requirements will be based on a factorial approach in infants, children, and adolescents and on serum Pi in adults.

There is a clear and absolute requirement for phosphorus during growth. At each growth stage, average net daily additions of bone and soft tissue mass can be approximated. Thus, the absorbed phosphorus intake needed to support such tissue accumulation can be readily estimated (see below for each relevant physiological state). In the mature adult, the requirement can be defined instead simply as the intake needed to maintain the plasma Pi within the normal range. As with all continuous clinical variables, there is a gray zone between the empirical normal range and values associated with evident deficiency disease. Thus, the lower end of the established normal range for serum Pi is 0.8 to 0.9 mmol/liter (2.5 to 2.8 mg/dl). Clear evidence of bony or soft tissue dysfunction are not common until serum Pi levels drop below 0.3 to 0.5 mmol/liter (0.9 to 1.6 mg/dl).

Although it is not likely that all levels of Pi within the normal range are equally salubrious for cell functioning, insufficient information exists to allow selection of any one value within the population normal range as superior to any other. The fact that growth and epiphyseal cartilage maturation in children are abnormal at even adult normal levels of Pi supports the assumption that subnormal Pi values are not adequate to sustain optimal tissue function. Therefore, in what follows, the requirement will be based on the intake associated with maintenance of serum Pi at the bottom end of the normal range. A limitation of this approach lies in the fact that available data on Pi apply mostly to the fasting state, whereas it is the integrated, 24-hour Pi that is most closely related to absorbed phosphorus intake and that constitutes the actual exposure that the tissues experience. Moreover, fasting serum Pi is only weakly correlated with current phosphorus intake (for example, Portale et al., 1987).

The approach is to define, as well as the available evidence will permit, the lower limit of normal for serum Pi in healthy individuals of various ages (Table 5-1). The exclusion of formula-fed babies from the infant data is essential since some formulas, and cow milk especially, have higher phosphorus levels than required, and hence they result in higher values for serum Pi. This fact precludes their use in establishing normative data for the requirement.

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