A substitution analysis can only lower a person’s intake of a food by up to their current intake, so the modelled “one serving/day” reduction of eggs is counterfactually available only to participants who already consume >=1 serving/day of eggs. Here that is 8.0% of the sample, versus 58.5% at >=1 serving/week (for whom the modelled reduction is only ~2-3%, HRs ~0.97-0.98).
The steep per-day hazard ratios (down to 0.79 for eggs→nuts, incident CVD) therefore describe a minority with high intake and “greater room for reducing intake.” Averaged over a population most of whom eat far fewer eggs, the attainable reduction from replacing eggs is closer to the small one-serving/week estimates. This is an applicability/weighting inference: it leaves every hazard ratio unchanged but caps the population-level effect implied by the large per-day numbers, and it locates the meaningful egg-reduction benefit at high habitual intakes rather than across the whole intake range.
Validity assessment (step 6)
Traced step: a one-serving/day substitution is counterfactually available only to someone whose current intake is >=1 serving/day; at 8% of the sample, the large per-day HRs describe that minority, and averaged over an intake distribution dominated by weekly-or-less eaters (for whom the attainable reduction, and thus the risk delta, is a small fraction of a serving/day) the population-average benefit is necessarily much smaller. No undercutting defeater survives: even under a fully linear per-serving dose-response the low-intake majority can only realise a proportionally tiny reduction, which is exactly the conclusion. The inference holds as stated (an applicability/weighting move, HRs untouched). approved / checked.