I don’t know if the body metabolizes materials from synthetic and “natural” sources differently. It’s not so much that nitrates/nitrites are inherently bad as it is that when subjected to heat (esp. high heat) they convert to nitrosamines which are bad! Michael Ruhlman has a really interesting “rant” on his blog about natural and synthetic nitrates. We don’t use them because we don’t need to for the kind of whole muscle cured meats we make.

A lot of processors use celery extract or juice, naturally high in nitrates, instead of synthetic nitrates both for safety (botulism) and fixing the color (that rosy hue. A lot of labels will say “No nitrates or nitrites added” and then “Except what is naturally occuring in celery.” It’s still nitrates and nitrites, but the source is different.