Nitrate is an important polyatomic ion with chemical formula NO3- Nitrates have a wide variety of uses, but they are probably most commonly used as fertilizer (for farming) and as a food additive (to preserve the red color of meats).
Unfortunately, nitrates tend to convert to nitrites due to the digestive process and bacterial action. Chemically, this only means that the molecule lost an oxygen atom. However, nitrites are known to be unhealthy to ingest. Although not officially tested in a scientific manner, many believe that nitrates themselves are also unhealthy to eat.
Additionally, nitrates are suspected to have a negative impact on mental health. It is now believed that nitrates kill off helpful bacteria in the body, which help the brain. "Those who have been hospitalized for an episode of mania (hyperactivity, euphoria and insomnia) [are] 3.5 times as likely to have ever eaten nitrate-cured meats as those with no history of a serious mental disorder."