There are no functional criteria for phosphorus status that reflect response to dietary intake in infants. Thus recommended intakes of phosphorus are based on Adequate Intakes (AIs) that reflect observed mean intakes of infants fed principally with human milk.
Human Milk. Human milk is recognized as the optimal source of milk for infants throughout at least the first year of life and as a sole nutritional source for infants during the first 4 to 6 months of life (IOM, 1991). Furthermore, there are no reports of exclusively human milk-fed, vitamin D-replete, full-term infants manifesting any evidence of phosphorus deficiency. Therefore, consideration of the AI for phosphorus in infants is based on data from term-born healthy infants fed human milk as the principal fluid milk during the first year of life. The approach was to set the AI at the mean of usual intakes of phosphorus as derived from studies where intake of human milk was measured using test weighing, and intake of food was determined by dietary records for 3 days or more. The following limited data on infants were also reviewed and considered as supportive evidence for the derived AIs.
Serum Pi. Using the term-born infant fed human milk as the model, the target range for serum Pi (the most appropriate biochemical indicator of dietary phosphorus adequacy during early life), is 2.42 to 1.88 mmol/liter (7.5 to 5.8 mg/dl). This wide range is a function of the precipitous fall in serum Pi during the first 6 weeks of life (Greer et al., 1982b). The reasons for the well-documented decline in serum Pi in infants are: (1) an increase in GFR as the infant's kidneys mature postnatally (McCrory et al., 1950), (2) a decline in phosphorus concentration in breast milk with advancing lactation (Table 5-2), and (3) possibly an inappropriate PTH response to rising serum Pi during early neonatal life (DeVizia and Mansi, 1992). However, there is evidence that the parathyroid glands can respond in early life since high phosphorus feedings (Ca:P molar ratio 1:2.5) resulted in serum PTH, which was significantly higher than in breast-fed infants at 2, 7, and 14 days of age