II . Urinary Values in Normal Black and White Subjects Urnary Urinary Urinary Urinay Urinary Crestinine

As compared with values in white subjects, bone mass is known to be increased and urinary calcium to be diminished in black individuals. To evaluate the possibility that these changes are associated with alterations in the vitamin Dendocrine system, an investigation was performed in 12 black subjects, 7 men and 5 women, and 14 white subjects, 8 men and 6 women, ranging in age from 20 to 35 yr. All of them were hospitalized on a metabolic ward and were given a constant daily diet containing 400 mg of calcium, 900 mg of phosphorus, and 110 meq of sodium. Whereas mean serum calcium, ionized calcium, and phosphate were the same in the two groups, mean serum immunoreactive parathyroid hormone (350±34 vs. 225±26 pg/ml, P < 0.01) and mean serum 1,25dihydroxyvitamin D (1,25(OH)2D) (41±3 vs. 29±2 pg/ml, P < 0.01) were significantly higher, and mean serum 25-hydroxyvitamin D (25-OHD) was significantly lower in the blacks than in the whites (6±1 vs. 20±2 ng/ml, P < 0.001). Mean urinary sodium and 24-h creatinine clearance were the same in the two groups, whereas mean urinary calcium was significantly lower (101±14 vs. 166±13 mg/d, P < 0.01) and mean urinary cyclic AMP was significantly higher (3.11±0.47 vs. 1.84±0.25 nM/dl glomerular filtrate, P < 0.01) in the blacks. Further, the blacks excreted an intravenous calcium load, 15 mg/kg body weight, as efficiently as the whites (49±3 vs. 53±3%, NS). Mean serum Gla protein was lower in blacks than in whites (14±2 vs. 24±3 ng/ml, P < 0.02), and increased significantly in both groups in response to 1,25(OH)2D3, 4 gg/ d for 4 d. There was a blunted response of urinary calcium to 1,25(OH)2D3 in the blacks, and mean serum calcium did not change. The results indicate that alteration of the vitamin Dendocrine system with enhanced renal tubular reabsorption of calcium and increased circulating 1,25(OH)2D as a result of secondary hyperparathyroidism may contribute to the increased bone mass in blacks. Their low serum 25-OHD is attributed to diminished synthesis of vitamin D in the skin because of increased pigment.