Association between fatty liver index and low back pain: A cross-sectional study from NHANES 1999–2004

Objective The Fatty Liver Index (FLI) is a non-invasive algorithm that estimates the presence of hepatic steatosis based on body mass index (BMI), waist circumference, triglycerides, and gamma-glutamyl transferase (GGT). In recent years, FLI has been increasingly recognized for its associations with various metabolic disorders. Whether FLI is linked to low back pain (LBP) remains unclear. This study aims to explore the association between FLI and LBP. Methods Participants from the NHANES 1999–2004 survey were included. LBP was defined by self-reports. FLI was calculated from metabolic indicators using a standard formula. Logistic regression models assessed the association between FLI and LBP, adjusting for potential confounders, with restricted cubic splines to check for nonlinearity. Subgroup analyses and sensitivity analyses using multiple imputation for missing data were also conducted. Results A total of 5339 participants were included. Higher FLI levels were significantly associated with increased odds of LBP. For each 20-unit increase in FLI, the risk of LBP increased by 9% (OR = 1.09, 95% CI: 1.03, 1.14, P = 0.0017). The highest quartile of FLI had a 42% higher risk of LBP compared to the lowest quartile (OR = 1.42, 95% CI: 1.13, 1.79, P = 0.0029). Subgroup analysis showed a stronger association among individuals with lower physical activity levels. Sensitivity analyses confirmed robustness. Conclusions Higher FLI values corresponded to an increased risk of LBP, especially in individuals with lower physical activity. Further studies are needed to validate this relationship.