In a Chinese cohort spanning wide socioeconomic conditions, consuming under one egg per week is an unusually low intake that tends to co-occur with poverty, food insecurity and prevalent poor health, all of which independently raise CVD and mortality risk. A single baseline FFQ also cannot separate people who habitually eat few eggs from those who reduced intake because of pre-existing illness (reverse causation).
Both mechanisms push the low-intake hazard ratios (CVD 1.22, mortality 1.29) upward relative to the true causal effect of low egg intake. The left arm of the U is thus the more confounding-prone half, and the apparent harm of very low intake should be discounted more heavily than the harm at the high-intake end, which is less tied to deprivation. This is a standard consideration for interpreting the low-exposure arm of a nutritional U-shape and does not depend on the exact numbers; it weakens (without eliminating) the low-intake half of the U-shaped reading.
Validity (step 6)
status: approved | reason_if_not_false: checked
Traced the step directly (standard confounding/reverse-causation direction-of-bias reasoning, no specialist formalism). Premises: (i) very low egg intake positively correlates with poverty/food insecurity/poor baseline health, which independently raise CVD and mortality; (ii) a single baseline FFQ conflates habitual low eaters with those who cut intake due to prevailing illness; (iii) the high-intake arm is less tied to deprivation. Conditional on these, a confounder that both raises the outcome and correlates with the exposure category biases that category’s HR upward, and reverse causation adds a further upward push - so the <1/week HR overstates the causal harm of low intake, and the comparative claim (discount the left arm more than the right) follows from premise (iii). The conclusion is already appropriately hedged (“liable to inflation”, “probably overstates”, “weakens without eliminating”). No undercutting defeater survives that does not deny a premise: the only route to reversing the sign of bias would require the SES/health confounders to be net-protective in this cohort, which contradicts premise (i). Holds as stated.