Vitamin D status of 51-75-year-old Irish women: its determinants and impact on biochemical indices of bone turnover

Objectives: To assess the vitamin D status of Irish postmenopausal women during wintertime, and to examine its relationship with serum parathyroid hormone (PTH) and biochemical markers of bone turnover. In addition, the determinants of wintertime serum 25-hydroxyvitamin D (25OH-D) levels in these women were investigated.

Design: A cross-sectional observational study. Setting: Cork City, Ireland (52 degrees N). Subjects: Ninety-five apparently healthy, free-living postmenopausal women (aged 51-75 years), not taking any medication and free from any condition likely to affect vitamin D status or calcium/bone metabolism.

Results: Forty-eight per cent and 7% of women had serum 25OH-D levels <50 nmol l(-1) and <25 nmol l(-1), respectively. 25OH-D levels in these women were positively associated with dietary calcium intake (P=0.0002) and use of vitamin D-containing supplements (P=0.031), and negatively associated with cigarette smoking (P=0.027) and body mass index (BMI) (P=0.030).

Low serum 25OH-D levels (<50 nmol l(-1)) were associated (P <0.01) with elevated serum PTH levels. There were no significant differences in urinary pyridinium crosslinks or serum osteocalcin, biochemical indices of bone turnover, between subjects with serum 25OH-D levels above or below 50 nmol l(-1).

Conclusion: A high proportion of Irish postmenopausal women had low vitamin D status (<50 nmol l(-1)) during late wintertime, which appeared to lead to elevated levels of serum PTH but not of bone turnover markers. Use of regular low-dose supplemental vitamin D, meeting daily calcium recommendations, cessation of smoking and maintaining BMI in the normal range are important factors that could help maintain adequate vitamin D levels during wintertime in these women.