18.1: Getting Enough

Scientists know a lot about how much of each of the nutrients we need. They also know a lot about how much of these nutrients are in various foods. In rigorously controlled studies, they make precise measurements of the nutrients taken in, and the amounts used, stored, and excreted. They also study how nutrients interact and function in the body. What they learn leads to recommendations for the amounts of nutrients we should include in our diet, amounts deemed “safe and adequate”— enough to meet our needs, yet not so much as to risk toxicity (another sort of balance).

Dietary Reference Intakes

Dietary Reference Intakes (DRIs) are a group of four reference values (Fig. 18-1): Estimated Average Requirement (EAR), Recommended Dietary Allowance (RDA), Adequate Intake (AI), and Tolerable Upper Intake Level (UL). The DRIs are a joint effort of the U.S. Dept. of Health and Human Services and Health Canada, providing a common set of values for both the U.S. and Canada. We as consumers are most concerned with:

• Recommended Dietary Allowance (RDA), the amount recommended in the diet to meet the needs of nearly all healthy people. When there isn’t enough information to set an RDA, an Adequate Intake (AI)—based mainly on customary intakes of healthy populations—is substituted. For simplicity, RDA is broadened in this book to include AI, since both values are amounts recommended in the diet.
• Tolerable Upper Intake Level (UL), the highest usual intake of a nutrient that’s unlikely to have adverse effects for nearly all healthy people. (For some nutrients there isn’t enough information to set a UL.) The aim is to not exceed the UL. For sodium, the UL has been replaced by the CDRR (Chronic Disease Risk Reduction Intake, e.g., to lower risk of chronic disease, those over age 13 are advised to reduce sodium intake if it’s above 2300 mg/day.

Recommended Dietary Allowance

The most familiar of the Dietary Reference Intakes (DRIs) is the Recommended Dietary Allowance (RDA). The first set was issued in 1941. (Many of the nutrients had just been identified in the previous decade.) The Food and Nutrition Board, assembled as part of the National Academy of Medicine, has the responsibility of updating the RDAs. The current set was issued as part of the Dietary Reference Intakes in 1997-2019.

The Board’s membership, which includes some of our most respected nutrition scientists, changes periodically, both to share the burden of the work, and to infuse a variety of expert opinions. Scientists periodically reassess what’s known of the nutrients, scouring the newest research reports. They also study the quantities of nutrients in our food supply, their effects on the body, and information about the public’s health and eating habits. Their objective is to recommend amounts of nutrients in the diet that will meet the needs of virtually all healthy people in the United States and Canada.

There are different RDAs for males and females, for different ages, and for pregnant women and nursing mothers (see Appendix A).

Are the RDAs Set High Enough for Good Health?

The RDAs are widely misunderstood to be minimums for physical health. They shouldn’t be confused with the amounts of nutrients actually required in the body. For one thing, RDAs allow for absorption rates. The RDA for iron, for example, is about ten times higher than the body’s requirement, because, on average, we absorb only about 10% of the iron in our diet.

The RDAs aren’t daily requirements in the sense that a deficiency will occur if they aren’t consumed daily. They’re for averages per day, since diets vary day to day. Practically speaking, it’s advised that nutrient intakes average the RDAs over several days.

Aside from what’s necessary to meet needs, is there such a thing as an “optimal intake”? In other words, do amounts higher than the RDAs provide health advantages for the average member of the general population? At the present time, such “optimal intakes,” if they actually exist, aren’t known. If such evidence were forthcoming, the RDAs would be revised accordingly.

How Useful are the RDAs for Assessing Individual Diets?

The Dietary Reference Intakes are used for such diverse purposes as evaluating diets of various population groups, establishing guidelines for food assistance programs (e.g., school lunch programs) and food-labeling information, and developing nutrition policy and education programs.

As an individual, however, you can only use the RDAs to target the likelihood that you’re meeting your nutrient needs, since your personal requirement isn’t known. If your diet meets the RDAs, the likelihood that you’re meeting your needs is high. The further your intake falls below the RDAs, the greater the likelihood that your intake is inadequate.

At any rate, planning a good diet in terms of the RDAs is too tedious a process for most of us— even when run on a computer. We need a shortcut for selecting a good diet. One such shortcut involves grouping foods by their nutrient content, and choosing appropriately from the groups.