of osteoporosis, but the panel concluded there is not sufficient evidence to firmly establish a relationship.
Chromium. A number of studies have shown that chromium stimulates insulin action in the body. However, the daily requirement for chromium could not be established because not enough information exists to determine a relationship between a particular dose of the nutrient and insulin response. Not all studies show that chromium supplementation has a positive effect on the regulation of glucose levels, the report notes. Based on current estimated consumption by the general population, an AI of 35 micrograms for men and 25 micrograms for women was recommended. Chromium is widely distributed throughout the food supply. Few serious side effects have been associated with excess intake of chromium from food, and little data are available on adverse effects resulting from chronically high intake of the chromium contained in supplements, so no UL was set. Some forms of chromium are known to be toxic, but those are not present naturally in foods or contained in currently available dietary supplements.
Copper. The new RDA for copper -- a nutrient necessary for proper development of connective tissue, nerve coverings, and skin pigment -- is 900 micrograms a day for both men and women. To protect against possible liver damage, the UL was set at 10 milligrams per day. Copper is widely distributed in foods such as organ meats, seafood, nuts, and seeds; some foods that are consumed in substantial amounts, such as milk, tea, chicken, and potatoes, also contain the nutrient, but at lower levels.
Iodine. Iodine is an important component of thyroid hormones and is stored in the thyroid gland. A deficiency can cause mental retardation, hypothyroidism, goiter, and dwarfism. Based on research into how much iodine the thyroid needs to properly regulate enzyme and metabolic processes, an RDA of 150 micrograms a day was established for both men and women. Most food sources have little iodine, though some plants grown in iodine-rich soil and seafood have higher concentrations because they absorb the nutrient from their environments. Iodized salt also is a dietary source. To avoid over-absorption of iodine by the thyroid, adults should not consume more than the UL of 1.1 milligrams daily.
Iron. Iron is vital for transporting oxygen in the bloodstream and for the prevention of anemia. Even more of the nutrient is needed during periods of growth and for the fetus during pregnancy. Women during pre-menopause years also need more, since iron is lost through menstruation. The report sets the RDA for men and post-menopausal women at 8 milligrams per day, and at 18 milligrams for pre-menopausal women. Pregnant women should consume 27 milligrams a day, which usually requires taking a small supplement since it is difficult to get that much iron through diet alone. The RDA for women who breast-feed and are not menstruating is 9 milligrams a day; for adolescents who breast-feed, it is 10 milligrams daily. Human milk only provides enough iron for infants until they are 6 months old, so the report recommends that older infants -- those between the ages of 7 months and 12 months -- who are breast-fed be given foods or formula containing additional iron; older infants receiving formula also should be given iron-fortified formula or foods. Oral contraceptives reduce menstrual blood losses, so women taking them need less daily iron. Post-menopausal women who are on hormone replacement therapy should consume more iron because the therapy often causes periodic uterine bleeding. Because the absorption of iron from plant foods is low compared to that from animal foods, vegetarians need to consume twice as much iron to meet their daily requirement.
The UL for iron is set at 45 milligrams a day for adults, above which gastrointestinal distress may occur, especially when consuming iron supplements on an empty stomach. Research has suggested a possible link between elevated iron stores and a higher incidence of heart disease and cancer. However, the report says that evidence for a relationship between dietary iron intake and increased risk of these diseases is inconclusive. In addition, individuals who inherit both genes for hereditary hemochromatosis, an iron absorption disorder, are at increased risk for accumulating harmful amounts of iron. The tolerable upper intake level was not set to protect these people since there is insufficient evidence to determine a specific maximum level that would provide significant protection against the development of the clinical symptoms of this disorder.
Manganese. This nutrient is involved in bone formation and in protein, fat, and carbohydrate metabolism. Nuts, legumes, tea, and whole grains are rich sources of manganese. The report sets an adequate intake level for manganese at 2.3 milligrams per day for men and 1.8 milligrams per day for women. The UL is set at 11 milligrams for adults, based on a recent study showing that no adverse health effects occurred when this amount was consumed on a chronic basis. Neurological side effects, similar to symptoms caused by Parkinson's disease, were observed in an earlier study among participants who consumed 15 milligrams a day.
Molybdenum. The new RDA for molybdenum is 45 micrograms per day for both men and women. Sources of this enzyme-enhancing nutrient include legumes, grain products, and nuts. The UL was set at 2 milligrams, based on studies showing impaired reproduction and growth in animals at high levels of chronic intake.
Zinc. Zinc is associated with more than 100 specific enzymes and is vital for protein function and gene expression. Many breakfast cereals are fortified with zinc and it is naturally abundant in red meats, certain seafood, and whole grains. The RDA for zinc was set at 11 milligrams per day for men and 8 milligrams per day for women. Vegetarians may need up to 50 percent more, however, since a chemical in plants, called phytate, as well as calcium, hinder zinc absorption in the body. As is the case with iron, human milk does not contain enough zinc for older infants between the ages of 7 months and 12 months to meet their RDA, so children this age should consume foods containing this nutrient if they consume human milk or be given formula containing zinc. A UL of 40 milligrams for adults was set, based on studies showing that zinc adversely affects copper absorption at high levels of intake.
Arsenic, Boron, Nickel, Silicon, and Vanadium. Although there is some evidence suggesting a beneficial role for these elements in animal and human health, not enough data exist to define with certainty what their specific roles may be. Therefore, recommended intake levels were not established. However, based on adverse effects noted in animal studies, tolerable upper intake levels were set for boron at 20 milligrams per day; for vanadium at 1.8 milligrams per day; and for nickel at 1 milligram per day. Arsenic in chemical forms is a known toxic element, but not enough data exist on chronic intakes at lower levels from food and supplements to set a UL. Data also were lacking upon which to base a UL for silicon.
A Research Agenda
The report identifies several gaps in what is known about these 14 micronutrients. For example, there is a dearth of studies designed specifically to estimate average nutrient requirements for healthy humans, especially infants, children, adolescents, the elderly, and pregnant women. In addition, there has been a lack of research aimed at studying the role of these micronutrients in reducing the risk of chronic diseases or detecting side effects from chronic overconsumption. High priority should be given to research that attempts to fill in this missing information, including studies to further identify factors that impair or enhance the absorption and metabolism of these nutrients and to further investigate the role of arsenic, boron, nickel, silicon, and vanadium in human health.
The study was sponsored 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; the Dietary Reference Intakes Private Foundation Fund, including the Dannon Institute and the International Life Sciences Institute; and the Dietary Reference Intakes Corporate Donors' Fund, which includes contributions from Roche Vitamins Inc., Mead Johnson Nutrition Group, Nabisco Foods Group, U.S. Borax, Daiichi Fine Chemicals Inc., Kemin Foods Inc., M&M/Mars, Weider Nutrition Group, and the Natural Source Vitamin E Association.
The study was undertaken by a group of more than 40 scientists from the United States and Canada under the auspices of the Institute of Medicine's Food and Nutrition Board. The Institute is a private, nonprofit organization that provides health policy advice under a congressional charter granted to the National Academy of Sciences.
INSTITUTE OF MEDICINE
Food and Nutrition Board
Standing Committee on the Scientific Evaluation of Dietary Reference Intakes (DRIs)
Panel on Micronutrients
Robert M. Russell, M.D. (chair)
Professor of Medicine and Nutrition
Department of Medicine and Nutrition
School of Medicine, and
Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center
John L. Beard, Ph.D.
Professor of Nutrition
Department of Nutrition
Pennsylvania State University
Robert J. Cousins, Ph.D.1
Boston Family Professor of Human Nutrition
Department of Food Science and Human Nutrition, and
Center for Nutritional Sciences
University of Florida
John T. Dunn, M.D.
Professor of Internal Medicine
School of Medicine
University of Virginia
Guylaine Ferland, Ph.D.
Associate Professor of Nutrition
Department of Nutrition
University of Montreal
K. Michael Hambidge, M.D., F.C.R.P.
Professor of Pediatrics
Department of Pediatrics
University of Colorado Health Sciences Center
Sean Lynch, M.D.
Professor of Medicine
Eastern Virginia Medical School
James G. Penland, Ph.D.
U.S. Department of Agriculture Human Nutrition Research Center, and
Adjunct Professor of Psychology
University of North Dakota
A. Catharine Ross, Ph.D.
Professor of Nutrition
Department of Nutrition, and
Pennsylvania State University
Barbara J. Stoecker, Ph.D., R.D.
Department of Nutritional Sciences, and
Head of Nutritional Sciences
Oklahoma State University
John W. Suttie, Ph.D.1
Frank Morgan Strong Professor of Biochemistry
Department of Biochemistry
University of Wisconsin
Judith R. Turnlund, Ph.D., R.D.
Research Nutrition Scientist
U.S. Department of Agriculture Western Human Nutrition Research Center
University of California
Keith P. West, Dr.P.H., R.D.
Professor of Human Nutrition
Division of Human Nutrition
School of Hygiene and Public Health
Johns Hopkins University
Stanley H. Zlotkin, M.D., Ph.D.
Departments of Pediatrics and Nutritional Sciences
University of Toronto, and
Senior Scientist and Medical Director of Nutrition Support, and
Division of Gastroenterology and Nutrition
Hospital for Sick Children
Subcommittee on Upper Reference Levels of Nutrients
Ian C. Munro, Ph.D. (chair)
Consultant Toxicologist and Principal
Subcommittee on Interpretation and Use of DRIs
Suzanne P. Murphy, Ph.D. (chair)
Cancer Research Center of Hawaii
University of Hawaii
Standing Committee on the Scientific Evaluation of DRIs
Vernon R. Young, Ph.D.1,2 (chair)
Professor of Nutritional Biochemistry
Laboratory of Human Nutrition
School of Science
Massachusetts Institute of Technology
Paula R. Trumbo, Ph.D.
Panel Study Director
Allison A. Yates, Ph.D., R.D.
DRI Study Director
1 Member, National Academy of Sciences
2 Member, Institute of Medicine
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Editor: Susan K. Boyer, RN
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