A two-sample Mendelian randomization study using 60,801 coronary artery disease cases from CARDIoGRAMplusC4D found that metabolically healthy adiposity is actually protective against coronary artery disease (OR 0.28, 95% CI 0.19–0.42), while metabolically dysfunctional obesity (OR 1.90) and metabolically dysfunctional leanness (OR 1.86) confer statistically indistinguishable coronary risk. Critically, the two high-risk phenotypes operate through divergent mediators: unhealthy obesity acts through triglycerides and diabetes, while unhealthy leanness acts through apolipoprotein B, blood pressure, and diabetes — with no shared conduit beyond diabetes itself.
This finding carries significant clinical implications that extend well beyond the abstract. The result directly challenges BMI-centric cardiovascular screening: lean individuals with metabolic dysfunction may be systematically under-identified as high-risk, precisely because current frameworks — including the landmark 2025 Lancet Commission obesity redefinition — require excess adiposity as a prerequisite. The apolipoprotein B pathway identified in lean dysfunction aligns with established atherogenic lipoprotein research, lending biological plausibility.
However, critical caveats apply. Mendelian randomization assumes no pleiotropy and valid instruments — both assumptions are contested in complex metabolic traits. The study is a preprint not yet peer-reviewed, meaning methodology and conclusions require independent scrutiny before clinical translation. That said, the replication with an independent instrument sharing only five of 53 variants strengthens confidence. If validated, this is a paradigm-shifting result: coronary risk reflects adipose storage failure, not adipose quantity.
