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Seasonal Variation in Bone Histomorphometry

Need et al. reported seasonal effects on histomorphometry characteristics in 121 bone biopsies obtained from osteoporotic patients. They showed increased osteoid thickness (OTh.) and mineralization lag time (MLT) and decreased 25(OH)D levels in the winter months. The authors reported their data as mineralization lag time, but it would seem the formula used was osteoid maturation time (Omt.) rather than MLT. Nevertheless, the fact of a seasonal difference seems real and is likely to have been present in MLT as well. The patients were mostly women (112), and they did not report menopausal status, but we assume from their median age (66 yr) that the majority were postmenopausal. We report here comparable findings from a previous study of the effect of menopause on bone histomorphometry. Our group obtained bone biopsies from 78 women before menopause (mean age, 49.4 ± 1.9 yr) and 12 mo after their last menses (mean age, 54.6 ± 2.2 yr). This study was conducted in Omaha, NE (41° N). Using the same approach taken by Need et al., we divided the biopsies into summer (April to September) and winter (October to March). We found similar seasonal differences in OTh. from biopsies obtained from postmenopausal women in winter compared with summer months (7.7 ± 1.9 versus 6.9 ± 1.0 m; p 0.001). True MLT also showed a seasonal difference (70.9 ± 6.28 days in winter versus 41.8 ± 26.2 days in summer; p 0.05). Omt. was also significantly different between winter and summer (14.2 ± 4.1 versus 12.5 ± 2.8 days; p 0.05). Biopsies obtained from premenopausal women in our study did not show a significant seasonal difference in OTh. or MLT. Because the preand postmenopausal women were the same women, this finding suggests that postmenopausal bone is more responsive to seasonal changes. The primary factor is likely vitamin D obtained from sun exposure in the summer months.