Background A low body mass index (BMI: weight (kg)/height (m)²) is an important risk factor for hip fracture. In Norway, hip fracture rates have declined in recent decades at the same time as BMI has increased in the population; however, to what extent increased BMI has contributed to the declining hip fracture trend is unknown. We aimed to estimate the number of incident hip fractures prevented or postponed (IFPP) attributable to increased BMI using an adaptation of IMPACT coronary heart disease model methods.

Methods Hip fractures in Norway from 1999–2019 were extracted from the Norwegian hip fracture database (NORHip) and prevalences of BMI > 25 in the Norwegian population was extrapolated from the fourth wave (1994–5) and the seventh wave (2015–16) of the Tromsø study. Hazard ratios of hip fracture according to age and sex were estimated using data from the Cohort of Norway (CONOR). We calculated the difference between the observed number of hip fractures in 2019 and the expected number of hip fractures given stable fracture rates. IFPPs in 2019 attributable to changes in prevalences of BMI > 25 were estimated according to sex and 5-year age groups from 50 to 85+.

Results Hip fracture rates decreased by 28% from 1999 to 2019 with 2,549 fewer hip fractures observed than expected in 2019. The prevalences of BMI > 25 had increased in all age groups in both men and women over the time period. Unadjusted estimates showed that the increased prevalences of BMI > 25 accounted for ~30% of the decline. Increased BMI explained ~50% of the total number of IFFPs in men and ~25% of the total IFPPs in women.

Conclusion The incident hip fracture rates in Norway declined between 1999 and 2019. These preliminary results suggest that increased BMI in the population, reflecting a shift in the population BMI distribution, has significantly contributed to the declining hip fracture incidence, particularly in men.