Jia et al., Metabolites, 2023, 13, 16 pages. DOI: 10.3390/metabo13101041
Background & Rationale
Inflammatory bowel disease (IBD), comprising Crohn’s disease (CD) and ulcerative colitis (UC), affects up to 0.5% of the western population. Omega-3 polyunsaturated fatty acids (PUFAs) have been proposed as preventive agents due to their anti-inflammatory properties, but epidemiological evidence remains conflicting. Previous randomised controlled trials combined eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) despite their distinct molecular functions, making it difficult to identify which component confers benefit. This study applied Mendelian randomisation (MR) to estimate the causal effects of individual omega-3 components on IBD risk and explore potential metabolic mediators.
Study Design
This was a two-sample Mendelian randomisation study using publicly available genome-wide association study (GWAS) summary statistics. Genetic instruments for total omega-3 fatty acid were derived from UK Biobank (approximately 500,000 participants aged 40 to 69 years, European ancestry). Genetic instruments for α-linolenic acid, EPA, and DHA were obtained from 8,866 participants of European ancestry from the CHARGE Consortium. Outcome data for IBD, CD, and UC were obtained from the International Inflammatory Bowel Disease Genetics Consortium (IIBDGC), comprising 59,957 European participants (25,042 IBD cases and 34,915 controls). Independent single nucleotide polymorphisms (SNPs) associated with exposures at genome-wide significance and in linkage disequilibrium r² less than 0.01 were selected as instruments. The primary endpoint was the odds ratio for IBD, CD, and UC per genetically predicted unit increase in each omega-3 component. Secondary analyses included colocalization analysis in the FADS2 gene region, bidirectional MR to assess reverse causation, and mediation MR analysis using 974 circulating metabolites. The inverse variance weighted (IVW) method was the primary analysis approach.
Patient Population
Summary-level genetic data were derived from populations of European ancestry. IBD cases were diagnosed using accepted endoscopic, histopathological, and radiological criteria. Mean F-statistics for all exposures ranged from 29.82 to 262.21, indicating strong instrument validity.
Key Findings
Using the IVW method, genetically predicted higher EPA concentrations were associated with decreased IBD risk (odds ratio 0.78, 95% confidence interval 0.63 to 0.98), with no evidence of heterogeneity or pleiotropy. For CD specifically, EPA showed a stronger protective effect (odds ratio 0.67, 95% confidence interval 0.50 to 0.91), but the effect on UC was weaker and non-significant (odds ratio 0.88, 95% confidence interval 0.68 to 1.14). There was little evidence supporting causal effects of total omega-3 fatty acid, α-linolenic acid, or DHA on IBD risk. Leave-one-out analyses indicated effects were largely driven by SNPs in the FADS2 gene region. Colocalization analysis provided strong evidence (posterior probability greater than 94%) that omega-3 PUFAs and CD shared causal variants in the FADS2 region, but little evidence for UC (posterior probability less than 7%). Bidirectional MR suggested genetic liability to UC was associated with lower DHA levels, but no reverse causation was detected for IBD or CD. Mediation analysis identified that EPA influenced IBD risk via α-linolenic acid and linoleic acid metabolism, and methylhistidine metabolism pathways. Linoleic acid mediated 58.33%, histidine mediated 66.67%, and DHA mediated 50.00% of the total effect of EPA on IBD.
Discussion
The authors report that EPA is the key component of omega-3 PUFAs conferring protective effects against IBD, particularly CD. This effect is mediated through lower levels of linoleic acid and histidine, both associated with increased IBD risk. The FADS2 gene, encoding delta-6 desaturase, plays a central role in omega-3 metabolism and mediates the relationship between omega-3 PUFAs and CD. The lack of effect on UC may reflect impaired intestinal absorption or abnormal omega-3 metabolism in UC patients. The colocalization analysis strengthens causal inference by demonstrating that the same genetic variants in the FADS2 region influence both omega-3 levels and CD risk.
Authors’ Conclusions
The authors conclude that EPA is the predominant active component of omega-3 fatty acids associated with decreased IBD risk, mediated via interaction with linoleic acid and histidine metabolites. The FADS2 gene appears to mediate the effects of omega-3 PUFAs on IBD risk. The findings suggest that supplementation or dietary intake of EPA, rather than α-linolenic acid or DHA, may be more effective for preventing IBD, particularly CD. The authors acknowledge limitations including different data sources for exposures, assumed linear relationships, and inability to directly assess the EPA-to-DHA ratio. Well-powered epidemiological studies and clinical trials are needed to explore potential benefits of high EPA concentration or EPA/DHA supplementation in IBD and its subtypes.
Reference
Jia X, Hu C, Wu X, Qi H, Lin L, Xu M, Xu Y, Wang T, Zhao Z, Chen Y, Li M, Zheng R, Lin H, Wang S, Wang W, Bi Y, Zheng J, Lu J. Evaluating the Effects of Omega-3 Polyunsaturated Fatty Acids on Inflammatory Bowel Disease via Circulating Metabolites: A Mediation Mendelian Randomization Study. Metabolites. 2023;13(10):1041. doi:10.3390/metabo13101041
This Scientific Publication Summary is an objective summary of the published trial for personal and educational use. It does not constitute clinical advice, endorsement of the intervention, or a recommendation to alter clinical practice.