4.15: Recommended Dietary Allowances (RDAs)
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These are the most familiar of the Dietary Reference Intakes. During World War II, the U.S. government needed a guide for feeding the troops (many of the nutrients had just been identified in the previous decade). The Food and Nutrition Board of the National Academy of Sciences, made up of distinguished scientists with expertise in various areas of nutrition, is in charge of making this guide.
The Board examines the scientific data to decide on the Recommended Dietary Allowances (RDAs)—amounts of nutrients in the diet expected to meet the needs of virtually all healthy people in the United States (97.5% of a normal population; Figure 4.1). RDAs don’t quite cover those with the highest need (about 2.5% of the population). But even these people would be close to meeting their needs at RDA levels (Figure 4.1).
The recommended amounts are typically higher than what our body actually requires. Iron, for example, is poorly absorbed from the diet, so the RDA for iron is about ten times more than the body’s requirement. For some nutrients, our requirements aren’t well established, and the RDAs are based mainly on customary intakes of healthy populations.
Our needs differ if we’re growing, menstruating, pregnant, etc., so there are several sets of RDAs. The set for infants, for example, is used by manufacturers of infant formula. The sets of adult RDAs for some nutrients are given in Appendix A-4.
RDAs aren’t daily needs in the sense that you’ll be deficient if you don’t meet them every day. They’re for daily averages because diets vary from day to day. They’re also generous, not minimums. RDAs allow for normal stresses of daily life and adequate reserves in the body— even for water-soluble vitamins.
Practically speaking, it’s advised that nutrient intakes average the RDAs over several days. Dietary assessments are commonly done on 3-day diet records. (Disease can change dietary needs, and dietitians give special advice in such cases.)
Since World War II, the use of RDAs has expanded from its original use as a guide to plan and procure food for national defense. RDAs are now used for such purposes as evaluating diets of various groups in the U.S. population, establishing guidelines for food-assistance programs (e.g., school lunch programs) and food labels, and developing nutrition policy and nutrition education programs. This diverse use of the RDAs has complicated their revision; there are now economic, environmental, and political issues.