[roel_v]

This is one of those things I have had on my 'need to look into further' list for a long time. Last status of my personal research is that it's clear that nitrite 'causes' (to just use the popular phrasing) cancer, and that many leafy vegetables are high in nitrate, and that nitrate gets converted to nitrite to some extent during fermentation.

People who make 'organic' sausages, for example, add celery as a 'preservative' so that they don't have to use the 'chemical' nitrite - except, the way it 'preserves' is that the nitrate in the celery is converted to nitrite during fermentation, but much less controlled than just measuring out nitrite in the first place, so those 'organic' or 'artisanal' sausages often have higher nitrite content than 'normal' sausages.

A quick google shows that this might happen for some vegetables too (e.g. https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1745-4549....) but I can't tell if the same would hold for all fermented food.