this post was submitted on 29 Aug 2025
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Msleading title: the study didn't look at switching, just at correlations in the population.
And being vegan/vegetarian also correlates with wealth. And wealth correlates with lower rates of disease.
So yeah unless we get longitudinal studies I wouldn’t read much into these population wide correlations.
You can see my comment below on interpreting primary medical literature, so I'm not going to argue for the merits of specific claims about health based on this study. What I will address is what makes you think this study isn't longitudinal when it's called "Longitudinal associations between vegetarian dietary habits and site-specific cancers in the Adventist Health Study-2 North American cohort"? I'm also perplexed what you think "longitudinal study" means as it pertains to wealth.
There are dozens of meta-analyses broadly confirming a reduction in chronic disease with vegetarian and vegan diets if that's what you're interested in. There's so much that this particular study barely makes a dent except for the novel association between PBD and a reduced risk of lymphoma. (This still needs to be shown in meta-analyses, so it doesn't matter much.)
If the meta-analysis is based on studies with flawed methodology and/or assumptions, then the meta-analysis itself will be of little value. Is that not the case?
When a systematic review and meta-analysis is published (a meta-analysis is usually a component of the systematic review), methodologies are taken into account. It isn't just mindlessly throwing studies into a hydraulic press and fusing them together. For example, I can show you a 2017 systematic review (not a meta-analysis, but again, these usually come in one package) exploring possible links between managed bees and: 1) competition with native bees, 2) effect on fauna populations, and 3) pathogen transmission.
It's open-access if you want to read it, and over and over again the authors grill the methodology of the studies they're reviewing. They basically say "hey, the studies say this, but their collective methodologies are too weak, and so further, more robust studies need to be done." If they were just going by the studies ignoring methodology, they probably would've concluded managed bees are harmful to the ecosystem. But they couldn't because scientists care deeply about methodology. That's a major part of synthesizing scientific literature.