Some studies have shown that altering the quantity or function of bacteria in the mouth blunts the beneficial effects of dietary nitrate. Such effects have been observed in animal studies of germ-free mice[1] and in clinical studies that have manipulated the oral microbial environment by adding anaerobic bacteria into the mouth[2] or by using antiseptic mouthwash.[3][4][5][4][6] The typical rise in plasma nitrite after consuming nitrate is also blunted after using antiseptic mouthwash.[7][4]
Interestingly, one epidemiological study that examined 945 individuals over 3 years found that frequent mouthwash users (at least twice daily) had a 55% greater risk of developing prediabetes or type 2 diabetes compared to less frequent users and a 49% greater risk compared to nonusers.[8] While this study did not examine mechanisms, the disruption of the nitrate-to-nitrite conversion pathway in the mouth, which is catalyzed by oral bacteria,[9] was discussed as one possible explanation for the findings.