Association between low back pain and body mass index in adult twins: an analysis of monozygotic and dizygotic twins of the Washington State Twin Registry.

BACKGROUND CONTEXT

Low back pain (LBP) is a common and significant cause of disability worldwide, however; questions about cause still remain.

PURPOSE

To investigate the association between LBP, body mass index (BMI), and moderate to vigorous physical activity (MVPA) in a twin sample.

STUDY DESIGN

Cross sectional study of monozygotic (MZ) and dizygotic (DZ) twins from the Washington State Twin Registry.

PATIENT SAMPLE

Monozygotic and dizygotic twins from the Washington State Twin Registry.

OUTCOME MEASURES

Self-report measures: Low back pain, body mass index, duration and intensity of exercise.

METHODS

The sample included 5,183 same-sex pairs (69% MZ). The outcome was self-reported diagnosis of LBP from a health care provider. A phenotypic model tested the association between BMI and LBP without including genetic or shared environmental confounds. We then re-estimated the association using a quasi-causal model which controls for those confounds. Finally, we used a mediation model to investigate if the association between LBP and MVPA was mediated by BMI.

RESULTS

In the phenotypic regression of LBP on BMI, there was a ~4-fold increase in the odds of having LBP with every one-unit increase in BMI (odds ratio [OR] =3.83; 95% confidence interval =3.28, 4.46). However, quasi-causal regression of LBP on BMI was reduced to zero (OR =0.95; 95% confidence interval =0.60, 1.49). A significant genetic background to BMI and LBP was present (b =1.66; p<.001), even after controlling for confounders. In another analysis there was a significant direct effect between MVPA and LBP (b=-0.092, standard error [SE] =0.017, p<.001). In mediation analysis, the effect of MVPA on LBP was partially mediated through MVPA effects on BMI ( [Formula: see text] =-0.043, SE=0.003, p<.001) and BMI effects on LBP ( [Formula: see text] =1.281, SE=0.079, OR=3.6, p<.001), however shared environmental factors confounded this relationship.

CONCLUSIONS

BMI was not associated with LBP, despite sharing a phenotypic association, but they may share a genetic influence. The effect of MVPA on LBP is, in part, mediated by BMI but shared environment confounds this relationship.

LEVEL OF EVIDENCE

Level 3.