A Mendelian randomization analysis conducted by Børge Nordestgaard of Copenhagen University Hospital, Denmark and colleagues, using data from observational studies, supports a causal relationship between body mass index (BMI) and risk for ischemic heart disease (IHD). The findings, published in this week's PLoS Medicine, have important implications for public health policy because they show that the association between BMI (which is modifiable by lifestyle changes) and IHD is continuous. This means that any increase in BMI increases the risk of IHD; there is no threshold below which a BMI increase has no effect on IDH risk.

The authors state: "This analysis demonstrates the value of observational studies and their ability to provide essentially unbiased results because of inclusion of genetic data avoiding confounding, reverse causation, and bias."