Obese adults have a lower risk of hip and vertebral fracture, but a greater risk of lower limb and proximal humerus fracture, compared to adults with a normal body mass index (BMI). Differences in fracture risk by skeletal site in obesity might be attributed to differences in bone mineral density (BMD), bone microstructure and bone strength between obese and normal BMI individuals and/or differences in physical function, possibly related to fall frequency and/or direction. The role of vitamin D in bone metabolism and physical function in obesity is unclear. The effect of obesity on bone microstructure and strength in young and older, men and women, has not been investigated in a matched case control design.
BMD, microstructure and strength were determined using novel imaging technologies. The roles of various adipose compartments and biochemical factors on BMD and microarchitecture were investigated. Physical function and vitamin D metabolism of obese and normal BMI individuals was compared.
Younger and older obese adults have higher BMD, favourable microstructure and greater bone strength, but there is a greater difference in BMD and microstructure between obese and normal BMI in older adults than in younger adults. Obese adults have greater bone strength than normal BMI adults, regardless of age. Higher BMD in obesity is associated with lower resorption, possibly mediated by circulating leptin and oestradiol which were, in turn, associated with subcutaneous abdominal adiposity. Obese adults have greater muscle mass but poorer physical performance than normal BMI adults. This might be associated with greater fall frequency and affect fall direction. Obese adults have lower total and free 25OHD and 1,25(OH)2D than normal BMI adults, likely due to greater volumetric dilution. Low 25OHD in obesity does not appear to affect BMD, microstructure or strength, or physical performance.
Despite greater BMD and bone strength, obese individuals are at greater risk of some fractures. This might be due to the effect of poor physical function on fall characteristics in obesity.
