BACKGROUND:

Low serum 25-hydroxyvitamin D ¿25(OH)D concentrations are commonly found in the elderly and are associated with hip fracture. Treatment with vitamin D and calcium can reduce the risk of fracture. The relation between the rise in parathyroid hormone (PTH) with age and the decrease in 25(OH)D is not clear. Neither is there any consensus on the serum concentration of 25(OH)D required for bone health.

OBJECTIVE:

Our objective was to study the relations between serum PTH, serum vitamin D metabolites, and other calcium-related variables in postmenopausal women.

DESIGN:

This was a cross-sectional study of 496 postmenopausal women without vertebral fractures attending our menopausal osteoporosis clinics.

RESULTS:

PTH was significantly positively related to age and serum 1, 25-dihydroxyvitamin D ¿1,25(OH)(2)D and inversely related to 25(OH)D and plasma ionized calcium. There was a step-like increase in PTH as serum 25(OH)D fell below 40 nmol/L. In women with 25(OH)D concentrations >40 nmol/L, 1,25(OH)(2)D was positively related to 25(OH)D; in women with 25(OH)D concentrations </=40 nmol/L, the relation was the inverse. In women with 25(OH)D concentrations </=40 nmol/L, 1,25(OH)(2)D was most closely related to PTH; in women with 25(OH)D concentrations >40 nmol/L, 1,25(OH)(2)D was most closely (inversely) related to plasma creatinine. Therefore, with serum 25(OH)D concentrations increasingly <40 nmol/L, serum 1,25(OH)(2)D becomes critically dependent on rising concentrations of PTH.

CONCLUSION:

The data suggest that aging women should maintain 25(OH)D concentrations >40 nmol/L (which is the lower limit of our normal range for healthy young subjects) for optimal bone health.