Estimating the causal influence of body mass index on risk of Parkinson disease: A Mendelian randomisation study
Fig 1
Directed acyclic graph of instrumental variable analysis using genetic variants as proxies for environmental exposures (adapted from Lawlor et al. [14]).
Genetic variants (Z) associated with an exposure such as BMI (X) can be used as proxies to determine the effect of the exposure (X) on the outcome (Y). The three IV assumptions are indicated by arrows or the absence of arrows: (1) the IV in this schematic (FTO gene variant) is robustly associated with the exposure; (2) the IV is not associated with confounding factors (C); and (3) there is no alternative way that the IV affects the outcome other than via the exposure. BMI, body mass index; IV, instrumental variable; PD, Parkinson disease.