Reasoning (A-10): Two features of the data jointly discriminate a causal from a marker interpretation. First, the candidate causal mediators of an egg effect are its cholesterol and protein content, which are constant across countries; yet in COSM, additionally adjusting the model for cholesterol and protein intake did not substantially alter the (already near-null) egg-T2D estimate. If cholesterol or protein were driving risk, controlling the association’s own proposed mechanism should have moved it - it did not, arguing the residual signal is not nutrient-mediated. Second, the association is present only in US cohorts and, notably, is internally consistent there (I2 0% at HR 1.18) while absent in non-US cohorts (0.97). A universal biological effect of eggs cannot switch on only in the US; what differs is the diet eggs accompany - a Western pattern (red/processed meat, fried potatoes, high-fat dairy, sweets, refined grains) in the US versus prudent/traditional patterns where eggs are eaten in Japan. Register-based outcome ascertainment in COSM (low misclassification) makes it unlikely the non-US null is an artefact of missed cases. Taken together, egg intake behaving as a proxy for the local dietary pattern explains both the geographic on/off pattern and the mechanism-adjustment null more parsimoniously than a direct causal effect. The argument is inferential, not deductive: unmeasured effect-modification by a genuinely causal egg effect remains logically possible.
Validity (step 6)
status: approved, reason_if_not_false: checked.
Traced both prongs. Prong 1 (mediator-adjustment): if eggs raise T2D risk through their cholesterol/protein content, conditioning on cholesterol and protein intake should attenuate the egg coefficient; it did not move → those specific nutrients are not the operative mediators. The step is a valid mediation inference. It is weak here (COSM is already near-null, so there is little to attenuate and the mediator test was run in a cohort showing no effect), but weakness is a matter of strength, not validity, and the statement is framed as “better explained by,” not a proof. Prong 2 (geographic on/off): a universal biological mechanism cannot switch on only in US cohorts, so an I2=0% signal at 1.18 confined to the US while non-US is 0.97 points to something that co-varies with geography — the accompanying diet — rather than the egg itself.
The statement is a comparative/parsimony claim (“better explained by … than by”), and both prongs push the same way, so the evidential step holds. The obvious undercutting defeater — effect-modification of a genuinely causal egg effect by the surrounding Western diet, which would also produce a US-only signal — is explicitly acknowledged and only downgrades the argument to evidential (which it already is), not refuted. Approved as stated.