Seasonal variation in bone density in postmenopausal women

We investigated seasonal variation in bone density by cross-sectional designs in two large cohorts of women (N = 2337) in Sweden. One cohort was strictly population-based (all 75 yr old, N = 1044), and one cohort was patient-based (age > or = 55 yr, mean age 68, N = 1293). Each woman was assessed once and the inclusion was continuous throughout almost all days of the year. Bone mineral density (BMD) of the total body, hip, and lumbar spine was determined by means of dual X-ray absorptiometry (DXA). For both cohorts, the BMD was similar in those assessed during the period April-September compared to those assessed during the period October-March. On the other hand, when women who had been assessed during July-December were compared with women assessed during January-June, we found the BMD to be up to 5% lower during July-December in all skeletal regions in the population-based sample, and 4% lower at the Ward's triangle and trochanter in the patient-based sample. After correction for small differences in age and weight, women of both cohorts who had been assessed during July-December still had lower BMD at the hip compared with women assessed during January-June. In conclusion, we found no variation in bone density during the period April-September compared to the period October-March in these two cohorts of postmenopausal women. In elderly women living at a northern latitude, BMD may be a few percent lower during the period July-December compared to the period January-June.