| This might suffer from the same issue the alcohol consumption study lacked. People that aren't consuming eggs might do so because of health issues. Meaning that they are more prone to feel the side effects. Similar effect happened in the alcohol study, where moderate consumption had a lower mortality than those consuming nothing. It turned out that those abstaining from alcohol were more prone to get side effects immediately. After adjusting for these errors in data reporting, the u-curve disappeared and the relationship was linear (more alcohol you consumed higher the mortality). I was also, embarrassingly, citing the alcohol study, since then I've realized the first reaction to a paper should be doubt and I'll definitely abstain from acknowledging epidemiological diet studies in the future. Even meta-analyses taking them into account. One of the best examples is that many meta-analyses conclude that dietary cholesterol does not increase serum cholesterol (proven risk in CVD). Disregarding the fact that many of the aggregated studies do not measure baseline cholesterol, and it is assumed that there's a linear response on serum cholesterol to dietary cholesterol when it's a 20 year old information that response is non-linear. |
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.