Johns Hopkins Magazine -- April 1998
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APRIL 1998




H E A L T H    A N D    M E D I C I N E

The ABC's of the RDA
Interview by Melissa Hendricks
Illustration by Melissa Grimes

Benjamin Caballero, director of the Center for Human Nutrition at the School of
Public Health.

Once each decade, a panel of national experts convenes to review and revise the nation's benchmark of sound nutrition known as the RDA, or Recommended Dietary Allowances.

The review process used to address questions concerning "minimum" requirements, such as, How much calcium is required to prevent deficiency? But thanks to advanced techniques in molecular and cell biology, and greater knowledge about epidemiology, during the past 20 years nutrition science has undergone what some authorities call "a quiet revolution."

Now, biomedical scientists are exploring how particular vitamins and minerals might protect against longer term assaults on the body, such as cancer, heart disease, or osteoporosis. For example, they are determining the levels of calcium that prevent osteoporosis. It is the task of the Food and Nutrition Board of the Institute of Medicine to sort through this mountain of new data for the latest revision of the RDA, which will be issued by the year 2000.

On the board sits Hopkins's Benjamin Caballero, a pediatrician and director of the Center for Human Nutrition at the School of Public Health. Caballero gamely agreed to sit down over a lunch of lean pork and sweet potatoes to talk with writer Melissa Hendricks about what the American public can expect from the revised RDA, and to explain why he believes vitamin supplements aren't necessarily the answer.

How did the RDA get its start?

The Food and Nutrition Board of the National Academy of Sciences created the RDA in 1941. The United States was at war, and was concerned with defining what the minimum vitamin and mineral requirements were to keep people working and fighting. The principle was to define the minimum need, and add a safety amount to it, to cover the wide range of people and age groups. The result was one level of nutrient intake, which covers the nutritional requirements of 95 percent of the population.

Over the years, the board added 10 minerals and vitamins to the initial eight. But the board has not significantly changed the amounts of recommended nutrients. This time around, the Food and Nutrition Board decided to do things differently.

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What's so different about the current effort to revise the RDA?

This time the board is setting up a different expert panel to review each nutrient group, instead of just one committee to review all the nutrients. For example, the first panel, which completed its report last August, reviewed studies on calcium and related nutrients (vitamin D, phosphorus, magnesium, and fluoride). Another will examine the antioxidants. Another will look at trace elements like iron and zinc. Since the science of nutrition is becoming more complex, the expertise really is split among different scientists.

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The panel that reviewed the evidence on calcium and related nutrients very clearly did consider disease prevention, particularly osteoporosis. For the first time, it recommended that people consume higher levels of calcium (See "Calcium,").

The next report will be for folic acid. I suspect the board will increase the present recommendation for folic acid. The goal is to assure that women of reproductive age consume the minimum amount to reduce neural tube defects. In some sense, the Food and Nutrition Board is behind the times: The FDA already passed a regulation requiring that flour be fortified with folic acid, which became effective January 1.

As for the other nutrients, those reports will come out in a staggered fashion. The overall plan is to issue the complete set of revised recommendations by the year 2000.

There's another change worth noting. The Food and Nutrition Board is introducing the Estimated Average Requirement, or EAR. This new measurement estimates the amount of each nutrient required to meet the needs of 50 percent of the population. It is based on the idea that not everyone needs to meet the 95 percent standard of the RDA to stay healthy. The EAR will also include breakdowns by age and gender.

Will the expert panels recommend increases for other vitamins and minerals, with an eye toward preventing chronic diseases?

That's hard to predict. I will say that the evidence for vitamin E is some of the strongest. Based on studies where people take a supplement, vitamin E seems to reduce the risk of some cancers by 60 to 70 percent. Increased levels of vitamin E also appear to decrease the amount of fat (lipids) in the arteries, and to reduce the risk of heart disease by 80 to 90 percent.

Can we expect a recommendation to increase vitamin E consumption then?

The current RDA for vitamin E is 10 milligrams per day for men. That's the equivalent to the amount found in about 1/8 cup of safflower oil. For women it is 8 milligrams. The protective effects for reducing risks of certain forms of cancer (colon, prostate, breast, lung) reported for vitamin E begin at around 200. That's 20 times the current recommended amount--the equivalent of consuming two and a half cups of safflower oil each day. It is virtually impossible to get 200 units of vitamin E from food. So that would mean that people would have to take a vitamin E supplement.

Frankly, I doubt the RDA is going to jump from 10 to 200. There is now no official consensus or recommendation on any high-dose vitamin for the general population. (With the exception of supplements for pregnant women and infants, who have higher nutritional needs than they can get from food.)

Before advising people to take any supplement, we need to define exactly what amount would be advisable for the whole population, when they should take it, and for how long. For example, if we find that 200 units of vitamin E a day has a protective effect, would you recommend that everybody take 200 units? First of all, I would ask, when do I start? As a child? As a middle-aged person who has a higher risk? Then, for how long should I take it? I don't think the data exist that show what the potential side effects would be of having 20 times what you basically need every day for life. Also, there is no evidence of vitamin E's protective effects in children or adolescents.

Another reason there is no official recommendation for the general population is that there are variables in the life of each individual--lifestyle, age, genetic background, family history, profession, level of physical activity--that reduce or increase risk of disease. So a single nutrient may have very little effect in one person and a significant effect in another, depending on what each person is doing.

For example, if a person is a smoker, trying to reduce the risk of lung cancer with some vitamin or antioxidant will have very little effect, relative to quitting smoking. So I think it's going to be very difficult to come up with uniform recommendations for preventing disease for the whole population.

The only official consensus is to eat five fruits and vegetables a day. We can recommend that people eat broccoli. We know empirically that people live longer, healthier lives if they eat more fruits and vegetables. But if we start talking about a compound in a capsule that is, for example, the equivalent of the amount of that compound in 100 pounds of broccoli, then we really need to understand the physiology of that compound before we make recommendations.

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Apart from what the official recommendations say, do you believe that people should routinely take vitamin supplements?

I don't think it's wise. I only recommend supplements if the diet is deficient, or people are not likely to consume what they need. Pregnant women and infants, for example. But people who consume a variety of food in their normal diet don't need supplements. Incidentally, most nutritionists tend not to recommend supplements. I don't take them.

I'm not too worried about the chemical effects of supplements. I am concerned with people's self-deception, thinking they are doing something good for their health, and thus excusing themselves from trying to improve their diet, be more physically active, quit smoking, etc. This may seem like moralizing, but think about this: We are the country that consumes the most vitamin supplements, and yet we have the worst rates of obesity, heart disease, and other chronic diseases. Does this tell you something about the value of using supplements?

There is also the matter of toxicity. Megadoses of certain vitamins or minerals can cause harm. If you take too much vitamin E, you might only cause a nosebleed. But if a pregnant woman takes too much vitamin A, it could cause birth defects.

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But the reality is that many people simply do not eat a balanced diet. Can't we get equivalent levels of nutrients through vitamin pills?

There is a national campaign to get people to eat five fruits and vegetables a day. Five a day gives you fiber, antioxidants, phytochemicals. The recommendation is not based on some desirable level of vitamin intake. Rather, it is based on studies showing that populations consuming at least that amount of fresh fruits and vegetables--not of supplements--have significantly lower rates of certain diseases. This is a fact. There is no evidence that consuming a supplement with the same amount of vitamins present in those five servings offers similar protection.

Take the example of beta carotene, the building block of vitamin A that is found in green and orange vegetables and fruits. Studies have shown that beta carotene reduces the risk of some forms of cancer through antioxidant effects. But some scientists now suspect that the actual powerful antioxidant may be other substances found in foods that are rich in beta carotene. Beta carotene may just be a proxy for something else. If you eat lettuce or spinach, you are eating maybe 50 or 100 different antioxidants, of which we know only a handful. So we measure what we can measure, but that doesn't mean that beta carotene is the most active compound.

So if that is the case, and if we recommend that people who don't eat vegetables take beta carotene in capsule form, then we may normalize the level of beta carotene in their blood to a level that makes us happy, but we won't necessarily protect them.

Whether "natural" foods are better or will always be preferable to supplements or enriched foods, I am not sure. I tend to doubt it. Our nutrient requirements depend on the equilibrium between ourselves and our environment. As our lifestyle continues to deviate from a "natural" state, I would be surprised if we can keep that equilibrium only with "natural" foods. For example, as the ozone layer is depleted, our exposure to oxidative stress increases, and so does our need for antioxidant nutrients. We may reach a point in which our exposure is so high that not even 20 servings a day can provide enough antioxidants!

I have nothing against fortifying foods with vitamins or asking people to take vitamin pills. But as a scientist, I need to see solid evidence before making a policy that will affect millions of people.

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What's so hard about definitively proving whether vitamins can prevent cancer and other chronic diseases?

For one thing, very few scientists are willing to do one experiment over 20 years before they can publish.

And even with good longitudinal studies, we are limited in our ability to point to cause-andeffect relationships. For instance, take the Nurses' Health Study, now in its 20th year, with an enrollment of around 80,000 nurses nationwide. The same participants are contacted every year or two by telephone and/or questionnaire. They are asked about weight, lifestyle factors, and what they ate lately. Like any poll, these studies are very good at showing trends: people doing X tend to get more of Y than people who don't do X, etc. But they are not experiments, there is no hypothesis, and there is no randomly assigned control group. So this study is not good at documenting cause and effect. The nurses who say they use 200 units of vitamin E may also be those who are quite concerned about being healthy, who exercise regularly, don't smoke, and so on.

The ideal study is a randomized clinical trial, in which you give one group of people a particular nutrient supplement and give a placebo to another group, and follow those people over time to see whether disease risk is lower in the group getting the supplement. But those studies are simply not possible, particularly in this country because supplements are so widely used. It would be very difficult to do a clean study.

Would you say the Food and Nutrition Board is fairly conservative in terms of what it is willing to recommend?

Whatever numbers the board recommends have tremendous policy implications. These recommendations are used to set targets for food aid programs, to gauge food labeling and health claims, to determine the adequacy of food staples donated worldwide, etc. International food aid programs are entirely based on the RDA. For example, say we recommend that people have 200 units of vitamin E per day. That could mean that food aid shipments would have to include capsules of vitamin E.

The RDA also helps us determine which countries need aid, who is deficient, how much food they need to import. Many countries have their own RDAs. But obviously, the U.S. RDAs are very influential, thoroughly researched, and based on a lot of data.

In what direction do you think the field of nutrition is moving?

I see a time when you go to the supermarket with your magnetic card, which has all your family history, risk factors, lifestyle preferences, whatever. You put it in a machine at the entrance, and the grocery list comes out. Some of the food will be fortified to a level where you wouldn't need to take a particular supplement. You'd just need to consume that food.

Melissa Hendricks is the magazine's senior science writer. She can be reached via e-mail at