Associations of fatty acids with the risk of biliary tract calculus and inflammation: a Mendelian randomization study
Authors: Xie Xing-Ming , Liu Tao , Wang Guo-Ying 
Published: January 2024

Abstract
Background
The presence of gallstones in both the gallbladder and bile ducts is referred to as cholelithiasis. The prevalence of cholecystolithiasis and bile duct stones differs. Observational and Mendelian randomization (MR) studies have elucidated the significant contributing role of numerous fatty acids (FAs) in the development of cholelithiasis. Despite numerous studies about cholelithiasis, evidence on the relationship between serum FA levels and cholecystolithiasis, as well as bile duct stones with or without inflammation, remains insufficient.

Methods
A two-sample MR study was designed to clarify the impact of serum FA levels on various bile duct inflammatory diseases. The summary statistics of single nucleotide polymorphisms (SNPs) associated with fatty acids were obtained from the UK Biobank (UKB) and included data from 114,999 participants. The researchers obtained GWAS summary statistics for cholecystolithiasis and bile duct stones in 463,010 and 361,194 European participants, including cases with and without inflammation. No sample overlap between the exposure and outcome was verified through the “mr-lap” package. The SNPs were screened to identify instrumental variables (IVs). Cochran’s Q test was applied for heterogeneity assessment. Inverse variance weighting (IVW) (fixed effects or random effects), MR-Egger regression and weighted median methods were used for MR. Multivariable MR was applied to determine the direct effect of each exposure on the outcome. A false discovery rate (FDR) was applied to adjust for multiple testing correction based on the Benjamini-Hochberg method. Finally, the FinnGen Consortium was used to validate some results.

Results
The overall concentration of polyunsaturated fatty acids (PUFAs) in the serum was negatively associated with the risk of calculus of the gallbladder with acute cholecystitis (IVW, OR = 0.996, P = 0.038, CI 0.992–0.999; weighted median, OR = 0.995, P = 0.025, CI 0.991–0.999). The percentage of PUFAs to total monounsaturated fatty acids(MUFAs) (IVW, OR = 0.998, P = 0.045, CI 0.997–0.999) and the percentage of PUFAs to total FAs (IVW, OR = 0.997, P = 0.025, CI 0.995–0.999) had a protective role against cholecystitis. The percentage of PUFAs to total FAs had a protective role against calculus of the gallbladder without cholecystitis (IVW, OR = 0.995, P = 0.026, CI 0.990–0.999; MR Egger, OR = 0.99, P = 0.03, CI 0.982–0.998; weighted median, OR = 0.991, P = 5.41e-06, CI 0.988–0.995). Conversely, the percentage of MUFAs to total FAs increased the risk for cholecystitis (IVW, OR = 1.001, P = 0.034, CI 1.0001–1.002). However, there were no causal effects of the above exposures on the outcomes through multivariable MR and multiple testing correction. Finally, the causal effects of the above exposures on cholecystitis were validated in the FinnGen Consortium, which suggested that the percentage of PUFAs to total FAs (IVW, OR = 0.744, P = 0.021, CI 0.579–0.957) had a protective role against cholecystitis.

Conclusion
These Mendelian randomization findings suggested that more attention should be focused on people who have low serum PUFA levels, which may have a potential role in the occurrence of calculus of the gallbladder or cholecystitis rather than calculus of the bile duct without cholangitis or cholecystitis.