The potential causal relationship between fat mass in different body areas and low back pain: Findings from NHANES and Mendelian randomization studies

The purpose of this study is to investigate and discuss the potential relationship between fat distribution in different body regions and low back pain (LBP). We used 2 research methods to study the impact of fat distribution on LBP. First, we conducted a cross-sectional study using National Health and Nutrition Examination Survey data to examine the relationship between fat levels in the trunk and lower limbs and LBP. Second, we employed a two-sample Mendelian randomization approach to investigate the causal relationship between trunk fat, lower limb fat, and LBP. In the cross-sectional study, the results of multivariable logistic regression analysis showed that for every unit increase in trunk fat mass (FM) (kg), the risk of LBP increased by 2.8% (odds ratios, OR = 1.028, 95% confidence intervals [CI] [1.021, 1.034], P < .00001). Similarly, there was a positive correlation between right leg FM, left leg FM, and LBP (fully adjusted model: OR = 1.075, 95% CI [1.053, 1.097], P < .00001; OR = 1.076, 95% CI [1.053, 1.099], P < .00001). Mendelian randomization analysis indicated that an increase in trunk FM had a detrimental effect on LBP (OR = 1.296, 95% CI [1.207-1.392], P = 9.86 × 10-13), while an increase in left lower limb FM had a protective effect on LBP (OR = 0.672, 95% CI [0.604-0.748], P = 2.61 × 10-13), and an increase in right lower limb FM also had a protective effect on LBP (OR = 0.655, 95% CI [0.590-0.728], P = 3.32 × 10-15). The study revealed a positive causal relationship between trunk FM and LBP. However, the causal relationship between FM in the lower extremities and LBP remains controversial. Nonetheless, investigating lower extremity FM as a potential protective factor in depth is a direction worth focusing on in the future.