First published in 1943 as a guide to wartime procurement of food supplies, recommended dietary allowances are now widely used by diet planners, marketers, nutritionists and consumers.
The RDAs are set by the Food and Nutrition Board of the National Research Council, an arm of the National Academy of Sciences, which reviews recommendations by a panel of experts. The guidelines are revised about every five years to take into account the latest research on dietary needs.
Easier said than done.
A new RDA report due out this year has been postponed repeatedly because of disagreement between the scientific panel and its independent reviewers over a draft proposal to reduce the RDA for vitamins A, C and B6 and the minerals magnesium, iron and zinc, while raising the RDA for calcium. The two groups reached an impasse last week.
Not only were the scientists unable to agree on whether specific RDAs for several nutrients should be revised, said NRC chairman Frank Press, but some participants also advocated "a more comprehensive approach" to dietary need that goes beyond merely avoiding nutritional deficiencies. The report has been postponed indefinitely, while the Food and Nutrition Board considers whether to start all over again or try to resolve the dispute.
Even defining the concept of an RDA is tricky.
The academy defines RDAs as "the levels of intake of essential nutrients considered . . . on the basis of available scientific knowledge, to be adequate to meet the known nutritional needs of practically all healthy persons."
The definition contains important qualifications:
RDAs do not guarantee an individual's health; they are recommendations for whole populations. They are deliberately designed to exceed the needs of the average person, to ensure that they meet the needs of "practically" everyone in the population.
RDAs are not minimum requirements. They usually include a surplus, since the exact requirement varies from person to person. The RDA also has some extra built in to allow for storage of reserves. The normal adult's need for vitamin C, for example, is between five and 10 milligrams a day, but the RDA is set at 60 milligrams.
RDAs apply to healthy people. They don't cover the special nutritional needs of premature babies or people with metabolic disorders, severe infections and chronic illnesses.
The most recent RDA report carefully lists other factors that affect nutritional needs, including physical activity, climate and age.
Of the 13 essential vitamins, the NAS sets RDAs for only 10. (RDAs for vitamin K and two of the B-complex vitamins, biotin and pantothenic acid, do not exist because of insufficient knowledge about the exact level needed.) The academy also sets RDAs for six minerals -- calcium, phosphorus, magnesium, iron, zinc and iodine.
The best way to meet the RDAs, the academy advises, is "with a wide variety of foods." To meet the RDAs simply by taking vitamin supplements or eating vitamin-fortified foods while ignoring overall diet is to risk running short of other basic nutrients for which there is no specific RDA.
Although it is not possible to set a specific RDA for every vitamin, the RDAs serve as a general dietary guideline, the academy says. A varied diet that meets the known RDAs "will probably be adequate in all other nutrients."
"The point is, when we rely on supplements, we are relying only on what we know is important, and there may be some vital nutrients we don't know about," says Dr. Walter Mertz, director of the Department of Agriculture's Human Nutrition Research Center in Beltsville and a member of the committee that compiled that current RDAs.
"When we rely on a balanced diet, we have a pretty good chance of getting everything we need."