EFFECT OF AGING ON THE METABOLISM OF PHOSPHORUS AND 1,25-DIHYDROXYVITAMIN-D IN HEALTHY-MEN

We tested the hypothesis that aging alters physiological regulation of the serum concentration of 1,25-dihydroxyvitamin D [1,25(OH)(2)D] by inorganic phosphorus. In seven elderly men [age 71 +/- 1 (SE) yr] and 9 young men (29 +/- 2 yr), dietary phosphorus was first normal, then i ncreased and decreased within its normal range. At each intake of phos phorus, serum concentrations of 1,25(OH)(2)D in the elderly did not di ffer from those in young men, but fasting and 24-h mean serum concentr ations of phosphorus were lower in elderly men. With phosphorus restri ction, in each group serum 1,25(OH)(2)D increased by 47%, and 24-h mea n serum phosphorus decreased by 0.6 +/- 0.1 mg/dl. Serum concentration s of 1,25(OH)(2)D varied inversely with 24-h mean serum phosphorus (R = -0.92, P < 0.0001). Thus, in healthy elderly men in whom glomerular filtration rate is normal or near normal, serum concentrations of 1,25 (OH)(2)D increase when dietary phosphorus is restricted; the magnitude of response at steady state is unaffected by aging, but the time cour se of response is delayed. At any level of serum phosphorus, serum 1,2 5(OH)(2)D is lower than that in young men, as reflected by a lower int ercept of the regression of serum 1,25(OH)(2)D on 24-h mean phosphorus .