Older people in China and the United Kingdom differ in the relationships among parathyroid hormone, vitamin D, and bone mineral status

Abstract

To explore whether there are ethnic differences in relationships among parathyroid hormone (PTH), vitamin D, and bone mineral status, 352 healthy volunteers, 60–83 years old, were studied in Shenyang, Peoples' Republic of China (108 men, 110 women), and in Cambridge, UK (67 men, 67 women), in late winter. Early morning fasting blood and 2-h fasting urine were analyzed for 25-hydroxyvitamin D (25OH-D), PTH, and free deoxypyridinoline (DPD). Hip bone mineral status was measured using dual-energy X-ray absorptiometry (Lunar). There were significant differences (P < 0.001) in plasma 25OH-D and PTH concentrations between Shenyang and Cambridge [25OH-D nmol/L: Shenyang = 29.0 (SD 12.7), Cambridge = 35.7 (12.9)]; PTH ng/L: Shenyang = 34.3 (13.4), Cambridge = 25.2 (11.0)]. PTH was negatively related to 25OH-D in both populations. The relationship was exponential, best described by an inverse log-log equation with no break point (P < 0.001), indicating that the exponential curve did not tend toward a low plateau. PTH was higher for a given 25OH-D and decreased less with increasing 25OH-D in Shenyang than in Cambridge (country-ln25OH-D interaction, P = 0.0005). After adjusting for bone area, weight, height, age, and sex, hip bone mineral content (BMC) was significantly related to PTH concentration in Cambridge but not in Shenyang [femoral neck coefficient: Cambridge = −0.064 (SE 0.027), P = 0.02; Shenyang = −0.027 (0.028), P = 0.3; trochanter: Cambridge = −0.116 (0.034), P = 0.001; Shenyang = −0.019 (0.027), P = 0.5]. There was a significant country–lnPTH interaction at the trochanter (P = 0.02), but not at the femoral neck (P = 0.7). A weak positive association between BMC at the femoral neck and 25OH-D concentration was found in Cambridge [coefficient: 0.054 (0.028), P = 0.05] but not in Shenyang (coefficient: −0.013, P = 0.5; country–ln25OH-D interaction, P = 0.07). Urinary DPD concentration was also positively related to plasma PTH concentration in Cambridge subjects only [coefficient: 0.2 (0.08), P = 0.02]. These data suggest that although PTH increases when 25OH-D decreases, and Chinese people have a higher PTH for a given 25OH-D, older Chinese adults may be more resistant than Britons to the effects of PTH on bone.