OVERVIEW

In the past, Recommended Dietary Allowances (RDAs) in the U.S. and Recommended Nutrient Intakes (RNIs) in Canada were the only values available to health professionals for planning and assessing diets of individuals and groups and for making judgments about excessive intake. However, the RDAs and RNIs were not ideally suited for many of these purposes (IOM, 1994). The Dietary Reference Intakes (DRIs) developed in this report—Estimated Average Requirements (EARs), RDAs, Adequate Intakes (AIs), and Tolerable Upper Intake Levels (ULs)—are a more complete set of reference values. Each type of DRI has specific uses. The most widespread uses of DRIs—diet assessment and planning—are described in this chapter. EARs, RDAs, AIs, and ULs refer to average daily intake over 1 or more weeks.

Three of the DRIs—the EAR, RDA, and AI—were set with reference to a specific criterion of adequacy. The criterion of adequacy may be the same for each gender and life stage group, but sometimes it is not. For example, the criterion of adequate calcium intake is desirable calcium retention for most age groups, but calcium balance is the criterion for men and women ages 31 through 50 years. Desirable calcium retention, in itself, would not be used to determine adequacy. Instead, it is the presumed relationship between desirable calcium retention and reduced risk of fracture in later life that provides a basis for selection of this indicator. Reduced fracture risk is the functional outcome chosen as the hallmark of adequacy for calcium, and its indicator is desirable calcium retention to the extent that retention can be affected by dietary intake. Each nutrient chapter (Chapters 4 through 8) identifies the primary indicator or criterion that defines adequacy for the specific life stage and gender group. (See also Tables S-1 to S-5.)

USING RECOMMENDED DIETARY ALLOWANCES

USING ADEQUATE INTAKES

An AI is based on observed or experimentally determined approximations of the average nutrient intake, by a defined population or subgroup, that appears to be sufficient to sustain a defined nutritional state in the specified population. It is emphasized that, in contrast to the EAR, which is an estimate of the requirement that applies to individuals, the AI is usually derived from mean intakes of groups—the group rather than the individual is the unit of observation. The AI is therefore higher than the EAR would be, if it could be determined, since by definition, the EAR is the intake that meets the nutrient need of only 50 percent of the individuals in a group. Because of uncertainties about the relationship of the AI to the actual average requirement, the AI provides an imprecise basis for the assessment of nutrient intakes of population groups. Thus, the applications of the AI must be quite different from those of the EAR. However, healthy individuals with an intake at or above the AI are assumed to have a low risk of intake inadequate for a defined state of nutrition.

USING TOLERABLE UPPER INTAKE LEVELS

The 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. As intake increases above the UL, the risk of adverse effects increases. In most cases it applies to usual intakes from all sources, but in the case of magnesium, it does not apply to intake from food or local water supplies.

Similar to the situation for nutrient requirements, the intake at which a given individual will develop adverse effects as a result of taking high amounts of a nutrient from food and/or nonfood sources cannot be known with certainty. If the individual's intake is below the UL, there is good assurance that the intake will not cause adverse effects, given current knowledge. At intakes above the UL, the risk of adverse effects increases. Although there is no established benefit for healthy individuals associated with consumption of nutrients or food components above the RDA or AI, there is little concern of an increased risk of harmful effects of consumption up to the UL.

Ordinarily, the UL refers to intake from food, fortified food, water, and supplements. Nutrients are often available from a variety of food and nonfood sources. For fluoride, for example, intake from all sources must be considered, including water and dental products. Combined with other sources of fluoride, the over-use of fluoride-containing dental products may place many young children at risk of dental fluorosis.

To avoid exceeding the adult UL for calcium of 2.5 g (62.5 mmol)/day, intake from food, fortified food, and supplements must all be considered. The UL for calcium was determined based in part on studies in which hypercalciuria was related to both dietary and supplemental intakes of calcium. For phosphorus, intakes from both food and supplements are to be considered when comparing intakes with the UL of ∼4.0 g (130 mmol)/day for adults. However, for individuals requiring very high energy intakes, phosphorus intake from diet alone may exceed the UL without adverse effects.

To avoid exceeding the UL for vitamin D of 50 μg (2,000 IU)/day, considering intake from all sources is important, but ordinarily only intake from food, fortified food, and nutrient supplements provides notable amounts. (Apart from fatty fish and liver, high concentrations of vitamin D do not occur naturally in commonly eaten foods in the United States and Canada.) Individuals who have high intakes of vitamin D-fortified foods (such as milk and margarine) and of vitamin D supplements (in multivitamin preparations, alone, or in fish oil) should keep the combined average daily intake below the UL of 50 μg (2,000 IU)/day to minimize the risk of developing hypercalcemia.

For many nutrients, such as magnesium, adverse effects associated with high levels of intake from food sources have not been reported. The UL of 350 mg (14.6 mmol)/day was set for magnesium from nonfood, over-the-counter pharmacologic products. This is the intake at which an individual might first experience an adverse effect (diarrhea) from ingestion of a nonfood source of magnesium for at least 1 week. Since the UL for magnesium applies only to nonfood sources, individuals should not be concerned with trying to avoid excessive amounts of magnesium from that occurring naturally in food or water.

USING ESTIMATED AVERAGE REQUIREMENTS

The EAR is particularly useful for evaluating the possible adequacy of nutrient intakes of population groups and for planning intakes of groups. In 1986, the National Research Council (NRC) proposed an approach to calculating the prevalence of inadequate nutrient intakes among a population group, using the mean (or EAR) and SD of the requirement (NRC, 1986). Beaton (1994) has noted that the prevalence of inadequate nutrient intakes can be estimated by the proportion of the population with intakes below the EAR, assuming that the variance of the intakes is greater than the variance of the EAR (which is generally the case). The EAR may also be used in setting a recommended mean intake for a population group, such that only 2 to 3 percent of the population falls below the EAR. These methods are described briefly below. A more detailed explanation of the methodology is given in Beaton (1994) and by the World Health Organization, Food and Agriculture Organization of the United Nations, and International Atomic Energy Agency (WHO/FAO/IAEA) Expert Consultation in Trace Elements in Human Nutrition and Health (WHO, 1996). It is anticipated that methods of using the EAR for these purposes will be addressed in a subsequent Food and Nutrition Board report.

Assessing the Adequacy of Nutrient Intake of Groups

An estimate of the prevalence of inadequate intakes of a nutrient by a specific gender or life stage group can be obtained by determining the percentage of the individuals in the group whose usual intakes are less than the EAR (Beaton, 1994) (see Figure 9-1). The estimate is most accurate if the requirements are symmetrically distributed and the SD of intakes is at least twice as large as the SD of requirements. It is also important that the intake data reflect usual intakes; methods have been developed to remove the day-to-day variance in the distributions of intake data (Nusser et al., 1996). This adjustment narrows the distribution and thus gives a better estimate of the proportion of the group with intakes below the EAR. This concept is illustrated in Figure 9-2. Estimates of the prevalence of inadequate intakes also can be affected by biases in the intake data (such as underreporting). (See Chapter 2 for a discussion of the many potential sources of error in self-reported dietary data.)

FIGURE 9-1

Prevalence of inadequate intakes in a population group. Adapted from Beaton (1994).

FIGURE 9-2

Effect of multiple days of observation on the apparent distribution of nutrient intake and how it changes the prevalence of inadequacy as estimated by comparison to EAR. Adapted from Hegsted (1972).

Table 9-1 illustrates the importance of adjusting distributions before calculating intake percentiles for groups. The means and seven percentiles of intakes of phosphorus from the 1994 CSFII are shown for teenage boys and for young women, with and without adjustment for usual intake using the Nusser et al. procedure (1996). If unadjusted 1-day intake data are used to estimate the prevalence of inadequate phosphorus intakes for young women, the actual prevalence of usual intakes below an EAR of 580 mg (18.7 mmol)/day will be overestimated. As shown in Table 9-1, between 10 and 25 percent of 1-day intakes fall below 580 mg (18.7 mmol)/day, while less than 5 percent of the adjusted intakes are below the EAR. For teenage boys, less than 10 percent would have inadequate intakes using the adjusted distribution and an EAR of 1,055 mg (34 mmol)/day, while almost 25 percent would have inadequate intakes if only the unadjusted distribution were examined.

TABLE 9-1

Adjusted and Unadjusted Phosphorus Intakes (mg/day) of Young Women and Adolescent Boys in the United States.

The adjustment procedure is also crucial when examining the percentage of the population that is above the UL. Using an unadjusted distribution for adolescent boys, 5 percent of the population would have phosphorus intakes above 3,350 mg (108.1 mmol)/day, a level that is approaching the UL of 4,000 mg (129 mmol)/day. This might be considered unacceptably high unless the adjusted distribution were also examined: the ninety-fifth percentile of usual intakes is just 2,413 mg (77.8 mmol)/day implying that intakes seldom exceed the UL.

When assessing the intake of populations, a number of other questions need to be considered:

• To what extent does the nutrient requirement affect intake? Such a relationship, described by WHO/FAO/IAEA (WHO, 1996), would limit the validity of the probability approach.
• What kinds of adjustments can be made, if any, for biases in the food intake data?
• What factors should be considered in interpreting the findings in different populations?
• What is the allowable level of inadequate intake in a population before concern is raised?

OTHER USES OF DIETARY REFERENCE INTAKES

For many years, the U.S. Recommended Dietary Allowances and the Canadian Recommended Nutrient Intakes have been used by many national and federal agencies for a variety of purposes. For example, they have been considered in setting regulations for feeding programs, setting standards for feeding in group facilities (nursing homes, school cafeterias, correctional facilities), developing recommended intakes for the military, and setting reference values for food labels. They have been used for comparative purposes in many computer programs for nutrient analysis and by dietitians in modifying diets for patients. Guidance for using DRIs for these and many other purposes is beyond the scope of this report, but should be addressed in future reports.

ADDRESSING DISCREPANCIES BETWEEN USUAL INTAKE AND THE AI

Chapter 4 reveals major discrepancies between estimates of usual intakes of calcium and the AI for calcium for some of the life stage groups. For example, mean calcium intake from foods for U.S. females aged 9 years and older is about 650 mg (16.3 mmol)/day, but the AI for calcium ranges from 1,000 to 1,300 mg (25 to 32.5 mmol)/day, depending on the age group. However, this discrepancy does not necessarily mean that dietary intakes are generally inadequate because the relationship between the AI and the distribution of requirements for the nutrient is not known. Nevertheless, for individuals who wish to increase their calcium consumption, there are several possible strategies. These include increasing intake of foods high in calcium, such as low- or nonfat milk products, and increasing consumption of foods fortified with calcium, such as calcium-fortified orange juice or breakfast cereals. For those individuals at high risk, use of calcium supplements may be desirable in order to meet the AI. Identifying the most appropriate strategies to improve nutrient intake should be the focus of a research agenda.

SUMMARY

The correct reference value must be used for its intended purpose, which usually involves either planning for an adequate intake or the assessment of adequacy of intake. It is anticipated that future publications will address the interpretation and appropriate uses of DRIs in more detail in order to assist both the health professional and those interested in nutrition policy and analysis.