According to the criteria for IV selection, a total of 2,818 SNPs were identified and selected as IV associated with gut microbiota. The F-statistics for these IVs all exceed 10, suggesting that the estimated coefficients are improbable to be influenced by the bias caused by weak instruments. Supplementary Table 1 and Supplementary Table 2 provides detailed information about the selected IVs.
The majority of gut microbiota showed no significant correlation with AD. However, using the IVW method, we identified 16 bacterial features that were significantly associated with the risk of AD (Supplementary Table 3).
We used three methods, IVW, weighted median and MR-Egger, and defined p < 0.05 for IVW method screening as a positive result. Therefore, a total of 16 bacterial genera were identified to have a statistically significant causal relationship with AD based on IVW method (Figs. 3 and 4). Nine of the bacteria are negatively correlated with AD, indicating that they may have a protective effect on AD. They are: class Methanobacteria (OR = 0.9933, 95% CI, 0.9880–0.9987, p = 0.0150), family Methanobacteriaceae (OR = 0.9933, 95% CI, 0.9880–0.9987, p = 0.0150), family Porphyromonadaceae (OR = 0.9789, 95% CI, 0.9616–0.9966, p = 0.0196), genus Blautia (OR = 0.9838, 95% CI, 0.9725–0.9952, p = 0.0056), genus Butyricicoccus (OR = 0.9859, 95% CI, 0.9739–0.9981, p = 0.0233), genus ErysipelotrichaceaeUCG003 (OR = 0.9914, 95% CI, 0.9833–0.9995, p = 0.0381), genus Parasutterella (OR = 0.9911, 95% CI, 0.9823–0.9999, p = 0.0478), order Methanobacteriales (OR = 0.9933, 95% CI, 0.9880–0.9987, p = 0.0150), phylum Proteobacteria (OR = 0.9867, 95% CI, 0.9759–0.9977, p = 0.0176). Supplementary Table 4 shows the completed data. In sensitivity analysis, MR-Egger, weighted median demonstrated consistent results, except for family Porphyromonadaceae and genus Erysipelotrichaceae UCG003, where the MR-Egger trend was in the contrary direction compared to IVW and weighted median.
Another seven species showed a positive correlation with AD, class Negativicutes(OR = 1.0131, 95% CI, 1.0022–1.0241, p = 0.0183), genus Eubacteriumbrachygroup (OR = 1.0068, 95% CI, 1.0010–1.0127, p = 0.0225), genus Coprococcus3 (OR = 1.0164, 95% CI, 1.0046–1.0285, p = 0.0065), genus Enterorhabdus (OR = 1.0117, 95% CI, 1.0027–1.0208, p = 0.0108), genus Oxalobacter (OR = 1.0067, 95% CI, 1.0009–1.0125, p = 0.0231), genus Ruminiclostridium6 (OR = 1.0129, 95% CI, 1.0048–1.0212, p = 0.0019), order Selenomonadales (OR = 1.0131, 95% CI, 1.0022–1.0241, p = 0.0183) (Supplementary Table 4). In the MR-Egger method, the trends of class Negativicutes, genus Eubacteriumbrachygroup and order Selenomonadales are different from those of the IVW and WM methods.
In horizontal pleiotropy analysis, we used the MR-Egger method and found p value of the MR-intercept were all greater than 0.05. In addition, further MR PRESSO analysis was conducted, ruling out the existence of horizontal pleiotropy (P > 0.05) (Supplementary Table 5 and Supplementary Table 6). To assess the heterogeneity of gut microbiome IVs, we employed Cochran's Q test statistics, which revealed no heterogeneity among the gut microbiome IVs (P > 0.05) (Supplementary Table 7).
Reverse MR analyses were conducted to examine the links between 16 bacterial traits and AD. No significant statistical relationship was observed using the IVW method: class Methanobacteria (OR = 0.7314, 95% CI, 0.2217–2.4124, p = 0.6075), class Negativicutes (OR = 0.7112, 95% CI, 0.4186–1.2085, p = 0.2077), family Methanobacteriaceae (OR = 0.7309, 95% CI, 0.2216–2.4106, p = 0.0150), family Porphyromonadaceae (OR = 0.8716, 95% CI, 0.4908–1.5480, p = 0.6392), genus Eubacteriumbrachygroup (OR = 1.4058, 95% CI, 0.4060–4.8674, p = 0.5909), genus Blautia (OR = 0.9453, 95% CI, 0.5572–1.6038, p = 0.8348), genus Butyricicoccus (OR = 0.9834, 95% CI, 0.5704–1.6952, p = 0.9518), genus Coprococcus3 (OR = 0.8886, 95% CI, 0.5040–1.5667, p = 0.6831), genus Enterorhabdus (OR = 1.0383, 95% CI, 0.4168–2.5868, p = 0.9356), genus Erysipelotrichaceae-UCG003 (OR = 0.6593, 95% CI, 0.3556–1.2221, p = 0.1858), genus Oxalobacter (OR = 1.2849, 95% CI, 0.4021–4.1051, p = 0.6724), genus Parasutterella (OR = 0.7245, 95% CI, 0.3713–1.4136, p = 0.3447), genus Ruminiclostridium6 (OR = 0.7095, 95% CI, 0.3825–1.3162, p = 0.2764), order Methanobacteriales (OR = 0.7314, 95% CI, 0.2217–2.4124, p = 0.6075), order Selenomonadales (OR = 0.7112, 95% CI, 0.4186–1.2085, p = 0.2077), phylum Proteobacteria (OR = 0.8970, 95% CI, 0.5284–1.5226, p = 0.6871) (Supplementary Table 8 and Supplementary Table 9).