[mrob]

>People that aren't consuming eggs might do so because of health issues.

The authors are aware of this, and excluded people suffering from several common health problems for this reason:

"In the present study, we excluded individuals reporting medical histories of cancer (n=2577), heart disease (n=15 472) or stroke (n=8884), or having prevalent diabetes (n=30 300) defined by self-reported diabetes or on-site plasma glucose testing (fasting blood glucose ≥7.0 mmol/L or random blood glucose ≥11.1 mmol/L). We made these exclusions to avoid a prevalence–incidence bias and minimise the effect of reverse causality led by potential confounders such as lifestyle factors."

This isn't all possible sources of reverse causality, but it does at least make it less likely.