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DRI DIETARY REFERENCE INTAKES FOR Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride
TABLE 5-1 Normative Values for Serum Inorganic Phosphorus (mmol/liter) for Age
higher Pi in newborn infants than in older children and adults is the lower glomerular filtration rate (GFR) of infants. GFR is about 32 ml/min/1.73 m2 at about 1 week of age, and rises to 87 at 4 to 6 months (Brodehl et al., 1982; Svenningsen and Lindquist, 1974). In the first months of life, plasma Pi concentration appears to be a reflection both of renal glomerular maturity and of the amount of dietary intake. Mean serum Pi appears to decline by about 0.3 mmol/liter (0.9 mg/dl) across the second half of the first year of life (Specker et al., 1986). Human milk-fed, compared with formula-fed, infants have a slightly lower plasma Pi (2.07 versus 2.25 mmol/liter or 6.4 versus 7.0 mg/dl) which is simply a function of differences in intake (Greer et al., 1982c; Specker et al., 1986); Gaucasian compared with African American infants have a slightly higher plasma Pi irrespective of type of milk feeding (Specker et al., 1986).
The general relationship between absorbed phosphorus intake and plasma Pi in adults is set forth in Figure 5-1, derived by Nordin (1989) from the infusion studies of Bijvoet (1969). (In Bijovet's studies, a neutral phosphate solution was infused intravenously at a steadily increasing rate and produced a controlled hyperphosphatemia.) The achieved plasma Pi could thus be directly related to the quantity entering the circulation. Plasma Pi rises rapidly at low intakes, since the filtered load will be below the TmP, and little of the absorbed phosphorus will be lost in the urine (Figure 5-1). The steep, ascending portion of the curve thus represents a filling up of the extracellular fluid space with absorbed phosphate. At higher intakes, urinary excretion rises to match absorbed input and plasma levels change much more slowly.