Dietary Reference Intakes
For Vitamin C, Vitamin E, Selenium, and Carotenoids
Institute of Medicine
April 10, 2000
Norman I. Krinsky
Professor of Biochemistry
Tufts University School of Medicine, Boston
Chair, Panel on Dietary Antioxidants and Related Compounds
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. Research into the health benefits of dietary antioxidants has received a great deal of public attention. Today we offer our findings and recommendations, 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 several years the board has updated and expanded the system for determining these values -- now called Dietary Reference Intakes or DRIs. Our report, Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids,
is the latest in a series on DRIs.
In recent years, researchers have focused on the possible role that dietary antioxidants play in promoting and maintaining health. Antioxidants help neutralize potentially damaging byproducts of the body's metabolism. These byproducts -- highly reactive forms of oxygen and nitrogen -- are sometimes called free-radicals. Because they react so easily with other molecules, they often interfere with the normal operation of cells and cause damage. That interference is known as oxidative stress, and antioxidants can compensate for it.
Beyond that, however, are the various claims that taking large doses of antioxidants might help prevent a variety of chronic ailments including cardiovascular disease, diabetes, Alzheimer's, various forms of cancer, and other diseases. Does taking larger doses of dietary antioxidants help limit the gradual buildup of cellular damage that one day may turn into chronic disease?
After a comprehensive review of the scientific evidence, our panel concluded that too little is known at present to provide a definitive answer. Too few studies have tested the effects of these dietary antioxidants in human subjects, and in some cases, where such human testing has been done, the results are contradictory. Although a large number of population studies reveal a link between a diet rich in foods containing antioxidants, such as fruits and vegetables, and a lower incidence of certain chronic diseases, we cannot conclude at this time that the antioxidants are the reason. Other nutrients or factors could be at play. While convincing scientific evidence may one day be available, the question of whether dietary antioxidants can prevent chronic disease still remains unanswerable. What is known, however, is that taking very large quantities of some antioxidants can actually cause health problems, as I will soon explain.
So how much of these nutrients does one need to take on a daily basis to maximize their effectiveness? To answer that question we offer two types of numbers. The recommended dietary allowance -- the RDA -- represents a daily intake goal for healthy individuals. We also offer what we call "tolerable upper intake levels." These represent a ceiling -- the largest amount of a nutrient that healthy individuals can take each day without risking adverse health effects. I should point out that this is the first time the Food and Nutrition Board has set upper limits for this group of nutrients.
We looked at vitamin C, vitamin E, selenium, and the carotenoids, including beta-carotene, and made the following recommendations.
We increased the recommended daily intake levels for vitamin C
to maximize its effectiveness in the body. Women should consume 75 milligrams per day; the daily amount for men should be 90 milligrams. Because smokers are more likely to experience higher levels of oxidative stress and cellular damage, they need more vitamin C, and therefore, we recommend an extra 35 milligrams each day. These RDAs can easily be met without taking supplements. Among foods containing vitamin C are citrus fruits, strawberries, broccoli, potatoes, peppers, and leafy green vegetables.
As an upper limit for vitamin C intake -- from both foods and supplements -- we recommend 2,000 milligrams per day for adults. Taking more can cause diarrhea in some people, and any amount above the RDA is likely to be excreted by the body unused.
We also increased the daily recommended levels for vitamin E
. The only form of vitamin E that can be fully utilized by the body is alpha-tocopherol, and we recommend that both men and women consume 15 milligrams from food, This is equivalent to 22 International Units from natural-source vitamin E or 33 I.U.s of the synthetic form. Nuts, seeds, liver, leafy green vegetables, and some vegetable oils are all sources of alpha-tocopherol.
The upper level for daily consumption of this form of vitamin E is 1,000 milligrams per day for adults, a level that would be reached only by taking supplements. That amount translates into 1,500 International Units of "d-alpha-tocopherol" -- sometimes labeled as the "natural source" of vitamin E supplements -- or 1,100 International Units of "dl-alpha-tocopherol," a synthetic version. Consuming greater amounts heightens the risk of bleeding, since at high levels this nutrient acts as an anticoagulant.
In the body, selenium
plays a role as a dietary antioxidant once it is inserted into enzymes that defend against oxidant damage. For selenium, our recommendations are set to maximize the activity of these enzymes. Both women and men should get 55 micrograms per day of selenium. Food sources include seafood, liver, meat, and grains. The upper limit for selenium is set at 400 micrograms per day, including amounts consumed from both foods and supplements. At higher levels, the risk of a toxic reaction increases. Called selenosis, it is marked by hair loss and fingernail and toenail damage.
In the case of beta-carotene and other carotenoids,
it is not clear from the evidence that they are truly dietary antioxidants, and we did not treat them as such. Laboratory tests have shown them to have antioxidant properties, but results in human trials have been inconsistent. At the moment, the only clear nutritional role played by the carotenoids is in the formation of vitamin A, and their contribution to meeting those requirements will be discussed in a future report.
Furthermore, data on the adverse effects of consuming too much supplementary beta-carotene are contradictory. For this reason we are not setting upper limits for the carotenoids. We urge people to use caution before taking these nutrients in high doses, and recommend supplementation only to prevent or control a vitamin A deficiency.
These recommendations represent a snapshot of a very active area of scientific research. Studies are ongoing, and yet, there is much to be learned. Our report highlights areas where more research is needed. In general, the impact on human health needs to be studied in well-controlled clinical trials. One very important area concerns the long-term effects of consuming antioxidants at daily levels above the upper limits. When properly done, this research should not pose health risks to subjects, and we encourage more of these studies. Similarly, much more research is needed to investigate the role of dietary antioxidants and carotenoids in the prevention of chronic diseases. Research should also focus on how these substances interact with each other and with other food components. 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 North American adults get enough vitamin C, vitamin E, and selenium from their normal diets to meet our recommendations. And those who don't could get enough of these nutrients simply by improving their diets. Therefore, we can and do recommend that adults eat more fruits and vegetables that are rich in these nutrients.
This concludes my opening statement. My colleagues and I will now take questions. This public briefing is being taped, so please step to a microphone to ask your question, and give your name and affiliation.