Serum P_i. The relationship between serum P_i and absorbed intake, as presented in Figure 5-1, allows estimation of the intakes associated with P_i values within the range typically considered normal.

The extrapolation from absorbed intake to ingested intake shown in Table 5-6 is based on an absorption efficiency for phosphorus of 60 to 65 percent, the value typically observed in studies of adults on mixed diets (Heaney and Recker, 1982; Stanbury, 1971; Wilkinson, 1976). This absorption estimate for phosphorus is fairly robust, since variation in absorptive performance for phosphorus in adults is narrow (Heaney and Recker, 1982; Wilkinson, 1976).

The estimates of ingested intake in Table 5-6 apply to a typical, mixed diet. They will underestimate intakes needed to achieve a given serum P_i value if the dietary phosphorus consists heavily of phytate phosphorus. Thus, diets in which major fractions of the phosphorus intake are derived from unleavened cereal sources will require higher intakes to produce the curve of Figure 5-1 (which, as already noted, is for typical mixed diets), that is, the curve will be expanded to the right when the x-axis is expressed as ingested intake. This is shown by the lower dashed curve in Figure 5-1. Conversely, diets in which much of the phosphorus is derived, for example, from the phosphoric acid in certain car-