Parameters For Dietary Reference Intakes

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Life Stage Groups

The life stage groups described below were choscn by keeping in mind all the nutrients to be reviewed, not only those included in

INTRODUCTION 37

this report. Additional subdivisions within these groups may be added in later reports. If data arc too sparse to distinguish differences in requirements by life stage or gender group, the analysis may be presented for a larger grouping.

Infancy

Infancy covers the period from birth through 12 months of age and is divided into two 6-month intervals. The first 6-month interval was not subdivided further because intake is relatively constant during this time. That is, as infants grow they ingest more food; however, on a body weight basis their intake remains the same. During the second 6 months of life, growth velocity slows, and thus total daily nutrient needs on a body weight basis may be less than those during the first 6 months of life.

For a particular nutrient, the average intake by full-term infants who arc born to healthy, well-nourished mothers and exclusively fed human milk has been adopted as the primary basis for deriving the Adequate Intake (AI) for most nutrients during the first 6 months of life. The value used is thus not an Estimated Average Requirement (EAR); the extent to which intake of human milk may result in exceeding the actual requirements of the infant is not known, and ethics of experimentation preclude testing the levels known to be potentially inadequate. Therefore, the AI is not an EAR in which only half of the group would be expected to have their needs met.

Using the infant fed human milk as a model is in keeping with the basis for estimating nutrient allowances of infants developed in the last Recommended Dietary Allowances (RDA) (NRC, 1989) and Recommended Nutrient Intakes (RNI) (Health Canada, 1990) reports. It also supports the recommendation that exclusive human milk feeding is the preferred method of feeding for normal full-term infants for the first 4 to 6 months of life. This recommendation has also been made by the Canadian Pacdiatric. Society (Health Canada, 1990), the American Acadcmy of Pediatrics (AAP, 1997) and in the Food and Nutrition Board report Nutrition During lactation (IOM, 1991).

In general, for this report special consideration was not given to possible variations in physiological need during the first month after birth or to the variations in intake of nutrients from human milk that result from differences in milk volume and nutrient concentration during early lactation. Specific. Dietary Reference Intakes (DRIs) to meet the needs of formula-fed infants arc not proposed in this report. The previously published RDAs and RNIs for infants

38 DTETARY REFERENCE INTAKES

have led to much misinterpretation of the adequacy of human milk because of a lack of understanding about their derivation for young infants. Although they were based on human milk composition and volume of intake, the previous RDA and RNI values allowed for lower bioavailability of nutrients from nonhuman milk.

Ages 0 through 6 Months. To derive the AT value for infants ages 0 through 6 months, the mean intake of a nutrient was calculated on the basis of the average concentration of the nutrient from 2 through 6 months of lactation with use of consensus values from several reported studies (Atkinson et al., 1995), and an average volume of milk intake of 0.78 L/day as reported from studies of full-term infants by test weighing, a procedure in which the infant is weighed before and after each feeding (Butte et al., 1984; Chandra, 1984; Hofvandcr et al., 1982; Neville et al., 1988). Because there is variation in both of these measures, the computed value represents the mean. It is expected that infants will consume increased volumes of human milk as they grow.

Ages 7 through 12 Months. Except for iron and /.inc., which have relatively high requirements, there is no evidence for markedly different nutrient needs during the period of infants' growth acceleration and gradual weaning to a mixed diet of human milk and solid foods from ages 7 through 12 months. The basis of the AT values derived for this age category was the sum of the specific nutrient provided by 0.6 L/day of human milk, which is the average volume of milk reported from studies in this age category (Hcinig et al., 1993), and that provided by the usual intakes of complementary weaning foods consumed by infants in this age category (Spcckcr et al., 1997). This approach is in keeping with the current recommendations of the Canadian Pacdiatric. Society (Health Canada, 1990), the American Acadcmy of Pediatrics (AAP, 1997), and Nutrition During Lactation (TOM, 1991) for continued feeding of human milk to infants through 9 to 12 months of age with appropriate introduction of solid foods.

One problem encountered in trying to derive intake data in infants was the lack of available data on total nutrient intake from a combination of human milk and solid foods in the second 6 months of life. Most intake survey data do not identify the milk source, but the published values indicate that cow milk and cow milk formula were most likely consumed.

INTRODUCTION 39

Toddlers: Ages 1 through 3 Years

The greater velocity of growth in height during ages 1 through 3 years compared with ages 4 through 5 years provides a biological basis for dividing this period of life. Because children in the United States and Canada from age 4 years onwards begin to enter the public school system, ending this life stage prior to age 4 years seemed appropriate. Data are sparse for indicators of nutrient adequacy on which to derive DRIs for these early years of life. In some cases, DRIs for this age group were derived from data extrapolated from studies of infants or of adults aged 19 years and older.

Early Childhood: Ages 4 through 8 Years

Because major biological changes in velocity of growth and changing endocrine status occur during ages 4 through 8 or 9 years (the latter depending on onset of puberty in each gender), the category of 4 through 8 years is appropriate. For many nutrients, a reasonable amount of data is available on nutrient intake and various criteria for adequacy (such as nutrient balance measured in young children ages 5 through 7 years) that can be used as the basis for the EARs and AIs for this life stage group.

Puberty/Adolescence: Ages 9 through 13 Years and 14 through 18 Years

Because current data support younger ages for pubertal development, it was determined that the adolescent age group should begin at 9 years. The mean age of onset of breast development (Tanner Stage 2) for white females in the United States is 10.0 ± 1.8 (standard deviation) years; this is a physical marker for the beginning of increased estrogen secretion (Herman-Giddens et al., 1997). In African-American females, onset of breast development is earlier (mean 8.9 years ± 1.9). The reason for the observed racial differences in the age at which girls enter puberty is unknown. The onset of the growth spurt in girls begins before the onset of breast development (Tanner, 1990). The age group of 9 through 13 years allows for this early growth spurt of females.

For males, the mean age of initiation of testicular development is 10.5 to 11 years, and their growth spurt begins 2 years later (Tanner, 1990). Thus, to begin the second age category at 14 years and to have different EARs and AIs for females and males for some nutrients at this age seems biologically appropriate. All children continue

40 DTETARY REFERENCE INTAKES

to grow to some extent until as late as age 20 years; therefore, having these two age categories span the period 9 through 18 years of age seems justified.

Young Adulthood and Middle Ages: Ages 19 through 30 Years and 31 through 50 Years

The recognition of the possible value of higher nutrient intakes during early adulthood on achieving optimal genetic, potential for peak bone mass was the reason for dividing adulthood into ages 19 through 30 years and 31 through 50 years. Moreover, mean energy expenditure decreases during this 30-ycar period, and needs for nutrients related to energy metabolism may also decrease. For some nutrients, the DRIs may be the same for the two age groups. However, for other nutrients, especially those related to energy metabolism, EARs (and RDAs) arc likely to differ for these two age groups.

Adulthood and Older Adults: Ages 51 through 70 Years and Over 70 Years

The age period of 51 through 70 years spans active work years for most adults. After age 70 years, people of the same age increasingly display variability in physiological functioning and physical activity. A comparison of people over age 70 years who arc the same chronological age may demonstrate as much as a 15- to 20-year age-related difference in level of reserve capacity and functioning. This is demonstrated by age-related declines in nutrient absorption and renal function. Bccause of the high variability in functional capacity of older adults, the EARs and AIs for this age group may reflect a greater variability in requirements for the older age categories. This variability may be most applicable to nutrients for which requirements arc related to energy expenditure.

Pregnancy and Lactation

Recommendations for pregnancy and lactation may be subdivided because of the many physiological changes and changes in nutrient needs that occur during these life stages. In setting EARs and AIs for these life stages, however, consideration is given to adaptations to increased nutrient demand, such as increased absorption and greater conservation of many nutrients. Moreover, nutrients may undergo net losses due to physiological mechanisms regardless of the nutrient intake. Thus, for some nutrients, there may not be a

INTRODUCTION 41

basis for EAR values that arc different during these life stages than they arc for other women of comparable age.

Reference Weights and Heights

The reference weights and heights selected for children and adults arc shown in Table 1-1. The values arc based on anthropometric data collcctcd from 1988-1994 as part of the Third National Health and Nutrition Examination Survey (NHANES III) in the United States. When extrapolation to a different age group was conductcd, these reference weights were used, cxccpt for iron which used weights with known coefficients of variation that were required for factorial modeling.

Using NHANES III data, the median heights for the life stage and gender groups through age 30 years were identified, and the median weights for these heights were based on reported median Body Mass Index (BMI) for the same individuals. Sincc there is no evidence that weight should changc as adults age if activity is maintained, the reference weights for adults ages 19 through 30 years arc applied to all adult age groups.

The most recent nationally representative data available for Canadians (from the 1970-1972 Nutrition Canada Survey [Dcmirjian, 1980]) were reviewed. In general, median heights of children from

TABLE 1-1 Reference Heights and Weights for Children and Adults in the United States"

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