Parathyroid response to vitamin D insufficiency: relations to bone, body composition and to lifestyle characteristics

Background Vitamin D insufficiency is very common and is known to cause secondary hyperparathyroidism (SHPT). However, in some subjects the PTH response to low vitamin D levels is blunted, which has been termed functional hypoparathyroidism (FHPT). Aim We compared indices of calcium homeostasis, bone metabolism and body composition in subjects with differential PTH responses to low vitamin D levels. Design Cross-sectional study. In 405 recent postmenopausal women with vitamin D insufficiency, we compared levels of bone turnover markers, bone mineral density (BMD), body composition, and body weight between subjects with SHPT and FHPT. Results Plasma 25-hydroxyvitamin D (P-25OHD) levels were slightly higher (P < 0.05) in SHPT compared with FHPT. SHPT was associated with higher levels of osteocalcin and bone-specific alkaline phosphatase, whereas whole body BMD and hip- and lumbar spine-BMD were significantly reduced. Subjects with SHPT had a 7% (P < 0.01) higher body weight and a 23% higher fat mass (P < 0.01) than subjects with FHPT, whereas lean tissue mass did not differ between groups. In SHPT, fat mass was increased by 14% (P < 0.001) at the upper and lower extremities and by 33% (P < 0.001) at the trunk. In a regression model, significant predictors of fat mass was P-PTH (r(p) = 0.248, P < 0.01) and P-osteocalcin (r(p) = -0.115, P = 0.02), with no effects of P-25OHD or P-creatinine levels. Conclusions Effects of vitamin D insufficiency on bone is associated with the PTH responses. The increased body weight and fat mass in SHPT compared with FHPT may imply that PTH excess contributes to fat accumulation.