As compared with values in white subjects, bone mass is known to be increased and urinary Ca to be diminished in black individuals. To evaluate the possibility that these changes are associated with alterations in the vitamin D-endocrine 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 20-35 yr. All of them were hospitalized in a metabolic ward and were given a constant daily diet contining 400 mg of Ca, 900 mg of P and 110 meq of Na. Whereas mean serum Ca, ionized Ca and phosphate were the same in the 2 groups, mean serum immunoreactive parathyroid hormone (350 .+-. 34 vs. 225 .+-. 26 pg/ml, P < 0.01) and mean serum 1,25-dihydroxyvitamin 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 Na and 24 h creatinine clearance were the same in the 2 groups, whereas mean urinary Ca was significantly lower (101 .+-. 14 vs. 166 .+-. 13 mg/d, P < 0.01) and mean urinary cAMP was significantly higher (3.11 .+-. 0.47 vs. 1.84 .+-. 0.25 nM/dl glomerular filtrate, P < 0.01) in the blacks. The blacks excreted as i.v. Ca load, 15 mg/kg body wt as efficiently as the whites (49 .+-. 3 vs. 53 .+-. 3%, not significant). 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 .mu.g/day for 4 day. There was a blunted response of urinary Ca to 1,25(OH)2D3 in the blacks, and mean serum Ca did not change. Alteration of the vitamin D-endocrine system with enhanced renal tubular reabsorption of Ca 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.
