Relationship between serum 25-hydroxyvitamin D and bone resorption markers in vitamin D insufficiency

It is known that nursing-home patients with vitamin D insufficiency have elevated serum parathyroid hormone (PTH) as well as raised serum alkaline phosphatase (ALP). Although it is well known that vitamin D insufficiency and secondary hyperparathyroidism are common among the elderly in western countries, there is continuing controversy over the level of serum 25-hydroxyvitamin D [25(OH)D] necessary for bone health. We approached this issue by examining the relationships between serum 25(OH)D, ionized calcium, PTH, and ALP and the urinary bone resorption markers hydroxyproline, pyridinoline, and deoxypyridinoline, corrected for creatinine (OHPr/Cr, Pyd/Cr, and Dpd/Cr, respectively), in 486 postmenopausal women of mean age 63 (SD 9.5) years, who were referred to our osteoporosis and menopause clinics for investigation. When the patients were divided into two groups with 25(OH)D above and below 20 nmol/L, 30 nmol/L, 40 nmol/L, 50 nmol/L, 60 nmol/L, or 70 nmol/L, the most significant differences between the two groups thus derived was found at a serum 25(OH)D level of 60 nmol/L (P < 0.001 for all markers). The most significant difference between groups for serum PTH was found when the patients were divided at a serum 25(OH)D of 50 nmol/L. PTH, OHPr/Cr, Pyd/Cr, and ALP were inversely related to serum 25(OH)D. PTH was inversely related to serum ionized calcium. There was a trend for ionized calcium to be positively related to 25(OH)D, but this did not reach statistical significance. We conclude that rises in three bone resorption markers and ALP can be detected in postmenopausal women when the serum 25(OH)D level falls below 60 nmol/L. Levels above this may be required for optimal bone health.