Causal Relationships between Iron Status and Nonalcoholic Fatty Liver Disease: Two-Sample, Multivariable, and Two-Step Mendelian Randomization

Nonalcoholic fatty liver disease (NAFLD) was clinically documented to be accompanied by iron homeostasis imbalances, however, the causal relationship between them remains unclear. Therefore, this study aimed to examine the relationship between iron homeostasis indicators (serum iron, ferritin, transferrin, total iron binding capacity (TIBC), and transferrin saturation (TSAT)) and NAFLD risk.

We applied two-sample Mendelian randomization (MR) to assess the effects of genetic liability to iron homeostasis indicators (N = 43,220-246,139) on NAFLD risk (N = 377,988) in individuals of European ancestry. Reverse direction MR, multivariable MR, and two-step MR were performed to estimate reverse association, causal effects independent of smoking or drinking, and the mediating effect of lipid metabolism, respectively. Smoking and drinking as confounders were considered confounders.

Genetically predicted serum iron, ferritin, and TSAT were significantly associated with a higher risk of NAFLD (odds ratio (OR): 1.286, 95% confidence interval (CI): 1.075-1.539; p = 0.0059; OR: 1.260, 95% CI: 1.050-1.500, p = 0.0195; and OR: 1.223, 95% CI: 1.067-1.402; p = 0.0039, respectively). Reverse direction MR analysis suggested that genetic liability to NAFLD had no significant causal effect on iron homeostasis. Sex-specific MR exhibited a stronger effect size for the association of elevated ferritin with NAFLD risk in males (OR: 1.723, 95% CI: 1.338-2.219; p = 2.48 × 10^-5). Two-step MR revealed that elevated triglycerides (TGs) mediated approximately 3%-5% of the observed effect of serum iron and TSAT on NAFLD risk, while decreased low-density lipoprotein cholesterol (LDL-C) mediated 9%-10%.

Genetic liability to iron status imbalance may causally affect NAFLD. This evidence may support the clinical treatment of NAFLD in the target population.