Liechty A, Tsang S, Turkheimer E, Duncan GE.
LBP is a common and significant cause of disability worldwide, however; questions about cause still remain.
To investigate the association between low back pain (LBP), body mass index (BMI), and moderate to vigorous physical activity (MVPA) in a twin sample.
Cross sectional study of monozygotic (MZ) and dizygotic (DZ) twins from the Washington State Twin Registry.
Monozygotic and dizygotic twins from the Washington State Twin Registry
Self-report measures: Low back pain, body mass index, duration and intensity of exercise
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.
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 (OR =3.83; 95% CI =3.28, 4.46). However, quasi-causal regression of LBP on BMI was reduced to zero (OR =0.95; 95% CI =0.60, 1.49). A significant genetic background to BMI and LBP was present (bA =1.66; p <0.001), even after controlling for confounders. In another analysis there was a significant direct effect between MVPA and LBP (bp =-.092, SE =.017, p<.001). In mediation analysis, the effect of MVPA on LBP was partially mediated through MVPA effects on BMI (bPMVPA=-0.043, SE=0.003, p<0.001) and BMI effects on LBP (bPBMI=1.281, SE=0.079, OR=3.6, p<0.001), however shared environmental factors confounded this relationship.
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.