The quality of observational studies may be upgraded if the studies are of the highest quality ― no confounding issues, large effect sizes, and a consistent dose-response relationship ― but this is rare in nutrition research.
In the processed meat analysis, the researchers evaluating reviews on this topic indicated that none of the studies involved met these standards. In fact, as the researchers explained, “The certainty of the effect estimate was downgraded to very low, due to serious risk of bias and/or serious imprecision.”
As with the studies in Annals of Internal Medicine, this study does not say that processed meat is “good for you” or that you should eat more of it. It simply says that guidance to reduce your consumption of processed meat is based on studies of very low quality and such conclusions are highly uncertain.https://www.dietdoctor.com/warnings-about-processed-meat-fail-the-test-of-science
Good article by Bret Scher (always worth listening to) on how utterly corrupt “nutrition science” is. Well worth the read.