Fig. 1

Study aims and assumptions. Study aims are to (1) estimate the total effect of adiposity on CRC risk using genetic instruments for BMI and WHR ((i) unadjusted for BMI) and (2) estimate the mediated effect of adiposity on CRC risk by metabolites from targeted NMR metabolomics. Aim 2 is addressed using two approaches: (1) two-step MR wherein effects are examined of adiposity on metabolites (ii) and of adiposity-related metabolites on CRC risk (iii) and (2) multivariable MR wherein effects of adiposity on CRC (i) are examined with adjustment for the effect of representative metabolite classes on CRC (iii). Sex-specific analyses were performed when sex-specific GWAS estimates for exposure and outcome were both available. When ≥ 2 SNP instruments were available, up to 4 MR models were applied: the inverse-variance-weighted (IVW) model which assumes that none of the SNPs are pleiotropic [28], the weighted median (WM) model which allows up to half of the included SNPs to be pleiotropic and is less influenced by outliers [28], the weighted mode model which assumes that the most common effect is consistent with the true causal effect [29], and the MR-Egger model which provides an estimate of association magnitude allowing all SNPs to be pleiotropic [30]. Analyses with metabolites as outcomes were conducted within discovery aims wherein P value thresholds are applied to prioritize traits with the strongest evidence of association to be taken forward into further stages of analysis (with CRC risk). Analyses with CRC as outcomes were conducted within estimation aims wherein P values are interpreted as continuous indicators of evidence strength and focus is on effect size and precision [31, 32]