Aging and Renal Function

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The serum concentration of Pi increases with a physiological decline in renal function associated with aging (but not renal disease per se). Healthy individuals excrete approximately 67% of their absorbed phosphate via the urine and the remainder via the gut as endogenous secretions. As the glomerular filtration capacity of the kidneys declines, the serum Pi concentration increases and more Pi is retained by the body. PTH secretions increase but the typical serum PTH concentrations, although elevated, remain within the upper limits of the normal range, at least for a decade or so. Thereafter, however, serum Pi and PTH both continue to climb as renal function declines and increased rates of bone turnover lead to measurable bone loss. This situation probably affects millions in the United States each year as they enter the 50s and proceed into the 60s; many of these individuals are overweight or obese and have the metabolic syndrome, which

Dietary Intake 1400 mg

1100 mg

Dietary Intake 1400 mg

1100 mg

6100 mg

5000 mg

5000 mg

Influx

Efflux

5000 mg

6100 mg

7000 mg

Stool 500 mg

Stool 500 mg

7000 mg

Kidneys

Urine 900 mg

Bone

Bone

Kidneys

Urine 900 mg

Figure 1 Phosphorus homeostasis and balance. The intestine, kidneys, and bone are organs involved in phosphate homeostasis. Fluxes of phosphate ions between blood and these organs are shown. Note the high fluxes in and out of bone each day. To convert phosphorus values from g to mmol, multiply by 32.29; from mg/dl to mmol/l, multiply by 0.3229. *Steps enhanced by parathyroid hormone. (Adapted with permission from Anderson JJB, Sell ML, Garner SC, and Calvo MS (2001) Phosphorus. In: Bowman BA and Russell R (eds.) Present Knowledge in Nutrition, 8th edn, p. 282. Washington, DC: International Life Sciences Institute Press.)

may negatively impact renal function. As the syndrome worsens, many of these individuals will progress to chronic renal failure and renal secondary hyperparathyroidism.

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