Bone mineral density in relation to body mass index among young women: a prospective cohort study

AIM: To identify important predictors among lifestyle behaviours and physiological factors of bone mineral density (BMD) in relation to body mass index (BMI) among young women over a 2-year period. DESIGN, SAMPLE AND MEASUREMENTS: Data were collected in 1999 and 2001. Healthy young women (n=152) completed a questionnaire. BMD measurements were performed by DEXA in the calcaneus. The women were subdivided into three categories according to baseline BMI. RESULTS: Baseline bodyweight explained 25% of the variability in BMD at follow-up in the BMI<19 category, and high physical activity seemed to hinder BMD development. In the BMI>24 category, a difference in time spent outdoors during winter between baseline and follow-up was the single most important factor for BMD levels. Overweight women with periods of amenorrhoea had lower BMD than overweight women without such periods. CONCLUSIONS: Predictors and lifestyle behaviours associated with BMD are likely to be based on women of normal weight. BMI should be considered when advising on physical activity, since high physical activity seems to impair BMD development among underweight young women, possibly due to energy imbalance. Among overweight women, sleep satisfaction is the greatest predictor associated with BMD change and may indicate better bone formation conditions. Energy balance and sleep quality may be prerequisites of bone health and should be considered in prevention.