Study design and MR assumptions
This study utilized a publicly available Genome-Wide Association Study (GWAS) to make causal inferences between exposure to 26 types of diets, including the consumption of coffee, tea, milk, yoghurt, cheese, cereals, bread, oily fish, non-oily fish, beef, mutton, pork, bacon, processed meat, cooked vegetables, raw vegetables, fresh fruit, dried fruit, pickled nuts, unsalted nuts, pickled peanuts, unsalted peanuts, red wine, beer, saturated fatty acids, and polyunsaturated fatty acids, and the outcome of aSAH. The primary method utilized in this study was the inverse-variance weighted (IVW) method, with multiple sensitivity analyses conducted to ensure the reliability of the results [16]. The present MR study was required to adhere to three fundamental assumptions of relevance, independence, and exclusivity: 1) instrumental variables must be strongly correlated with exposure factors; 2) instrumental variables could not be associated with any confounding factors associated with "exposure-outcome"; and 3) instrumental variables can solely influence outcome variables through exposure factors [17] (Fig. 1).
Since the data utilized in this study were derived from published summary statistics of GWASs, ethical approval and informed consent were not deemed necessary. The current study adhered to the guidelines set forth by the Strengthening the Reporting of Observational Studies in Epidemiology-Mendelian Randomization (STROBE-MR) for its design. A step-by-step flowchart illustrating the study design is shown in Fig. 2.
Data sources
GWASs data for different diets were obtained from the UK Biobank [18]. Summary-level data of aSAH were extracted from recent GWASs on intracranial aneurysms [19]. A pooled data set of SAH (5140 cases and 71,934 controls) was extracted from the meta-analysis of GWASs from the International Stroke Genetics Union [19]. The analysis was limited to 77,074 European individuals to reduce the population stratification bias. Detailed information on the summary-level data included in this study is provided in Table 1.
Table 1
The summary of information regarding data resources used in our study.
Exposure | Sample Size | p-value | Consortium | Access Link |
Milk intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-2966/ |
Yogurt intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-7753/ |
Salted peanuts intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-1099/ |
Unsalted peanuts intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-15555/ |
Salted nuts intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-15960/ |
Unsalted nuts intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-12217/ |
Coffee intake | 428,860 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-5237/ |
Tea intake | 447,485 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-6066/ |
Cheese intake | 451,486 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-1489/ |
Cereal intake | 441,640 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-15926/ |
Bread intake | 452,236 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-11348/ |
Oily fish intake | 460,443 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-2209/ |
Non-oily fish intake | 460,880 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-17627/ |
Beef intake | 461,053 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-2862/ |
Lamb intake | 460,006 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-14179/ |
Pork intake | 460,162 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-5640/ |
Bacon intake | 64,949 | 5 × 10− 6 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-4414/ |
Processed meat intake | 461,981 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-6324/ |
Cooked vegetable intake | 448,651 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-8089/ |
Raw vegetable intake | 435,435 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-1996/ |
Fresh fruit intake | 446,462 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-3881/ |
Dried fruit intake | 421,764 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-16576/ |
Red wine intake | 327,026 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-5239/ |
Beer intake | 327,634 | 5 × 10− 8 | MRC-IEU | https://gwas.mrcieu.ac.uk/datasets/ukb-b-5174/ |
Saturated fatty acids | 114,999 | 5 × 10− 8 | NA | https://gwas.mrcieu.ac.uk/datasets/met-d-SFA/ |
Polyunsaturated fatty acids | 114,999 | 5 × 10− 8 | NA | https://gwas.mrcieu.ac.uk/datasets/met-d-PUFA/ |
Outcome | | | | |
aSAH | 77074 (5140 cases, 71934controls) | NA | International Stroke Genetics Union | http://www.cerebrovascularportal.org/ |
Note: aSAH: aneurysmal subarachnoid hemorrhage; MRC-IEU: Medical Research Council-Integrative Epidemiology Unit; NA: not available. |
Selection of IVs
To identify single-nucleotide polymorphism (SNP) loci that were significantly associated with different dietary habits, a threshold of P < 5e− 8 was set. Because the SNPs contents of milk, yoghurt, salted nuts, salt-free nuts, salted peanuts, salt-free peanuts, and bacon were low, a loose threshold (P < 5e− 6) was set for these diets to include more IVs [20]. To reduce the linkage disequilibrium of different SNP loci, screening conditions of a clumping window < 10,000 kb and linkage disequilibrium level R2 < 0.001 were set [21]. To test the second key hypothesis, the phenotype scanner database (P < 5e− 8) was used to evaluate the subphenotypes of selected SNPs [22]. According to clinical guidelines [23], blood pressure is a risk factor for aSAH. Therefore, the SNPs associated with blood pressure were excluded by screening the Phenoscanner website (http://www.phenoscanner.medschl.cam.ac.uk/). Simultaneously, SNPs related to aSAH were excluded to prevent contravening the third key hypothesis that an IV was not directly related to the outcome. The F statistic was computed to assess the presence of a weak instrumental variable offset in the chosen instrumental variable. The following formula used was used: F = [R2 × (N-2)]/(1-R2), where N represents the sample size of exposure factors and R2 denotes the proportion of exposure factors explained by IVs. To further test the correlation assumption, an F value of SNPs higher than 10 indicated that our study avoided weak instrumental bias [24].
MR analysis
In the two-sample MR, the IVW method with multiplicative random effects was used to estimate the causal effect of different diets on aSAH, by performing a meta-analysis of the Wald ratio of a single SNP and assuming that genetic variation could only affect the results through exposure of interest rather than through other pathways [25]. The IVW method may be affected by instrumental bias or pleiotropic effects. Therefore, two complementary analysis methods were employed: the weighted median and MR-Egger methods. The weighted median method is capable of tolerating high levels of pleiotropy and yields robust estimates, particularly when more than half of the SNPs serve as valid IVs [26]. The MR-Egger method was used to obtain estimates after correcting for pleiotropic effects.
MR sensitivity analysis
MR-Egger regression and MR pleiotropic repetition and outliers (MR-PRESSO) were used to assess and correct the potential level pleiotropy among the selected IVs [27]. The MR-Egger intercept and homodyne can provide evidence of directional multi-directionality. To mitigate the bias stemming from horizontal pleiotropy, which affects outcomes via causal pathways other than exposure, MR-PRESSO was used to detect a wide range of horizontal pleiotropies for all outcomes. To evaluate the robustness of the results, further heterogeneity tests were performed on statistically significant results using the MR-Egger intercept test, sensitivity analysis, and Cochran’s Q statistics [28]. The leave-one-out method was used to determine whether causal inference was affected by a specific SNP locus. In addition, forest plots, scatter plots, funnel plots, and leave-one-out analysis plots were used to visualize the results with high reliability. In particular, the forest plots intuitively showed the influence of each SNP on the results, the leave-one-out method determined whether the results were visually robust, and the scatter plot showed the fitting outcomes of various MR analyses. The funnel plot intuitively judged the heterogeneity of IVs [29].
Statistical analysis
All statistical tests were two tailed. For dichotomous variables, effect estimates were converted into odds ratios (OR) to observe the relationship between diet and aSAH more intuitively. For the MR analysis, the P-value of the IVW method was key to demonstrating a causal association. The Bonferroni correction was used to mitigate the risk of false-positive results due to multiple tests, with a statistical significance of 0.0019 (0.05/26 [26 exposures and 1 outcome]). A P < 0.05, but higher than the statistical significance of the Bonferroni correction, was considered as implied evidence of potential causality. For sensitivity analysis, a P < 0.05 indicated significant heterogeneity and horizontal pleiotropy. All MR analyses performed in this study were carried out using the R software (version 4.2.1) and analyzed utilizinging the "Two Sample MR" package (version 0.5.6 ) [30].