FIGURE 5-1 Relation of serum Pi to absorbed intake in adults with normal renal function. (See Nordin [1989] for further details.) The solid curve can be empirically approximated by the following equation: Pi = 0.00765 × AbsP + 0.8194 × (1 − e(−0.2635 × AbsP), in which Pi = serum Pi (in mmol/liter), and AbsP = absorbed phosphorus intake (also in mmol). Solving this equation for the lower and upper limits of the normal range for Pi, as well as for its midpoint, yields the values presented in Table 5-1. The dashed horizontal lines represent approximate upper and lower limits of the normal range, while the dashed curves reflect the relationship between serum Pi and ingested intake for absorption efficiencies about 15 percent higher and lower than average. (© Robert P. Heaney, 1996. Reproduced with permission.)

The relationship shown in Figure 5-1 holds only in adult individuals with adequate renal function; that is, the slow rise of plasma Pi with rising phosphorus intake over most of the intake range applies only so long as excess absorbed phosphate can be spilled into the urine. However, in individuals with reduced renal function, phosphorus clearance remains essentially normal so long as GFR is at least 20 percent of mean adult normal values, largely because tubular reabsorption is reduced to match the reduction in filtered load. Below that level, excretion of absorbed phosphate requires higher and higher levels of plasma Pi to maintain a filtered load at least

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