Correlation of serum cotinine with fatty liver index in adults: data from the NHANES March 2017 and 2018

The correlation between serum cotinine and fatty liver index (FLI) needs further investigation for the early identification, prevention, and treatment of metabolic dysfunction-associated steatotic liver disease (MASLD).

Data from the NHANES database spanning from March 2017 to 2018 was used to perform the population-based study to assess the relationship between serum cotinine and FLI. A variance estimation strategy was applied to address the data volatility. To examine the correlation between serum cotinine and FLI, a weighted multivariate logistic regression model was used. Initial normality assessment through the Kolmogorov-Smirnov test indicated non-normal distribution. Median and interquartile range were employed for description of non-normally distributed measurement data, and group comparisons were made using the Kruskal-Wallis H test. Proportions were used for ordinal data description and comparisons, with the chi-square test used for categorical data. Smooth curve fittings and generalized additive models were used to explore the non-linear relationship between serum cotinine and FLI.

Finally, 2350 subjects (mean age: 49.83 ± 18.30, 1135 males and 1215 females) were selected for analysis. After adjusting for confounders, serum cotinine showed positive correlation with FLI in adults (β = 0.009, 95% CI: 0.003 to 0.014, P = 0.001). Additionally, individuals in the unexposed and passively exposed groups had lower FLI compared to those in the actively exposed group (β = -3.041, 95% CI: -4.728 to -1.353, P < 0.001; β = -2.159, 95% CI: -4.231 to -0.087, P = 0.041; respectively). Subgroup analyses by gender revealed positive associations between serum cotinine and FLI in both males (β = 0.007, 95% CI: 0.000 to 0.014, P = 0.048) and females (β = 0.012, 95% CI: 0.003 to 0.021, P = 0.007). Additionally, a positive correlation was found in "other races" subgroup (β = 0.017, 95% CI: 0.004 to 0.029, P = 0.008) rather than the subgroups of "Mexican American, Other Hispanic, Non-Hispanic White, and Non-Hispanic Black". The relationship between serum cotinine and FLI exhibited an inverted U-shaped curve with the turning point occurring at 521 ng/mL.

This study of a nationally representative sample demonstrates a positive association between serum cotinine and FLI, characterized by an inverted U-shaped curve. Both active and passive smoking emerge as a risk factor for the development and progression of MASLD. Smoking cessation is recommended to manage MASLD and support liver and cardiovascular health.