Conceptual Basis of Dietary Reference Intakes

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Brief descriptions of each category of reference intakes included in the DRI reports are provided in table 2, along with their typical uses. Initially, only three categories of DRIs for each nutrient were planned: an estimated average requirement (EAR), an RDA, and a tolerable upper intake level (UL). Early on, however, it became apparent that for nutrients with little dose-response data, a reference value would still be needed as a recommended level of intake, established on a different basis than the RDA (fig. 1). This

Table 1. Past uses of recommended dietary allowances

Examples

Basis of planning guides for eating patterns to achieve recommended nutrient intakes e.g., MyPyramid (USDA.gov); Food Guide to Healthy Eating (Health Canada) Basis for planning meals for groups e.g., nursing homes, correctional facilities Reference point for evaluating adequacy of the dietary intake of population subgroups e.g., WIC participants vs. non-participants Component of food and nutrition education programs e.g., Five-A-Day Program (US National Cancer Institute/NIH) Basis for nutrient intake goals for individuals e.g., Daily Values in Nutrition Facts Panel and Dietary Supplement Facts Panel in the US (adopted via regulations established by the US Food and Drug Administration, January 6, 1993, Fed. Register 58 FR 2079-2205) Maximum nutrient levels for fortification/dietary supplement formulation e.g., proposed by country representatives in the revision of the Nutrient Reference Values (NRV) of the Codex Committee on Nutrition and Foods for Special Dietary Uses (www.codexalimentarius.net)

A comprehensive list of uses of dietary reference standards is included in the DRI report on assessing adequacy [7] and on planning [16].

surrogate recommended intake, the adequate intake (AI), was not called an RDA to explicitly show that it was less conclusive and that more judgment was involved in its determination.

Model for Establishing Recommended Intakes

The DRI reports explicitly review the usefulness and limitations of all possible functional endpoints considered in determining adequacy, justifying those selected in establishing the requirement for the nutrient. Typically animal data are not used. While attention is paid to observed intakes in healthy populations, recommended intakes are based on epidemiological observations, human balance study data, depletion/repletion studies, and accepted surrogate markers or biochemical indicators of adequacy, when functional outcomes, such as decreased risk of chronic disease, are not available.

The basis of both the RDA and the use of the EAR in the assessment of adequacy of group intakes requires establishing a dose response (fig. 1). A normal or symmetrical distribution of requirements in a group of individuals with similar age and gender is also needed [7] (fig. 2). The departure from past derivations of most nutrient recommendations (with the exception of protein) is that in order to have an RDA as the recommended intake for an individual, there must be data available to establish an EAR. The EAR is defined as the best estimate of the average (actually, median) requirement for a group of similar individuals. Thus, half of individuals in the subgroup will have their needs met

Table 2. Dietary reference intakes: definitions and uses

Category of dietary reference intake (DRI)

Use to assess dietary plan adequacy1/ diets excess

Recommended intakes for individuals Adequate intake2 (AI) = the recommended average daily intake level based on observed or experimentally determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people that are assumed to be adequate - used when an RDA cannot be determined Recommended dietary allowance (RDA) = the average daily dietary nutrient intake level sufficient to meet the nutrient requirement of nearly all (97-98%) healthy individuals in a particular life stage and gender group

RDA = EAR + 2 X CVEAR Acceptable macronutrient distribution range (AMDR) = the range of intakes for an energy yielding macronutrient associated with reduced risk of chronic disease while providing adequate intakes of essential nutrients; given as a percent of energy intake Reference intakes for individuals Tolerable upper intake level (UL) = the highest average daily nutrient intake level that is likely to pose no risk of adverse health effects to almost all individuals in the general population. As intake increases above the UL, the potential risk of adverse effects may increase

Reference intakes for groups

Estimated average requirement3 (EAR) = the average daily nutrient intake level estimated to meet the requirement of half the healthy individuals in a particular life stage and gender group Estimated energy requirement (EER) = the EER is defined as the average dietary energy intake that is predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity consistent with good health. In children and pregnant and lactating women, the EER is taken to include the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health

Table 2. (continued)

Category of dietary reference intake (DRI) Use to assess dietary plan adequacy1/ diets excess

AMDR: use to estimate the proportion of the population that XX falls outside the range to assess adherence to recommendations and determine concern level about adverse consequences

Evaluating an individual's nutritional status requires data on biochemical, clinical, and anthropometric measures.

2The AI for infants is based on the average intake of the nutrient from human milk for infants at the midpoint of the age range, and the corresponding average composition of the nutrient from analyses of human milk obtained during the same stage of lactation. It should be used as a guide for infants, but actual intake and needs may vary depending on growth rate, etc.

3Requires statistically valid approximation of distribution of usual intakes. Source: The Institute of Medicine, Food and Nutrition Board [7, 11, 16].

Increase

Fig. 1. Conceptual model of dietary reference intakes (DRIs). The DRI model relating nutrient risk of inadequacy and excess includes four DRI categories. The estimated average requirement (EAR) can be used to assess adequacy of population intakes if assumptions are met and is the basis for the recommended dietary allowance (RDA). The adequate intake (AI) lies somewhere in the area depicted in the diagram, in that it is not directly related to the EAR, but may be an observed intake that appears adequate for all in the population. Its relationship to the EAR and thus the RDA is not known as it is only provided when it is not possible to determine an EAR from the available data. The goal in setting the tolerable upper intake level (UL) is that it is as high as possible without increasing the potential for adverse effects due to excess intake. It may actually be less than indicated in the conceptual model, if a great deal of uncertainty exists in the available data used to set the UL.

Increase

Fig. 1. Conceptual model of dietary reference intakes (DRIs). The DRI model relating nutrient risk of inadequacy and excess includes four DRI categories. The estimated average requirement (EAR) can be used to assess adequacy of population intakes if assumptions are met and is the basis for the recommended dietary allowance (RDA). The adequate intake (AI) lies somewhere in the area depicted in the diagram, in that it is not directly related to the EAR, but may be an observed intake that appears adequate for all in the population. Its relationship to the EAR and thus the RDA is not known as it is only provided when it is not possible to determine an EAR from the available data. The goal in setting the tolerable upper intake level (UL) is that it is as high as possible without increasing the potential for adverse effects due to excess intake. It may actually be less than indicated in the conceptual model, if a great deal of uncertainty exists in the available data used to set the UL.

Frequency distribution of individual requirements

Frequency distribution of individual requirements

of adequacy

Increasing Intake -►

Fig. 2. Model for dietary reference values. The theoretical model for establishing the estimated average requirement (EAR) and the recommended dietary allowance (RDA). The model assumes that the distribution of requirements is normal (or symmetrical); that members of the group are of similar age, gender, and size; and that each individual's requirement for the nutrient is independent of that individual's intake (not true for energy). Within the dietary reference intake (DRI) conceptual model, the RDA is derived from the EAR + 2 standard deviations of the EAR; thus the RDA should meet the needs of almost all (97.5%) of the individuals in the population. In this diagram, two members of the group, ab and ay, each from a similar subpopulation and age, have different requirements. While consumption of the EAR would provide an adequate intake of the nutrient for individual ab, it would be inadequate for individual ay, who needs more than the EAR. Note that the EAR could be different (either greater or less) if a different indicator of adequacy was chosen, resulting in a higher or lower RDA.

of adequacy

Increasing Intake -►

Fig. 2. Model for dietary reference values. The theoretical model for establishing the estimated average requirement (EAR) and the recommended dietary allowance (RDA). The model assumes that the distribution of requirements is normal (or symmetrical); that members of the group are of similar age, gender, and size; and that each individual's requirement for the nutrient is independent of that individual's intake (not true for energy). Within the dietary reference intake (DRI) conceptual model, the RDA is derived from the EAR + 2 standard deviations of the EAR; thus the RDA should meet the needs of almost all (97.5%) of the individuals in the population. In this diagram, two members of the group, ab and ay, each from a similar subpopulation and age, have different requirements. While consumption of the EAR would provide an adequate intake of the nutrient for individual ab, it would be inadequate for individual ay, who needs more than the EAR. Note that the EAR could be different (either greater or less) if a different indicator of adequacy was chosen, resulting in a higher or lower RDA.

at the EAR, half will not.1 In order to estimate a recommended intake that will provide almost all healthy individuals in the group with enough to meet their needs, the EAR is increased by two standard deviations2 of requirements to obtain the RDA for the nutrient (fig. 2).

Adequate for What?

A key question is the determination of the criterion or criteria of adequacy. Which functional outcome, surrogate marker for disease, or biochemical or

1The definition of the EAR is that of a median, while use of the word 'average' indicates a mean. The decision to call this DRI 'EAR' was made to follow precedent set by the UK [6], and because if the distribution of requirements is symmetrical, then the mean and median are the same.

2For some nutrients whose requirements are not symmetrically distributed such as iron, other statistical methods are used to establish both an average requirement (EAR) and RDA (amount that would be adequate for 97.5% of individuals in the group).

physiological indicator best reflects adequacy for a nutrient is not a trivial matter. The DRI reports outline the possible candidates, and describe in detail choices made for establishing adequacy, explicitly recognizing that if other criteria were chosen, the average requirement and the related RDA might well be significantly different. It is at this step, choosing the indicator, that geographic and policy-based decisions may direct different choices.

When Dose-Response Data Are Not Available

For well-studied nutrients, response data from individuals fed varying levels of intake are available to develop EARs and, from that RDAs. For many nutrients, however, available human data may only be at markedly deficient levels of intake or at amounts known to be more than adequate, with no information on intermediate levels needed to construct a dose response. For this situation, or where there are conflicting data regarding the appropriate criterion, or where responses are not uniform, an additional reference value, the AI, is established as a recommended intake for individuals [8].

Determination of the Coefficient of Variation of Requirements

While the average requirement is important in order to establish the EAR and thus the RDA, so too is the variation in requirements. In order to determine a reference value that will meet the needs of almost all in the population, the distribution of requirements must be estimated. In most cases where there were adequate data points to establish an EAR, there was not enough information to estimate the standard deviation (SDEAR) of the average requirement for the group; errors in estimating the distribution of requirements were considered sizable enough to not attempt to estimate it [8]. If cases where data on the variability of requirements for a nutrient was insufficient to calculate an SDEAR, then the coefficient of variation (CV) for the EAR is assumed to be 10% [8].

The use of 10% as a default CV comes from variation in measured basal metabolism of similar individuals estimated to be 10% [9], and on the distribution of protein requirements estimated to be 12.5% [9]. Seventeen nutrients have EARs; for six of them, the CV applied differs from the default CV of 10%: 15% for copper, molybdenum, niacin, and carbohydrate [10-12], and 20% for iodine and vitamin A [12]. For iron and protein [11, 12], where nonnormal distributions of requirements were identified, statistical modeling was used to establish the EAR and the RDA.

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