Background: Osteoporosis is a multifactorial disease. The body mass index (BMI) can affect bone density (BMD) with higher BMI scores that seems to be protective for osteoporosis in postmenopausal women. Primary aim of this study was to relate BMI and lumbar BMD in pre-menopausal females.
Methods: We obtained from our local registry demographic and anthropometric data of females aged 20 years or older that performed their first DEXA in our Hospital between 2006 and 2014. Criteria for patient selection were: absence of known risk factors of Osteoporosis (e.g., smoke, alcohol, metabolic disease that affects bone, fractures), a concomitant or previous treatment of osteoporosis or corticosteroids or other drugs affecting bone (e.g., thyroid hormones, oral anticoagulant, immunosuppressant drugs). Data collection included also Calcium, PTH and 25-OH vitamin D levels and a structured data sheet covering the medical and drug use history, and lifestyle habits, including smoking, consumption of alcohol and coffee, physical activity and calcium intake.
Patients with low or high BMI (over 95 percentile) were eligible. Osteopenia (low bone density) and osteoporosis (very low bone density) were defined by T- score <-1 and <-2.5, respectively.
Results: 1197 DEXA scan analysed. Mean age was 34.2 ± 1.8 years (range 20.8 to 45.7). Mean BMI was 27.2 ± 3.9 kg/m2 (range 15 to 36). Mean menarche age was 16.3 ± 5 years (range 11-17). The prevalence of low bone density (T score between -1 and -2.6) and very low bone density (T score <2.6) were 24.1% and 13.8%, respectively. Mean calcium and 25-OH vitamin D levels were 9.3 mg/dl (range 8.7-10.4 mg/dl) and 27 ng/ml (range 6-75 ng/ml) respectively. Using a statistical model adjusted all variables collected, we found a reverse correlation between BMI and calcium dietary intake. BMI values were related to BMD in a non-linear way with the lowest BMD observed with the extreme BMIs < 15 kg/m2 and > 30 kg/m2.
Conclusion: Our study showed a peculiar relationship between BMI and BMD values in pre-menopausal women that look like a parabolic curve, with lower BMD at extreme BMI levels.