It is a complicated story as nitrates ARE required in cured food like bacon to prevent botulism!! You really don't want that in your food. So if we don't add it in it's pure chemical form how is it added? It comes from plant based nitrates.
We find this article by Kris Kresser, one of our favorite health and nutritions sources to be very insightful on this topic. If you want to dive down this rabbit hole and learn the how story about Nitrates give it a look: The Nitrate and Nitrite Myth: Another Reason Not to Fear Bacon
In fact, nitrites are produced by your own body in greater amounts than can be obtained from food, and salivary nitrite accounts for 70-90% of our total nitrite exposure. In other words, your spit contains far more nitrites than anything you could ever eat.