When the AI is based on observed mean intakes of population groups, it is likely to always exceed the average requirement that would have been experimentally determined.

In the Dietary Reference Intake (DRI) nutrient reports (IOM, 1997, 1998b, 2000), the AI has been estimated in a number of different ways (see Appendix F). Because of this, the exact meanings and interpretations differ. In some cases, the AI was based on the observed mean intakes of groups or subpopulations that are maintaining health and nutritional status consistent with an apparent low incidence of inadequacy. In other cases, the AI was derived from the lowest level of intake at which all subjects in an experimental study met the criterion of adequacy; this is different from (and generally lower than) the group mean intake that is consistent with all subjects meeting the criterion of adequacy. The AI was sometimes estimated as an approximation of intake in a group with knowledge of actual requirements of only a few individuals.

The methods of derivation of the AI may differ substantially among nutrients and among life stage groups for the same nutrients; it follows that interpretation and appropriate use of the AI must differ also. In Table 5-1, AIs that represent estimates of desirable group mean intakes are identified. Note that the indicators of adequacy are not always indicators of a classical nutrient deficiency state; in some cases they also include factors that may be directed to decreasing risk of chronic, degenerative diseases. Following, and shown in detail in Appendix F, are some examples of nutrients with an AI and the basis for their derivation:

• Calcium: For infants the AI is a direct estimate of a suitable intake based on average content of human milk for an assumed volume of intake. For adolescents and adults the AI is an approximation of the calcium intake that would be sufficient to maintain desirable rates of calcium retention, as determined from balance studies, factorial estimates of requirements, and limited information on bone mineral content and bone mineral density (IOM, 1997).

• Vitamin D: The AI is a value that appears to be needed to maintain —in a defined group with limited, but uncertain, sun exposure and stores—serum 25-hydroxyvitamin D above the concentration below which vitamin D deficiency rickets or osteomalacia occurs. This concentration is rounded to the nearest 50 IU and then doubled as a safety factor to cover the needs of all people regardless of sun exposure.

• Fluoride: For infants the AI is based on reported group mean intakes; for children and adults the AI is based on factorial estimates of suitable group mean intakes. The criterion of adequacy was an intake that would be associated with low occurrence of dental caries.