Small vessel disease is a common cause of white matter damage in older people. It is strongly associated with age, hypertension, diabetes, and obesity. We have investigated the associations between body mass index (BMI) and diffusion changes (adjusted for other risk factors) in data from UK Biobank. BMI was more strongly related to diffusion changes in inferior brain regions, depending on whether the white matter tracts were within the anterior or posterior circulation. Obesity affects white matter integrity, beyond the effect of cardiovascular risk factors.