Low bone mineral density (BMD) is a major risk factor for hip, spine, and other osteoporotic fractures. Therefore, BMD testing using dual-energy X-ray absorptiometry (DXA) is recommended as part of fracture risk assessment in postmenopausal women aged 65 years and older in most treatment guidelines, including the National Osteoporosis Foundation (NOF), the Endocrine Society, the International Society for Clinical Densitometry (ISCD), Associations of Clinical Endocrinologists (AACE), and United States Preventive Services Task Force (USPSTF). Greater postmenopausal bone loss more quickly leads to osteoporotic BMD and so screening with serial DXA is important. However, the optimal interval for repeating DXA scans to reassess BMD and fracture risk is uncertain, and this should be informed by the expected change in BMD over time. In this issue of the JBMR, Moilanen and colleagues (1) attempt to fill this evidence gap by examining the long-term changes in femoral neck BMD after menopause in 2695 women from the Kuopio Osteoporosis Risk Factor and Prevention (OSTPRE) study. Baseline mean age was 53 years. Participants had DXA scans at baseline and every 5 years for a duration of up to 25 years—the longest timeframe reported in the literature. Baseline femoral neck BMD was normal. Careful cross-calibrations were performed between DXA scans over time to account for changes in densitometers. Self-reported risk factors and health conditions were also obtained by questionnaires at 5-year intervals. Although follow-up was fairly complete over the first 15 years, about 50% of women were lost to follow-up by 20 years and 75% by 25 years. Mortality and long-term care institutionalization account for 35% of all dropouts. The study reported a mean femoral neck bone loss of 10%(0.4%/year), which was constant over the 25years. Notably, menopausal hormone therapy (HT) along with larger weight gain were protective of bone loss. The Study of Women’s Health Across the Nation (SWAN) has shown that bone loss begins 1 year before the final menstrual period, accelerates over the next 2 years, and continues at a slightly reduced rate over the next 5 years.(2-4) In various studies including SWAN, the rate of femoral neck bone loss appears largely steady up to 15 years after menopause.(5-10) However, the degree of bone loss with aging, particularly after age 65 years, is less certain. A study by Ensrud and colleagues,(11) which included 5689 women over 65 years of age with longitudinal BMD measurements by DXA over an average of 3.6 years, found that annual rate of loss in hip BMD almost tripled between ages 65 and 69 years (À0. 35%/year) and ages over 85 years (À1. 00%/year). Other studies have also shown similar accelerations in rates of bone loss with aging after age 65 years.(12, 13) Various processes likely contribute to the accelerated bone loss including decreased physical activity, comorbidities, and lower metabolic functions. This acceleration in femoral neck bone loss is consistent with the exponential increase in hip and spine fracture risk after age 65 years. Although Moilanen and colleagues (1) found an overall linear trend in femoral neck BMD loss over 25 years, there was a slightly higher loss between 15 and 25 years after baseline (À6. 0%) than the first 10 years from baseline (À4. 7%), which is consistent with the study by Ensrud and colleagues.(11)

The study by Moilanen and colleagues (1) is unique in having 25 years of follow-up, whereas most other studies have had only as much as 3 to 5 years of follow up. The only other long-term study was by Zhai and colleagues,(8) which followed a cohort of 1003 …