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Titolo	Age, Renal Tubular Phosphate Reabsorption, and Serum Phosphate Levels in Adults
Autore	Massimo Cirillo, M.D. Second University of Naples, Carolina Ciacci, M.D. Federico II University of Naples, Natale G. De Santo, M.D. Second University of Naples
Referenza	New England Journal of Medicine 2008; 359 (8) :864-866
Contenuto	To the Editor: A relation between age and serum phosphate levels in adults has been recognized since 1969.1 We used data from the Gubbio Population Study to analyze serum phosphate levels and renal phosphate handling in relation to age in 2107 men and 2560 women (age range, 18 to 97 years).2 Information concerning medical history and diet was collected by questionnaire. Overnight urine samples were obtained for analysis of albuminuria and markers of protein and salt intake. Early-morning samples of blood and urine were collected under fasting conditions to examine renal tubular function. Renal tubular phosphate handling was assessed as the ratio of the maximum rate of tubular phosphate reabsorption to the glomerular filtration rate (TmP:GFR).3 Laboratory analyses were performed with the use of an autoanalyzer, with an intraassay error of less than 5% for serum variables and of less than 10% for urinary variables. Thus, in adults, serum phosphate levels decline with age, except for a transient increase during the perimenopausal period in women.4 The age-associated decline in serum phosphate levels reflects changes in tubular phosphate reabsorption, which, in turn, might be explained by age-dependent changes in tubular phosphate handling or in its hormonal modulators (e.g., parathyroid hormone, phosphatonins, and growth hormone). The lack of association between the decrease in serum phosphate levels with hypocalcemia or hypocalciuria does not support a role for hyperparathyroidism caused by vitamin D deficiency. A possible mechanism might be the age-dependent decrease in growth-factor secretion and related stimulation of phosphate reabsorption.5 The practical implication is that phosphate-depleting disorders might induce hypophosphatemia more readily in older persons because the tubular capacity for phosphate reabsorption and the level of serum phosphate before the development of such disorders are already diminished. For parallel reasons, disorders causing increases in serum phosphate levels should more readily induce hyperphosphatemia in younger persons.
Data	27.08.2008

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