Recommended Dietary Allowances (RDAs) have been prepared by the Food and Nutrition Board since 1941. The first edition was published in 1943 to provide ''standards to serve as a goal for good nutrition." Because RDAs are intended to reflect the best scientific judgment on nutrient allowances for the maintenance of good health and to serve as the basis for evaluating the adequacy of diets of groups of people, the initial publication has been revised periodically to incorporate new scientific knowledge and interpretations. This is the tenth edition.
RDAs are defined in Chapter 2 as 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. This definition has remained essentially unchanged since 1974 (eighth edition). Individuals with special nutritional needs are not covered by the RDAs.
In principle, RDAs are based on various kinds of evidence: (1) studies of subjects maintained on diets containing low or deficient levels of a nutrient, followed by correction of the deficit with measured amounts of the nutrient; (2) nutrient balance studies that measure nutrient status in relation to intake; (3) biochemical measurements of tissue saturation or adequacy of molecular function in relation to nutrient intake; (4) nutrient intakes of fully breastfed infants and of apparently healthy people from their food supply; (5) epidemiological observations of nutrient status in populations in relation to intake; and (6) in some cases, extrapolation of data from animal experiments. In practice, there are only limited data on which estimates of nutrient requirements can be based.
In preparing this tenth edition of the RDAs, the subcommittee operated from the general assumption that modifications to the RDAs are justified mainly on the basis of substantive new information or where there were inconsistencies in the way evidence was evaluated in previous editions. The subcommittee reviewed the scientific literature published since the ninth edition as well as older studies on which the previous RDAs were based in cases where it was deemed important to reexamine the original data. For most nutrients, RDAs were established by first estimating the average physiological requirement for an absorbed nutrient. The subcommittee exercised judgment in adjusting this value by factors to compensate for incomplete utilization and to encompass the variation both in the requirements among individuals and in the bioavailability among the food sources of the nutrient. Therefore, the RDAs provide a safety factor appropriate to each nutrient and exceed the actual requirements of most individuals. The RDA for energy, however, reflects the mean population requirement for each group, since consumption of energy at a level intended to cover the variation in energy needs among individuals could lead to obesity in most persons.
MAJOR REVISIONS IN THE TENTH EDITION AND THEIR BASES
The Summary Table in the back of this book summarizes the RDAs established by the subcommittee. It contains several changes that reflect advances in scientific knowledge in the past 9 years or new interpretations of data by the subcommittee. Changes include the following:
Age Groupings Because peak bone mass is probably not attained before age 25 years, the age class of 19 to 22 years has been extended through age 24 for both sexes.
Reference individuals Heights and weights of reference adults in each age-sex class are the actual medians for the U.S. population of the designated age, as reported in the second National Health and Nutrition Examination Survey (NHANES II). In the previous edition, reference heights and weights were set at an arbitrary ideal. Therefore, differences from the ninth edition in allowances for nutrients based on body weight may simply reflect the difference in reference body weights.
Nutrients RDAs for women during pregnancy and lactation are tabulated as absolute figures rather than as additions to the basic
allowances. This is a convenience and reflects the subcomnittee's judgment as to the precision with which the additional costs of reproduction and lactation are known. RDAs during lactation are now provided for the first and second 6-month periods to reflect the differences in the amount of milk produced (750 ml and 600 ml, respectively). In the ninth edition, a single allowance was provided throughout lactation based on secretion of 850 ml of milk. RDAs for infants who are not breastfed are based primarily on the amounts of nutrients provided by 750 ml (rather than 850 ml) of human milk plus an additional 25% (a result of adding 2 standard deviations) to allow for variance. In the ninth edition, allowances during the first 6 months of life did not include a consistent increment for individual variability.
Recommended Dietary Allowances
RDAs for some nutrients remain unchanged or were revised only slightly from the ninth edition. The following are major changes in this edition:
Energy Because reference weights are now actual medians rather than arbitrary ideals, the allowances are not directly comparable with values in the previous edition. Recommended allowances for adults were calculated by using empirically derived equations recently developed by the Food and Agriculture Organization for estimating resting energy expenditure and then multiplying the results by an activity factor representing light-to-moderate activity. Energy allowances range from 2,300 to 2,900 kcal/day for adult men and 1,900 to 2,200 kcal/day for adult women. Energy allowances in this edition and the previous one are similar, despite the different methods used to derive them.
Protein Protein allowances for adults are based on nitrogen balance studies, as recently recommended by the Food and Agriculture Organization, rather than on the factorial method used in the past. Despite this difference in the derivation of RDAs, the allowance for adult men and women remains at 0.8 g/kg of body weight per day. The increment estimated for pregnancy is reduced from 30 to 10 g/ day; this revision is more heavily influenced by theory of nitrogen gain and efficiency with which dietary protein is converted to fetal, placental, and maternal tissues than by new evidence.
Vitamin K RDAs for vitamin K are established for the first time in this edition; they are based on recently published work. The RDA
for adults and children is set at approximately 1 µg/kg of body weight. There is no recommended increment during pregnancy and lactation, because the effects of pregnancy on vitamin K requirements are unknown and lactation imposes little additional need for this nutrient.
Vitamin C Allowances for vitamin C are largely unchanged from the ninth edition; for example, the RDA for adults of both sexes remains at 60 mg/day. An increment of 10 mg/day has been added for pregnant women to offset losses from the mother's body pool to the fetus; this is half the increment recommended in the previous edition. The subcommittee recommends that regular cigarette smokers ingest at least 100 mg of vitamin C per day, since smoking seems to increase metabolic turnover of the vitamin, leading to lower concentrations in the blood.
Vitamin B6 An RDA of 0.016 mg of vitamin B6 per gram of protein appears to ensure acceptable values for most indices of nutritional status in adults of both sexes; in the ninth edition, the RDA was 0.020 mg of vitamin B6 per gram of protein. The RDA is established in relation to the upper boundary of acceptable levels of protein intake, i.e., twice the RDA for protein. The resulting vitamin B6 allowances of 2.0 and 1.6 mg/day for adult men and women, respectively, are lower than those in the previous edition.
Folate Folate allowances in this edition are much lower (often by 50% or more) than those in the ninth edition for all the age-sex groups. The basis for lowering the RDA is the recognition that diets containing about half the previous RDA maintain adequate folate status and liver stores. The folate allowance of approximately 3 µg/ kg body weight for adults and adolescents translates to 200 µg/day for the adult male and 180 µg/day for the adult femalean amount typically consumed in the United States and Canada by adults who show no evidence of poor folate status. During pregnancy, the RDA for folate is 400 µg/dayhalf the RDA in the ninth edition. The subcommittee considers this amount sufficient to build or maintain maternal folate stores and to support rapidly growing tissue.
Vitamin B12 Vitamin B12 allowances in this edition are one-third to one-half lower than those in the ninth edition for all the age-sex groups. For example, the RDA for adults and adolescents of both sexes is now 2 rather than 3 µg/day and for children 1 to 10 years ranges from 0.7 to 1.4 rather than 2 to 3 µg/day. Reductions are
based on recent data suggesting that the new allowances adequately sustain metabolic function and allow for biological variation, the maintenance of normal serum concentrations, and the build up or maintenance of substantial body stores; the latter is especially desirable in view of the increased prevalence of achlorhydria (which diminishes vitamin B12 absorption) and pernicious anemia beyond age 60.
Calcium In the ninth edition, the RDA for calcium for all adolescents was set at 1,200 mg/day to age 18, the approximate age at which longitudinal bone growth ceases. However, because peak bone mass is probably not attained before age 25, the subcommittee has extended this allowance through age 24 to promote full mineral deposition. For older ages, the allowance of 800 mg in the ninth edition is maintained. The subcommittee believes the most promising nutritional approach to reducing the risk of osteoporosis in later life is to ensure a calcium intake that allows the development of each individual's genetically programmed peak bone mass. It urges that special attention be paid to calcium intakes throughout childhood to age 25 years. The subcommittee emphasizes that the RDAs for calcium do not address the possible increased needs of persons who may have osteoporosis and should receive medical attention. RDAs for phosphorus parallel those for calcium except in infancy. In addition, the allowance for vitamin D, which promotes calcium absorption, is maintained at 10 µg/day throughout childhood to age 25 years.
Magnesium Increments of magnesium during pregnancy and lactation are far lower than in previous editions (reduced to +20 from + 150 mg/day during pregnancy and to +75 and +60 from +150 mg/day during lactation); these amounts should be sufficient to meet the needs of the fetus and maternal tissue growth and to allow for individual variation. The allowance for children of both sexes between 1 and 15 years of age is 6.0 mg/kg, an amount above the levels that were found to be sufficient to support a positive magnesium balance in adolescent boys and girls. This allowance translates into RDAs for children that are considerably lower than the RDAs in the ninth edition, especially for preadolescent children.
Iron In setting RDAs for iron, it was the subcommittee's judgment that a dietary intake that achieves a target level of 300 mg of iron stores meets the nutritional needs of all healthy people, since, over several months, this level of stores provides for the iron needs of a person consuming a diet nearly devoid of iron. Using population-
based data on iron intakes and status, turnover data, estimates of variability of iron losses among individuals, and distribution analysis, the subcommittee concluded that an RDA of 15 mg/day should meet the needs of essentially all healthy adolescent and adult women following usual dietary patterns and should provide a sufficient margin of safety. This allowance is a reduction from the 18 mg/day recommended in the ninth edition. The allowance for adult men and postmenopausal women remains at 10 mg/day.
A daily iron increment of 15 mg/day averaged over the entire pregnancy should be sufficient to meet maternal and fetal needs. Daily iron supplements are usually recommended, since the total need cannot be met by the iron content of habitual U.S. diets or by the iron stores of at least some women. No additional allowance of iron is recommended during lactation, since losses of iron in milk are less than menstrual loss, which is often absent during lactation. In contrast, the ninth edition recommended the continued use of the iron supplements prescribed during pregnancy for 2 to 3 months after birth to replenish iron stores.
The RDAs for iron are adequate for essentially all healthy people who daily consume diets containing 30 to 90 g of meat, poultry, or fish (containing highly absorbable heme iron) or foods containing 25 to 75 mg of ascorbate after preparation (to improve absorption of nonheme iron). People who eat little or no animal protein and whose diets are low in ascorbate may require higher amounts of food iron or vitamin C.
Zinc In the ninth edition, the RDA for adults of both sexes was set at 15 mg/day. In the present edition, the allowance remains at 15 mg/day for adult men, but is reduced to 12 mg/day for adult women on the basis of their lower body weight.
Selenium RDAs for selenium, established for the first time in this edition, are based on recent studies of Chinese men. The ninth edition provided a safe and adequate range for selenium intake, which for adults was 50 to 200 µg/day. In the present edition, the RDA for selenium in adults is set at 70 µg/day for men and 55 µg/ day for women. RDAs for infants, children, and adolescents are extrapolated from adult values on the basis of body weight, and a factor is added for growth.
Estimated Safe and Adequate Daily Dietary Intakes
The ninth edition established a category of safe and adequate intakes for essential nutrients when data were sufficient to estimate a
range of requirements, but insufficient for developing an RDA. This category, together with the caution that upper levels in the safe and adequate range should not be habitually exceeded because the toxic level for many trace elements may be only several times usual intakes, is maintained in the present edition. The table that includes this group of nutrients is similar to the corresponding table in the ninth edition, but incorporates several changes reflecting advances in scientific knowledge or new interpretations of data by the subcommittee.
Since vitamin K and selenium have been advanced to RDA status, they have been moved to the main Summary Table. Safe and adequate ranges are no longer provided for sodium, potassium, and chloride, since they are difficult to justify. Estimated minimum requirements for these electrolytes are provided for healthy persons at various ages (see Chapter 11). Minimum sodium requirements are estimated to range from 120 mg in the first 6 months of life to 500 mg/day in adulthood, and to increase during pregnancy and lactation; there is no known advantage in consuming large amounts of sodium and clear disadvantages for those susceptible to hypertension. Potassium requirements are estimated to range from 500 mg/day in early infancy to 2,000 mg/day in adulthood. Dietary recommendations for increased intake of fruits and vegetables, made in the recent Food and Nutrition Board report entitled Diet and Health, would yield a potassium intake of approximately 3,500 mg/day for adultsa level that could reduce the prevalence of hypertension and stroke.
Biotin In this edition, the estimated ranges of safe and adequate intakes for biotin are much lower for all age-sex groups than in the ninth edition (e.g., 30 to 100 µg/day for adolescents and adults compared to 100 to 200 µg/day). Improved analytical methods for biotin have reduced the estimates of daily intakes that are compatible with good health.
Copper Recent data on whole body surface losses of copper, along with data on urinary and fecal losses, indicate that a total dietary copper intake of approximately 1.6 mg/day is required to maintain balance in adult men. Therefore, 1.5 to 3 mg/day is recommended as a safe and adequate range of intake for adults and adolescentsa wider range than the 2 to 3 mg/day recommended in the ninth edition.
Manganese In this edition, a manganese intake of 2 to 5 mg/ day is recommended for adolescents and adults of both sexes. This is a wider range of safe and adequate intakes compared to the range
of 2.5 to 5 mg/day in the ninth edition. Since current dietary intakes of manganese appear to satisfy requirements, a recent survey showing the mean manganese intake of 2.7 and 2.2 mg for adult men and women, respectively, provides justification for the change.
Molybdenum The estimated safe and adequate range of molybdenum intake for adults and adolescents of 75 to 250 µg/day is based on average reported intakes; the ranges for other age groups are derived from extrapolation on the basis of body weight. These provisional intakes are half the amounts recommended in the ninth edition, which were based on human balance studies; the present subcommittee believes these studies are inappropriate to use in estimating requirements for trace elements.
USES AND IMPLICATIONS OF THE RDAs
Over the years, RDAs have become widely known and applied. They are typically used 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, and for developing new products in industry. The seventh edition of the RDAs (published in 1968) became the basis for establishing guidelines for the nutritional labeling of foods (known as the U.S. Recommended Daily Allowances, or USRDAs). Because of the wide use of the RDAs, it is important to understand their appropriate applications and limitations. These are discussed in Chapter 2. Three points are of' particular importance:
· The recommended allowances for nutrients are amounts intended to be consumed as part of a normal diet. If the RDAs are met through diets composed of a variety of foods derived from diverse food groups rather than by supplementation or fortification, such diets will likely be adequate in all other nutrients for which RDAs cannot currently be established.
· RDAs are neither minimal requirements nor necessarily optimal levels of intake. It is not possible at this time to establish optima. Rather, RDAs are safe and adequate levels (incorporating margins of safety intended to be sufficiently generous to encompass the presumed variability in requirement among people) reflecting the state of knowledge concerning a nutrient, its bioavailability, and variations among the U.S. population.
· Although RDAs are most appropriately applied to groups, a comparison of individual intakes, averaged over a sufficient length of time, to the RDA allows an estimate to be made about the probable risk of deficiency for that individual.
The subcommittee reemphasizes that RDAs can typically be met or closely approximated by diets that are based on the consumption of a variety of foods from diverse food groups that contain adequate energy. Such diets are entirely consistent with the type of dietary patterns advocated in the Food and Nutrition Board's report entitled Diet and Health to promote health and reduce risks of developing major chronic diseases. Together, the RDAs and the Diet and Health recommendations should be considered the appropriate basis for diet planning.