Have you met your RDA lately?

Have You Met Your RDA 

One by one and by the thousands, young Americans signed up at the outbreak of World War II to joint the military forces to fight for freedom. But to the consternation of the nutrition scientists of that day, many men were rejected for military services--poor nutritional status was affecting their health. Controversy arose. How could such a thing be in the great land of America, where nutritional deficiencies were considered a thing of the past? What did it mean? Did we have enough food supplies to feed our troops and keeps the workers at home well fed and healthy? How could we know?

In response to these and other questions, a group of 25 scientists met in 1941 and formed the first Food and Nutrition Board. The board took on the responsibility of establishing dietary standards. These standards were to be "a guide for planning and procuring food supplies for national defense." Since that time their use has expanded.

The standards the board developed and published in 1943 were the first Recommended Dietary Allowances (RDAs). Since that time and up to 1980, the RDAs were updated every four or five years. The tenth and latest edition of the RDAs was published in 1989 after a nine-year interval.

The RDAs are developed on the basis of what is known at the time of preparation. They change as we learn more about the nutrients our bodies need. Nutrients are added to the RDA list when enough information becomes available to estimate needs and make reliable recommendations.

"Recommended." Sometimes there is confusion about just what the RDAs are and how they are to be used. The RDAs are recommendations for nutrient intake, not requirements. If we look at how they are developed, it will help us to understand what that means.

First, scientists estimate the average requirements of a nutrient in a population, that is, the least amount a person must have to be healthy. Below that amount a person will begin to develop a deficiency. But we are all different, and we do not all need the same amount of every nutrient. So the scientists must then calculate just how the requirement varies from person to person in a population. Third, they investigate how well the body can actually use the nutrients from food. Some nutrients are not well absorbed; some must be converted to a different form to be used by the body.

Next, using the information about how the need for a particular nutrient varies in a population and how well that nutrient can be used by the body, they increase the requirement by an amount that will meet the needs of almost all the people in the population. This is usually a 30 to 50 percent increase over the requirement level. In statistical terms, the recommendation is set at two standard deviations above the mean value. Figure 1 illustrates this concept. We can see that all but 2.5 percent of the population are covered when the RDA is established in this way. We can also see that a few people in the population require very little of a particular nutrient and a few require high levels; most of the population is in the midrange.

Because of the generous nature of the recommendations, an intake below the RDA for a nutrient is not necessarily inadequate (see illustration at right). However, we have no assurance that the person who consumes low amounts of a nutrient has a low requirement for that nutrient. The farther below the RDA the actual intake is, the greater the possibility that the intake will be inadequate.

One exception to the generous allowance made in establishing nutrient recommendations is energy. In this case, average values are used (Figure 2) without the adjustments that would cover 97.5 percent of the population.

"Dietary." Sometimes people think that the D in RDA stands for daily. Instead, it really stands for dietary. It is not necessary to consume the RDA for every nutrient every day. Body stores or a surplus consumed shortly afterward will make up for the inadequacy. Averaging the intake of a nutrient over several days or a week will give us a better idea about what we are eating than if we look at just one day.

That word dietary should remind us about something else, too. The RDAs are intended to be met by eating a diet containing a wide variety of foods, rather than by using supplements or depending on just a few fortified foods. RDAs have not been established for all nutrients. Eating a varied diet is the best assurance that the needs for all nutrients will be met, whether or not an RDA has been established for them.

"Allowances." The RDA committees tend to be generous in the allowances because there is little evidence that small surpluses of nutrients are detrimental. On the other hand, all nutrients can become harmful at some level of intake. The idea that if some is good more must be better is just not true for nutrients. Figur 3 shows that both too little and too much of a nutrient can be detrimental. The RDAs are designed to meet the needs of healthy people, and the allowances have been set high enough so that no improvement in health will result from consuming higher levels. Any possible benefits from the nutrients will be achieved by using a varied diet.

In 1980 the RDA committee established a new category for certain nutrients, the Estimated Safe and Adequate Daily Dietary Intake (ESADDIe. There was not enough information about these nutrients to establish an RDA for them, but there was enough to suggest a range of intake. The range suggests that eating less than the lowest value may result in a deficiency, but it also suggests that habitually consuming more than the upper amount can be harmful.

1989 RDA. Several changes were made in the latest edition of the RDAs. The recommendations have always been made on the basis of age and sex. The 1989 edition expanded the 19- to 22-year-old group to include 24-year-olds. The reason for this change is that bone continues to increase in density even after adult height is reached. The highest bone mass that a person will have is called the peak bone mass, and it is not developed before the mid-20s or early 30s.

Calcium intake is very important during this time to assure that the bone is properly mineralized. At some time after the peak is reached, we begin to lose bone. Consuming enough nutrients to support the development of an adequate peak bone mass when we are young can help prevent osteoporosis later. It's like putting money in the bank to draw from as needed.

The new RDAs for 23-and 24-year-olds are higher for calcium, phosphorus, and vitamin D than they were before the age adjustment. Hopefully, this will encourage young women especially to consume adequate amounts of this important nutrient.

Nutrient recommendations are based on a "reference" person for each age and sex category. A major change took place in the tenth edition in setting the height and weight of the reference persons in the different categories. In the past they were set at an arbitrary ideal value, but in the 1989 edition the heights and weights are the actual medical values for the U.S. population of each category. Just as many people weigh less than the medial value as weigh more than it. The median values are not ideal values, but they do reflect what is found in the population.

This change from ideal to actual median weight caused a change to a higher weight for the reference persons in the adult age categories. Because some nutrient recommendations are based on the weight of the reference individual, the RDA for those nutrients are greater in the new edition. For instance, the previous RDA for protein for adult males was 56 grams and for adult females it was 46 grams. The 1989 RDA for adult males is 63 grams; for adult females it is 50 grams. However, both the 1980 and the 1989 values are based on 0.8 grams per kilogram body weight. The difference exists only because the reference individual weighs more in the later edition. In addition to protein, the higher reference body weights resulted in higher recommendations for energy, thiamin, riboflavin, and niacin.

Changes were also made in the Estimated Safe and Adequate Daily Dietary Intake list. First, as we can see in the table, enough data have accumulated to add vitamin K and selenium to the nutrients with an established RDA. Second, the electrolytes sodium, potassium, and chloride were removed from the ESADDI list and recategorized as Estimated Minimum Requirements (EMR).

The EMR for sodium is only 500 milligrams per day, which is markedly lower than the former ESADDI range of 1,100 to 3,300 milligrams per day. Even this amount is much more than is actually needed by the body. One teaspoon of table salt (sodium chloride) weighs about 5,000 milligrams. Since salt is about 40 percent sodium, that means that there are 2,000 milligrams of sodium in that teaspoon. Therefore, we need no more than a quarter of a teaspoon of salt a day to meet the EMR.

Studies show that many people consume a good deal more than the EMR, some as much as 20 times more! Much of the sodium that is eaten comes from salt that is added to food during processing and manufacturing. The highest salt intakes are found in diets that rely heavily on processed foods. The lowest intakes are found in diets that emphasize fresh fruits, vegetables, and dried peas and beans.

The RDAs for folate, vitamins [B.sub.6] and [B.sub.12], magnesium, and women's iron and zinc were lowered. The 50 to 55 percent decrease in the RDA for folate was the most dramatic. The changes were made because diets containing the lower amount can maintain adequate stores in the body.

We should mention one other change. The RDA for vitamin C was increased for smokers, although it remains largely the same for nonsmokers. The increase is because somkers seem to metabolize or break down vitamin C faster than nonsmokers do. This results in lower levels in their blood and body tissues. The RDA is higher to compensate for the deficit and help them meet their basic needs, but it is not a hedge against lung cancer.

The major changes in the new RDAs are for more calcium but less sodium. WE can expect more changes in future editions of the RDAs. Sometimes people are dismayed at the changes and wonder if the scientists know what they are doing. But the changes we see from time to time reflect greater understanding of how our bodies work. We still do not know the very best level of intake, the optimal amount, for every person at every age. And so nutrition research is a continuing exploration, an ongoing adventure. But while the research continues we can use the best information we have available to guides us in our food choices.

The 1989 RDA committee states that the RDAs should be used together with recommendations from Diet and Health in planning what we eat. The RDAs talk about the need for milligrams or micrograms of a nutrient. Sometimes it's hard for the person on the street to know just what all that means. Diet and Health talks about diet in terms we are more used to. You'll find those recommendations in the box above.

Patricia K. Johnston, Dr.P.H., M.S., R.D., is director of the Doctor of Public Health program and associate professor of nutrition at Loma Linda University, Loma Linda, California.