Objective To compare the relationship between body mass index (BMI) and physical activity for the risk of ankle, wrist and hip fractures in a large prospective study of postmenopausal women in the UK.

Methods In 1996–2001, women recruited into the Million Women Study completed a self-administered questionnaire asking about body size, physical activity, disease history, and lifestyle and reproductive factors. Incident ankle, wrist and hip fractures were identified through self-report in a follow-up questionnaire completed an average of 3.2 years after recruitment. RRs and CIs for each fracture site in postmenopausal women by BMI and physical activity at recruitment were calculated using Cox regression models, adjusted for socio-economic status, smoking and other health and lifestyle factors.

Results Among 599 550 postmenopausal women, 5114 reported an ankle fracture, 8565 reported a wrist fracture, and 754 reported a hip fracture during follow-up. When compared to lean women (BMI<22.5 kg/m²), obese women (BMI≥30 kg/m²) had an increased risk of ankle fracture (RR 2.47; 95% CI 2.32 to to 2.63), but a decreased risk of wrist fracture (RR 0.68; 95% CI 0.64 to to 0.72) and hip fracture (RR 0.29; 95% CI 0.23 to to 0.37) (χ²<0.001 for the effect of BMI for each fracture site). Physical activity had little influence on the risk of either ankle fracture or wrist fracture, but women who reported strenuous physical activities up to once per week or more often were at a lower risk of hip fracture than women who reported less frequent activity (RR 0.65; 95% CI 0.56 to to 0.76).

Conclusions BMI and physical activity have different effects on the incidence of fracture at different sites. While obese women are at increased risk of ankle fracture they are at lower risk of wrist fracture and hip fracture. Physical activity has no marked influence on ankle and wrist fracture but is protective against hip fracture.