Since the age groups used in recent US RDAs  differed from those used in the Canadian Recommended Nutrient Intakes , a number of experts were initially queried on appropriate age cutoffs. New data on the onset of menarche indicated a need to revise female grouping. The new age categories now take into account the age at which young children enter institutional feeding settings (pre-kindergarten) in Canada and the United States, potentially affecting energy requirements, as well as the change in onset of adolescent growth spurts .
In spite of the significant progress in research related to human nutritional needs, data were not usually available with multiple levels of nutrient intake to directly determine nutrient requirements for each gender and life-stage group, including children. Thus available data from other subgroups were extrapolated to develop EARs, AIs, and ULs for each subgroup (each DRI report discusses extrapolation methodologies employed for nutrients included in the report). Nutrients involved in energy metabolism were extrapolated on the basis of metabolic body weight (B.Wt.3/4). Extrapolation based directly on body weight was used for nutrients involved in bone maintenance and growth. The size of compartment or tissue weight was used as the basis for extrapolation for nutrients primarily distributed in the water space or other specific tissues.
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