We examined the effect of aging on the relationship between the concentrations of blood ionized calcium and of serum parathyroid hormone (PTH) in 22 healthy men [9 elderly (age 74 +/- 2 yr) and 13 young (age 39 +/- 1 yr)] in whom the glomerular filtration rate was > 70 ml/min. Throughout a 24-h period, serum concentrations of PTH in the elderly men were twice those in the young men, whereas blood ionized calcium did not differ between the two groups. With intravenous infusion of calcium gluconate, the minimum PTH concentration was two- to threefold higher in the elderly men. With infusion of NaEDTA. the maximum PTH concentration was 20% higher in the elderly men. The calcium set point for PTH release was higher in the elderly than in the young men (4.71 +/- 0.04 vs. 4.54 +/- 0.03 mg/dl, respectively, P < 0.005). In these healthy men, the age-related increase in serum PTH could not be attributed to a sustained decrease in concentration of either blood ionized calcium or 1,25-hydroxyvitamin D. These findings suggest that, with aging, the relationship between calcium and PTH is altered such that at any given level of calcium, the concentration of PTH is higher.

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To determine whether aging alters the metabolism of 1,25-dihydroxyvitamin D [1,25-(OH)2D] in men, we measured the serum concentrations, MCRs, and production rates of 1,25-(OH)2D in healthy old (age, 72 +/- 5 yr; n = 9) and young men (age, 34 +/- 5 yr; n = 9) consuming a constant metabolic diet and in whom the glomerular filtration rate was greater than 1.2 mL/s.1.73 m-2. The results indicate that when dietary calcium and phosphorus are normal and glomerular filtration rate is not reduced, the serum concentrations, MCRs, and production rates of 1,25-(OH)2D in old men [83 +/- 22 pmol/L; 0.62 +/- 0.10 mL/s.70 kg ideal BW (IBW); 51 +/- 12 fmol/s.70 kg IBW, respectively] and young men (90 +/- 20 pmol/L; 0.56 +/- 0.09 mL/s.70 kg IBW; 52 +/- 13 fmol/s.70 kg IBW, respectively) are equivalent. Indices of serum PTH, however, were elevated in the elderly men. These results suggest that aging per se has little or no effect on the serum concentration, MCR, or production rate of 1,25-(OH)2D in men. Maintenance of a normal production rate of 1,25-(OH)2D in elderly men, however, may require increased circulating PTH. Most observed declines in serum 1,25-(OH)2D in elderly men are probably a consequence of decreased functional renal mass.

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