Figure 1.

Analytical model and three hypotheses based on Mendelian randomisation (MR) analysis used to study the causal relationship between different modalities of vitamin B supplementation and Alzheimer’s disease and the moderating role of gut bacteria. SNP: single-nucleotide polymorphism. A. The instrumental variables (IVs) must be significantly connected to the exposure. B. The IVs cannot be connected to any known confounders that could alter the association between an exposure and an outcome. C. The IVs must be unrelated to the outcomes and may only affect the outcomes through their effects on the exposure. D: Reverse causation from the outcome to exposure. This figure presents a diagram of our MR study. The dashed lines indicate irrelevance, and the solid lines indicate relevance. In general, we had already determined the causal effect of exposure on the outcome (assuming that it was β0) before performing the two-step MR analysis. Step (1): Significant SNPs were identified in the exposed GWAS findings, SNPs with cascade imbalances were removed, and then the remaining SNP information was extracted from the GWAS findings of the mediator variable; here, it was necessary to make sure that the remaining SNPs could not be directly correlated with the confounders or the mediator variable. Finally, we could calculate the causal effect of exposure on the mediator variable (assuming that it was β1). Step (2): Significant SNPs were identified in the GWAS results of the mediator variable, SNPs with cascade imbalances were removed, and then, the remaining SNP information was extracted from the GWAS results of the outcome; here, it was necessary to ensure that the remaining SNPs could not be directly related to the confounders or the outcome. Finally, we could calculate the causal effect of the mediating variable on the outcome (assuming that it was β2).