considered in developing the EAR or AI were assessed. Chapter 4, Chapter 5, Chapter 6, Chapter 7 through Chapter 8 describe the rationale for the inclusion or exclusion of evidence. Among the factors considered were the methods used to determine intake from food and supplements; methods used for measuring the indicator of adequacy; relationships among the indicator, dietary intake, and functional or physiologic outcome; any allowances for adaptation to changes in intake; and other aspects of the study design.
When applicable, the strength, consistency, and preponderance of the data and the degree of concordance among epidemiological, clinical, and laboratory evidence determined the strengths of the indicators that were used as the basis for EARs and AIs in each stage of the lifespan. As was adopted by the Surgeon General's Report on Nutrition and Health (DHHS, 1988) and the Food and Nutrition Board's Diet and Health (NRC, 1989b), the assessment of the strength of the data supporting a nutrient's role in decreasing risk of chronic debilitating disease or developmental abnormalities was based on the following criteria (Hill, 1971):
strength of association, usually expressed as relative risk;
temporally correct association, with exposure preceding the onset of disease;
consistency of association;
specificity of association; and
The greatest weight was given to studies, if available, that were directly related to a determination of nutrient needs and that used an appropriate experimental design and outcome measure. Less weight was given to studies in which observed levels of nutrient intake were related to a specific criterion or criteria of nutriture. Neither average dietary intake data nor indicator of adequacy data alone provided a sufficient basis for deriving an EAR, although this approach, of necessity, was applied to the development of AIs for infants and for fluoride. If necessary, a factorial model could be used as a basis for estimating the physiological requirement, which could then be used to estimate the dietary requirement for a nutrient. This process was used to estimate the phosphorus requirements for some life stage groups.
In developing estimates of average requirements for minerals such as calcium, phosphorus, and magnesium, the available literature until the late 1980s consisted primarily of balance studies in which