For infants, evaluation of evidence to establish the DRIs consistently revealed a paucity of appropriate studies on which to base an Estimated Average Requirement (EAR) or UL. A Recommended Dietary Allowance (RDA) could not be calculated if a value for the EAR was not established, in which case the recommended intake was based on an Adequate Intake (AI). The nutrient recommendations for infants from birth through 6 months of age for all nutrients except for energy and vitamin D were set as an AI, a value that represents ''the mean intake of a nutrient calculated based on the average concentration of the nutrient in human milk from 2 to 6 months of lactation using consensus values from several reported studies,'' multiplied by an average volume (0.780 l/day) of human milk. The predicted daily volume of breast milk ingested by an infant was based on observational studies that used test weighing of full-term infants. For infants aged 7-12 months, the AI for many nutrients was based on mean observed nutrient intake from human milk in the second 6 months (0.6 l/day) in addition to published values for intake of nutrients from complementary or weaning foods if such data were available.
Assuming an adequate intake of milk for all infants was considered a valid approach since there is evidence that the volume of milk produced during the early months of lactation is very consistent among women irrespective of racial, cultural, or nutritional diversity or variations in body size. The volume of milk produced increases with greater size of the infant, when twins are nursing, and in response to greater frequency of nursing.
Using consensus values for the nutrient content of human milk was deemed appropriate since for many nutrients—energy, macronutrients, and macrominerals—maternal diet does not influence the nutrient content of the milk. The exceptions to this include the fatty acid profile, selenium, iodine, and the water-soluble vitamins. Although human milk is known to contain many nonnutrient bioactive factors, such as immune and growth factors and live enzymes, these were not considered to impact on nutrient needs per se.
For nutrients for which intake data were not available for ages older than 6 months, the EAR or AI was derived by extrapolation from estimates of intakes from older children or adults using the formula with adjustments for metabolic body size, growth, and variability:
EARinfant or child = EARchild or adult x(F)
where F = (weight^« or child/weighted or adult) (1 + growth factor); or occasionally by extrapolating up from intake of breast-fed infants with similar adjustments using the formula
where F = (weight6-n months/weight0 -5 months)
For a few nutrients, such as iron and zinc, sufficient metabolic data were available to derive an EAR using modeling or factorial methods.
Because no specific AIs were derived for formula-fed infants, it is incumbent upon industry to design formulas with a quantity and quality of nutrients that when fed will provide an amount of nutrients that meets the RDA or AI. An approach to establishing the amount of nutrient needed by formula-fed infants is addressed under the section titled ''Special Considerations'' in each DRI report.
When possible, a DRI called the tolerable upper level was defined as ''the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals in the general population'' (Institute of Medicine, 2002). Chronic consumption of nutrients above the UL increases the potential risk of adverse effects, the latter varying by nutrient. For infants, data were only available to reliably estimate ULs for vitamins A and D and the minerals fluoride, selenium, zinc, and iron. Although adequate data were not available to define a UL for infants for other nutrients, it is important to note that intake for nutrients for which a UL does not exist should only be consumed from food or formula and not from supplements. Also notable is that the UL for iron for infants is only relevant to intake from supplements and not foods.
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