Evidence that obesity does not influence the vitamin D-endocrine system in blacks

Abstract

As compared to nonobese white men and women, age-matched nonobese black subjects and obese white individuals show alterations in the vitamin D–endocrine system that are characterized by increases in mean serum immunoreactive parathyroid hormone (PTH), serum 1,25-dihydroxyvitamin D [1,25-(OH)₂D], and urinary cyclic adenosine 3,5-monophosphate (cAMP) and by decreases in mean serum 25-hydroxyvitamin D (25 OHD) and in urinary calcium. Thus, both groups show secondary hyperparathyroidism which is associated with increased renal tubular reabsorption of calcium and increased renal synthesis of 1,25-(OH)₂D. In view of these findings, studies were conducted in 10 obese black subjects (3 men and 7 women) and in 12 nonobese black individuals (7 men and 5 women), ranging in age from 20 to 35 yr, to determine whether obesity influences the vitamin D–endocrine system in blacks. Body weight averaged 99 ± 4 kg in the obese and 73 ± 3 kg in the nonobese subjects (p < .001). 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, 110 meq of sodium, 65 meq of potassium, and 18 meq of magnesium. Whereas mean serum Gla protein was significantly higher in the obese than in the nonobese black subjects (23 ± 4 vs. 14 ± 2 ng/ml, p < .05), mean serum total calcium, ionized calcium, phosphorus, magnesium, immunoreactive PTH by two different radioimmunoassays (399 ± 23 vs. 312 ± 26 pg/ml and 357 ± 26 vs. 392 ± 27 pg/ml), 25 OHD (5 ± 1 vs. 6 ± 1 ng/ml) and 1,25(OH)₂D (43 ± 3 vs. 41 ± 3 pg/ml) were the same in the two groups. Also, mean urinary calcium (105 ± 13 vs. 101 ± 14 mg/d), phosphorus, sodium, potassium, magnesium and cAMP (3.12 ± 0.24 vs. 3.11 ± 0.47 nM/dl GF) and creatinine clearance were the same in the obese and nonobese black men and women. The results indicate that obesity does not produce alterations in the vitamin D–endocrine system that are over and above the changes present in nonobese black men and women.