Study design: A cross-sectional postal survey of 29,424 twin subjects aged 12-41 years obtained from a population-generated panel.
Objectives: To determine whether obesity is associated with low back pain.
Summary of background data: Despite a large number of epidemiologic studies in this area, it is unclear whether obesity and low back pain are positively associated, and if so, whether there is a causal association.
Methods: The association and dose-response connection between body mass index and nonspecific low back pain experienced by subjects in the preceding year were studied. Possible modifying effects of age, gender, type of work, and smoking were investigated. The prevalence of nonspecific low back pain was also studied in monozygotic twin pairs who were dissimilar in body mass index.
Results: There was a modest positive association between body mass index and low back pain that increased with the duration of low back pain. The underweight subjects consistently reported lower prevalence of low back pain (odds ratios < 1) than did those higher in weight. The dose-response curve was usually A-shaped. A positive monotonic dose response was apparent mainly in those with long-lasting or recurrent low back pain. The positive association between body mass index and low back pain disappeared when monozygotic twins who were dissimilar in body weight classification were studied.
Conclusions: Obesity is modestly positively associated with low back pain, in particular with chronic or recurrent low back pain. However, because the association is weak, because there is no consistent positive monotonic dose response, and because the link disappears in monozygotic twins who are dissimilar in body mass index, it is unlikely that this association is causal. It is possible, however, that obesity plays a part in the chronicity of simple low back pain. Therefore, those with recurring or long-term low back pain deserve further attention.