Skip to main content
Fig. 1 | BMC Medicine

Fig. 1

From: Exploring causality in the association between circulating 25-hydroxyvitamin D and colorectal cancer risk: a large Mendelian randomisation study

Fig. 1

a Conceptional framework of Mendelian randomisation (MR). The instrumental variable is based on genome-wide significant single nucleotide polymorphisms from independent studies of the association between the exposure of interest (serum 25-hydroxyvitamin D (25-OHD) concentrations) and the outcome (colorectal cancer (CRC)). The effect of an instrumental variable should be independent from the confounding factors and should affect CRC risk only through exposure. In the presence of a causal relationship, the association between instrumental variable and CRC would be expected to be proportionate to its association with the serum 25-OHD concentrations, given the relationship between the serum 25-OHD concentrations and CRC risk. Figure adapted from Timpson et al. [65]. b Basic design of our MR on the causal effect of 25-OHD on CRC risk. The blue text of outer contour showed individual level MR analysis. β1 is the regression coefficient of instrumental variable (IV) on exposure (25-OHD level) using controls from the Scotland Colorectal Cancer Study (SOCCS); β2 is the regression coefficient of IV on outcome (CRC) using SOCCS series, Croatia and UK biobank case control studies. Causal effect is estimated by the ratio of β2 and β1. The red text of inner contour showed summary statistics MR analysis. Effect sizes of IVs on 25-OHD and CRC are extracted from two GWAS meta-analyses and causal estimate is derived from an inverse variance-weighted MR analysis

Back to article page