In the past, indicators have not generally been used for the phosphorus requirement. Instead, phosphorus recommendations have been tied to calcium, usually on an equimass or equimolar basis. As noted above, that approach is unsatisfactory. The two indicators that will be considered here for the Estimated Average Requirement (EAR) are phosphorus balance and serum Pi.
Although phosphorus balance might seem to be a logical indicator of nutritional adequacy, it is not an adequate criterion, since an adult can be in zero balance at an intake inadequate to maintain serum Pi within the normal range. Even during growth, balance will be positive in direct proportion to soft tissue and bony accumulation, but so long as plasma Pi is high enough, the degree of positive balance will be limited either by the genetic programming or by availability of other nutrients. Furthermore, the balance data that would be needed to bracket the requirement during growth are not available. And during senescence, if there is loss of bone or soft tissue mass, phosphorus balance will be negative. However, so long as plasma Pi remains within normal limits, these balances will reflect other changes occurring in the body and will not be an indicator of the adequacy of dietary phosphorus.
Because phosphorus intake directly affects serum Pi, and because both hypo- and hyperphosphatemia directly result in dysfunction or disease, the most logical indicator of nutritional adequacy of phosphorus intake is serum Pi. If serum Pi is above the lower limits of normal for age, phosphorus intake may be considered adequate to meet cellular and bone formation needs of healthy individuals. The relationship between Pi and phosphorus intake has, however, been clearly established only for adults (see “Regulation of the Serum Inorganic Phosphate Concentration”), and while the adverse effects of low serum Pi are well understood during growth, it is harder to define the critical values for phosphorus intake associated with the normal range of Pi values in infants and children. For that reason, estimates of