Are obesity and body fat distribution associated with low back pain in women? A population-based study of 1128 Spanish twins

To investigate the relationship between different measures of obesity and chronic low back pain (LBP) using a within-pair twin case-control design that adjusts for genetics and early shared environment. A cross-sectional association between lifetime prevalence of chronic LBP and different measures of obesity (body mass index-BMI; percent body fat; waist circumference; waist-hip ratio) was investigated in 1128 female twins in three stages: (i) total sample analysis; (ii) within-pair case-control analysis for monozygotic (MZ) and dizygotic (DZ) twins together; (iii) within-pair case-control analysis separated by DZ and MZ. Odds ratios (OR) and 95 % confidence intervals (CI) were calculated. BMI (OR 1.12; 95 % CI 1.02-1.26) and percent body fat (OR 1.15; 95 % CI 1.01-1.32) were weakly associated with lifetime prevalence of chronic LBP in the total sample analysis but were absent when shared environment and genetic factors were adjusted for using the within-pair case-control analysis. Greater waist-hip ratios were associated with smaller prevalence estimates of chronic LBP in the within-pair case-control analysis with both MZ and DZ twins (OR 0.67; 95 % CI 0.47-0.94). However, this association did not remain after the full adjustment for genetic factors in the MZ within-pair case-control analysis. BMI, percent of fat mass and greater depositions of fat and mass around the hips are associated with increases in chronic LBP prevalence in women but these associations are small and appear to be confounded by the effects of genetics and early shared environment. Therefore, our results do not support a causal direct relationship between obesity and chronic LBP.