The Recommended Dietary Allowances (RDAs) are the levels of intake of essential nutrients that, on the basis of scientific knowledge, are judged by the Food and Nutrition Board to be adequate to meet the known nutrient needs of practically all healthy persons [1].

<< The first edition of the Recommended Dietary Allowances (RDAs) was published in 1943 during World War II with the objective of “providing standards to serve as a goal for good nutrition.” It defined, in “accordance with newer information, the recommended daily allowances for the various dietary essentials for people of different ages” (NRC, 1943). The origin of the RDAs a has been described in detail by the chairman of the first Committee on Recommended Dietary Allowances (Roberts, 1958). The initial publication has been revised at regular intervals; this is the tenth edition.

From their original application as a guide for advising “on nutrition problems in connection with national defense, ” RDAs have come to serve other purposes: for planning and procuring food supplies for population subgroups; for interpreting food consumption records of individuals and populations; for establishing standards for food assistance programs; for evaluating the adequacy of food supplies in meeting national nutritional needs; for designing nutrition education programs; for developing new products in industry; and for establishing guidelines for nutrition labeling of foods. In most cases, there are only limited data on which estimates of nutrient requirements can be based.

ESTIMATION OF PHYSIOLOGICAL REQUIREMENTS

Where possible, the subcommittee established an RDA by first estimating the average physiological requirement for an absorbed nutrient. It then adjusted this value by factors to compensate for incomplete utilization and to encompass the variation both in requirements among individuals and in the bioavailability of the nutrient among the food sources.

Thus, there is a safety factor in the RDAs for each nutrient, reflecting the state of knowledge concerning the nutrient, its bioavailability, and variations among the U.S. population. It is the intent of the subcommittee that the RDAs be both safe and adequate, but not necessarily the highest or lowest figures that the data might justify.

There is not always agreement among experts on the criteria for determining the physiological requirement for a nutrient. The requirement for infants and children may be equated with the amount that will maintain a satisfactory rate of growth and development; for an adult, it may be equated with an amount that will maintain body weight and prevent depletion of the nutrient from the body, as judged by balance studies and maintenance of acceptable blood and tissue concentrations. For certain nutrients, the requirement may be the amount that will prevent failure of a specific function or the development of specific deficiency signs—an amount that may differ greatly from that required to maintain body stores. Thus, designation of the requirement for a given nutrient varies with the criteria chosen.

Ideally, the first step in developing a nutrient allowance would be to determine the average physiological requirement of a healthy and representative segment of each age and sex group according to stipulated criteria. Knowledge of the variability among the individuals within each group would make it possible to calculate the amount by which the average requirement must be increased to meet the need of virtually all healthy people. Unfortunately, experiments in humans are costly and time-consuming, and even under the best of conditions, only small groups can be studied in a single experiment. Moreover, certain types of experiments are not possible for ethical reasons. Thus, estimates of requirements and their variability must often be derived from limited information.

If population requirements follow a normal, or Gaussian, distribution pattern (Figure 2-1), adding 2 standard deviations (SDs) to the observed mean requirement would cover the needs of most (i.e., 98%) individuals. With the possible exception of the protein requirement, however, there is little evidence that requirements for nutrients are normally distributed. The distribution of the iron requirements for women, for example, is skewed (NRC, 1986). In this report, therefore, each nutrient is treated individually to allow for variability within a population, as explained in the relevant chapters of this report.

(...) CONDITIONS THAT MAY REQUIRE ADJUSTMENT IN APPLICATION OF RDAs

Clinical Considerations

RDAs apply to healthy persons. They do not cover special nutritional needs arising from metabolic disorders, chronic diseases, injuries, premature birth, other medical conditions, and drug therapies. >> [1]

References:

1 National Research Council (US) Subcommittee on the Tenth Edition of the Recommended Dietary Allowances. Recommended Dietary Allowances: 10th Edition. National Academies Press (US), 1989. doi:10.17226/1349

The Tolerable Upper Limit (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all individuals in the general population.

<< Unless otherwise specified, the UL represents total intake from food, water, and supplements. Due to a lack of suitable data, ULs could not be established for vitamin K, thiamin, riboflavin, vitamin B12, pantothenic acid, biotin, and carotenoids. In the absence of a UL, extra caution may be warranted in consuming levels above recommended intakes. Members of the general population should be advised not to routinely exceed the UL. The UL is not meant to apply to individuals who are treated with the nutrient under medical supervision or to individuals with predisposing conditions that modify their sensitivity to the nutrient.>> >> [2]

<< Traditionally, RDAs have been established for essential nutrients only when data are sufficient to make reliable recommendations. The subcommittee that prepared the ninth edition of the RDAs created the category “Safe and Adequate Intakes” for nutrients with data bases insufficient for developing an RDA, but for which potentially toxic upper levels were known. In this category were three vitamins (vitamin K, biotin, and pantothenic acid), six trace elements (copper, chromium, fluoride, manganese, molybdenum, and selenium), and three electrolytes (sodium, potassium, and chloride). In this, the tenth edition, only minimal requirements are given for the electrolytes, and vitamin K and selenium have been advanced to RDA status.>> [1]

References:

1 National Research Council (US) Subcommittee on the Tenth Edition of the Recommended Dietary Allowances. Recommended Dietary Allowances: 10th Edition. National Academies Press (US), 1989. doi:10.17226/1349

2 ods.od.nih.gov/HealthInformation