The longest study to assess bone mineral density of postmenopausal women has found lower bone loss than previously suggested.

The study, published in the Journal of Bone and Mineral Research, followed 686 women in Finland for more than two decades. Postmenopausal bone loss was found to be constant and appeared to be approximately 10% over 25 years.

The aim of this study was to monitor long-term changes in bone mineral density after menopause and factors affecting bone mineral density.

It found taking hormone replacement therapy (HRT) was linked with lower postmenopausal bone loss, as was having a lower weight at the start of the study and gaining weight during follow-up. Higher baseline bone mineral density was associated with a higher bone loss rate.

The study population consisted of a random sample of 3,222 women from the Kuopio Osteoporosis Risk Factor and Prevention (OSTPRE) study, of which 62.1% were postmenopausal at the beginning of the study. This group of women underwent dual-energy X-ray absorptiometry (DXA) measurements at the femoral neck every five years from baseline (in 1989) up to 25-year follow-up.

They also responded to risk-factor questionnaires at 5-year intervals. During the 25-year follow-up, the baseline cohort decreased to 686 women. 

Long-term bone loss was much lower than in previous studies

Lower baseline body mass index (BMI) and a greater increase in BMI were found to protect against postmenopausal bone loss (p<0.001). The lowest bone loss quartile included 15.2% more HRT users than the highest bone loss quartile (p = 0.004). The number of diseases/bone-affecting diseases, use of vitamin D/calcium supplementation, use of corticosteroids, smoking or alcohol use had no statistical significance for annual bone loss rate. 

Previous studies have shown that bone loss decreases after perimenopause, and that over a 15-year period, bone mineral density at the femoral neck steadily decreases with age in postmenopausal women. 

This study found that bone mineral density at the femoral neck in postmenopausal women decreased during the 25-year follow-up with a steady pattern between ages 53.3 and 77.4 years. However, long-term bone loss was much lower than in previous studies.